Damage evolution in nano- reinforced carbon fiber composites

Transcription

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Damage evolution in nanoreinforced carbon fiber composites
Niels De Greef
Thesis voorgedragen tot het
behalen van de graad van Master
in de Ingenieurswetenschappen:
materiaalkunde
Promotoren:
prof. dr. ir. S. Lomov
dr. ir. L. Gorbatikh
Assessoren:
prof. dr. sc. M. Seo
prof. dr. ir. B. Verlinden
Begeleiders:
dr. ir. L. Gorbatikh
dr. ir. A. Godara
Academiejaar 2009 - 2010
© Copyright by K.U.Leuven
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ii
Foreword
After four years of heavy, theoretical courses, it is time for the students of the
Faculty of Engineering of the Katholieke Universiteit Leuven to show what they
have learned during this four years and how to apply all this knowledge into
practice. What better way to do this than a master thesis? This master thesis report
presents the results of one year of hard work, both in the laboratory as at home.
I want to take this opportunity to extend a word of thanks to some people. First of
all, I would like to thank my parents for giving me the opportunity to start the five
year during educational program and for supporting me during these five
sometimes difficult years.
Of course, I cannot forget to thank my girlfriend Kelly for the permanent support
and the good and loving cares. I know it was not always easy, especially near the
end of this thesis.
Also a word of appreciation to my colleague students, for the five great years in
Leuven and for their help with everything. Michael, Dorien, Mathieu, Lennart,
Yentl, Linde, Koen, Dries and Pieter: it was a pleasure to study together with you
all.
I also want to show my appreciation to my master thesis supervisors, Larissa and
prof. Lomov, for their professional help and assistance. Larissa, thank you for the
assistance with the SEM (even on Saturdays) and for reading and correcting my
master thesis. Without your help and advice, this master thesis would just not be
possible.
Last but not least, a big thank you to the technical staff of the department
Metallurgy and Materials Engineering (MTM), which was always available for
technical assistance during this very busy year.
Niels De Greef
iii
Table of Contents
Foreword............................................................................................................................iii
Table of Contents ............................................................................................................... v
Abstract ..............................................................................................................................ix
Samenvatting .....................................................................................................................xi
List of figures and tables ................................................................................................. xiii
List of abbreviations and symbols ................................................................................. xxi
Chapter 1: Motivation ....................................................................................................... 1
Chapter 2: State of the art ................................................................................................. 3
2.1 Introduction to CNTs............................................................................................... 3
2.1.1 What are CNTs? ................................................................................................ 3
2.1.2 Types of CNTs ................................................................................................... 4
2.1.3 Properties of CNTs ........................................................................................... 6
2.1.4 Synthesis of CNTs ............................................................................................. 7
2.2 Two-phase composites: CNT-reinforced polymers ............................................ 10
2.2.1 Limitations of CNTs as reinforcements ........................................................ 10
2.2.2 Synergistic effect ............................................................................................ 12
2.2.3 Mechanical properties of nanocomposites................................................... 13
2.2.4 Challenges for processing .............................................................................. 17
2.3 Three-phase composites: Nano-engineered fiber-reinforced composites ....... 21
2.3.1 Challenges for processing .............................................................................. 21
2.3.2 Mechanical properties of three-phase composites ...................................... 23
2.4 Concluding remarks .............................................................................................. 27
Chapter 3: Materials and methods ................................................................................. 29
3.1 Materials ................................................................................................................ 29
v
3.2 Production of carbon fiber composite plates ...................................................... 31
3.3 Quality control ....................................................................................................... 33
3.4 Methods .................................................................................................................. 34
3.4.1 Static tensile test............................................................................................. 35
3.4.2 Post-mortem X-ray investigation .................................................................. 38
3.4.3 Post-mortem microscopical investigation ................................................... 39
3.4.4 Fatigue tests .................................................................................................... 39
3.5 Conclusion.............................................................................................................. 39
Chapter 4: Quality control results .................................................................................. 41
4.1 Matrix viscosity ..................................................................................................... 41
4.2 Optical microscopy ................................................................................................ 42
4.3 Thickness measurements ..................................................................................... 44
4.4 Conclusion .............................................................................................................. 45
Chapter 5: Static tensile tests results ............................................................................. 47
5.1 Tensile tests in the fiber direction ....................................................................... 47
5.1.1 Mechanical properties ................................................................................... 48
5.1.2 AE measurements........................................................................................... 50
5.2 Tensile tests in bias direction ............................................................................... 54
5.2.1 Mechanical properties ................................................................................... 55
5.2.2 AE measurements........................................................................................... 56
5.3 Conclusion .............................................................................................................. 60
Chapter 6: Post-mortem X-ray investigation results .................................................... 61
6.1 Samples tested in the fiber direction ................................................................... 61
6.1.1 Samples loaded up to ε1 ................................................................................. 62
6.1.3 Failed samples ................................................................................................ 64
6.1.4 Crack density .................................................................................................. 64
6.2 Samples tested in the bias direction .................................................................... 66
6.2.3 Failed samples ................................................................................................ 67
6.3 Conclusion.............................................................................................................. 71
vi
Chapter 7: Post-mortem SEM investigation results...................................................... 73
7.1 Damage patterns in the fiber direction................................................................ 73
7.1.1 Damage at ε1 ................................................................................................... 73
7.1.2 Damage at ε2 ................................................................................................... 76
7.1.3 Damage at failure............................................................................................ 78
7.2 Damage patterns in the bias direction ................................................................. 80
7.2.1 Damage at ε1 ................................................................................................... 80
7.2.2 Damage at ε2 ................................................................................................... 81
7.2.3 Damage at failure............................................................................................ 83
7.3 Conclusion .............................................................................................................. 86
Chapter 8: Fatigue tests results ...................................................................................... 87
8.1 Motivation .............................................................................................................. 87
8.2 Technical difficulties ............................................................................................. 87
8.3 Fatigue tests at 600 MPa ....................................................................................... 88
8.4 Conclusion .............................................................................................................. 89
Chapter 9: General conclusion........................................................................................ 91
Bibliography..................................................................................................................... 93
Appendix A: Datasheets .................................................................................................. A1
A.1 Textile reinforcement: Hexforce G0986 Injectex ............................................... A2
A.2 Carbon fibers: Hextow AS4C GP ........................................................................... A3
A.3 Epoxy resin: Epikote resin 828LVEL ................................................................... A5
A.4 Hardener: Dytek DCH-99 ...................................................................................... A7
A.5 CNTs: Nanocyl NC7000 series .............................................................................. A8
Appendix B: Data processing of tensile tests ................................................................ B1
B.1 Raw data ................................................................................................................ B1
B.2 Steps in the data processing ................................................................................. B2
B.2.1 Stress-strain curve ......................................................................................... B2
B.2.2 Calculation of mechanical properties ........................................................... B3
B.2.3 AE events and cumulative energy curve ...................................................... B4
B.3 Final result ............................................................................................................. B4
vii
Abstract
In this study, the damage initiation and evolution in woven carbon/epoxy
composites modified with carbon nanotubes (CNTs) is investigated and compared
with those in the composite without CNTs (virgin composite). The CNT-modified
composite, also called nano-engineered carbon fiber-reinforced composites
(nFRC), contains 0.25 wt% CNTs in the epoxy matrix.
To study the damage evolution in the material, tensile tests are performed in the
fiber and bias directions, accompanied by acoustic emission (AE) measurements
and full-field strain mapping (SM). The mechanical properties of the composite in
the fiber direction are hardly influenced by the CNTs, a slight improvement of the
strain to failure is noticed. The AE measurement in the fiber direction have
indicated that CNTs can shift the damage initiation threshold strain ε min towards
higher strains. Also the first damage threshold strain ε 1 and second damage
threshold strain ε2 are improved with respectively 42% and 56%. Post-mortem Xray investigation of the tensile samples loaded up to strains just above ε 1 and ε2,
and loaded till failure is done to visualize the damage inside the materials.
Transversal cracks can be seen after ε2. At a given strain level, the crack density is
lower in the nFRC composite in comparison with the virgin composite. To study
the damage patterns in both materials in more detail, also SEM investigation is
done on cross-sections of the loaded tensile samples. This investigation has
indicated that, at the same strain level, the nFRC composite has less fiber/matrix
debonding in the beginning stage of damage, less transversal cracks after ε2 and
also less transversal cracks at failure. Preliminary fatigue tests in the fiber
direction show an insignificant increase in the number of cycles to failure at high
load level for the nFRC composite.
The tensile tests in the bias direction show a significant improvement of the strainto-failure (+18%). First AE events in the nFRC composite initiate however at a
lower strain (-30%) in comparison with the virgin composite. Also ε1 and ε2 are
negatively influenced by CNTs. X-ray investigation, on the other hand, indicates
less cracks in the nFRC composite after ε2, which is confirmed by SEM investigation
of the cross-sections of loaded tensile samples. The crack density at failure is also
lower in the nFRC composite.
ix
Samenvatting
In deze masterproef is de schade-initiatie and –evolutie van geweven
koolstofvezel/epoxy composieten, gemodificeerd met koolstof nanobuizen (carbon
nanotubes, CNTs), onderzocht en vergeleken met die in composieten zonder CNTs
(referentiecomposiet). Het CNT-gemodificeerde composiet, ook wel nanoengineered koolstofvezelversterkt composiet (nFRC) genoemd, bevat 0.25 wt%
CNTs in de epoxymatrix.
Om de schade-initiatie en –evolutie in het materiaal te onderzoeken, zijn
trektesten uitgevoerd in de vezel- en biaxiale richting, bijgestaan door akoestische
emissie (AE)-metingen en full-field rekbepaling. De mechanische eigenschappen in
de vezelrichting zijn nauwelijks beïnvloed door de CNTs, een kleine verbetering
van de breukrek is opgemerkt. De AE-metingen in de vezelrichting hebben
aangetoond dat CNTs de rek van schade-initiatie εmin kunnen opschuiven naar
hogere rekken. Ook de eerste en tweede schadedrempelrekken ε1 en ε2 zijn
verbeterd met respectievelijk 42% en 56%. Post-mortem X-stralen onderzoek op
trekmonsters belast tot rekken net boven ε1 en ε2, en belast tot breuk is uitgevoerd
om de schade in beide materialen te visualiseren. Transversale scheuren zijn
zichtbaar vanaf ε2. Voor een gegeven rek is de scheurdichtheid lager in het nFRCcomposiet, in vergelijking met het referentiecomposiet. Om het schadepatronen in
beide materialen in meer detail te bestuderen, is ook SEM-onderzoek gedaan op
doorsneden van de belaste trekmonsters. Dit onderzoek heeft aangetoond dat,
voor eenzelfde rekniveau, het nFRC-composiet minder van de matrix ontbonden
vezels bevat in het beginstadium van schade, minder transversale scheuren na ε2
en ook minder transversale scheuren bij breuk. Vermoeiingstesten in de
vezelrichting tonen een insignificante verbetering van het aantal cycli tot breuk bij
een hoog belastingsniveau voor het nFRC-composiet.
De trektesten in de biaxiale richting tonen een significante verbetering van de
breukrek (+18%). De eerst registraties van AE in het nFRC-composiet initiëren
nochtans bij een lagere rek (-30%), in vergelijking met het referentiecomposiet.
Ook ε1 en ε2 zijn negatief beïnvloed door de CNTs. X-stralen onderzoek, anderzijds,
toont minder scheuren in het nFRC-composiet na ε2. Dit is bevestigd door SEMonderzoek op doorsneden van de belaste trekmonsters. De scheurdichtheid bij
breuk is ook lager in het nFRC-composiet.
xi
List of figures and tables
List of figures
Figure 1: schematic diagram showing how a hexagonal sheet of graphene sheet is
rolled to form a CNT [6] ......................................................................................................................... 5
Figure 2: illustration of the atomic structure of (a) an armchair type and (b) a zig-zag
CNT [6] .......................................................................................................................................................... 5
Figure 3: A) SEM image of two AFM tips holding a MWCNT, which is attached at both
ends on the AFM silicon tip surface by electron beam deposition of carbonaceous
material. B) high-magnification SEM image of the indicated region in (A), showing
the MWCNT between the AFM tips. [9] ........................................................................................... 6
Figure 4: fracture of an individually loaded MWCNT captured in SEM images. A) a
MWCNT having a section length of 6.9 µm under tensile load just before breaking. B)
after breaking, one fragment of the same MWCNT was attached on the upper AFM
tip and had a length of 6.6 µm. C) the other fragment of the same MWCNT was
attached on the lower AFM tip and had a length of 5.9 µm. The increase of the total
can be explained by the ‘sword-in-sheath’ telescoping failure mechanism of
MWCNTs. [9]............................................................................................................................................... 7
Figure 5: schematic representation of the carbon-arc discharge method for the
production of CNTs [14] ........................................................................................................................ 8
Figure 6: schematic representation of the laser-ablation method for the production
of CNTs [14] ................................................................................................................................................ 9
Figure 7: schematic representation of the CVD method for the production of CNTs
[14] ................................................................................................................................................................. 9
Figure 8: influence of particle volume content on separation between fibers of the
same diameter in hexagon arrangement [15] ............................................................................ 11
xiii
Figure 9: ratio of particle surface and volume for spherical and fibrous particles as a
function of the particle diameter (FS=fumed silica, CB=carbon black, CF=carbon
fiber, GF= glass fiber, BG=glass balls, l/d= aspect ratio) [15] .............................................. 12
Figure 10: comparison between the experimental data for the normalized Young’s
modulus (EN=EComposite/EResin) and the model of Krenchel for SWCNT-reinforced
epoxy [16] .................................................................................................................................................. 13
Figure 11: comparison between the experimental data for the normalized Young’s
modulus (EN=EComposite/EResin) and the model of Krenchel for MWCNT-reinforced
epoxy [16] .................................................................................................................................................. 13
Figure 12: Ultimate tensile strength of epoxy-based composites containing nonfunctionalized nanoparticles [17].................................................................................................... 14
Figure 13: experimentally obtained fracture toughness of epoxy-based composites
containing nano-particles, as a function of the weight fraction [17] ................................ 15
Figure 14: SEM micrograph of a DWCNT/epoxy composite. A surface crack, induced
by etching, is bridged by the nanotubes [16] .............................................................................. 16
Figure 15: TEM micrograph of a MWCNT bridging a matrix crack [16] .......................... 16
Figure 16: characterization of the fatigue properties of the nanocomposite and
baseline epoxy. Fatigue crack propagation tests for showing crack growth rate
(da/dN) plotted as a function of the stress intensity factor amplitude. .......................... 17
Figure 17: typical TEM micrographs of CNT/epoxy composites: (a) sonication of
MWCNT/epoxy; (b) sonication of DWCNT/epoxy [15] .......................................................... 19
Figure 19: typical TEM micrographs of CNT/epoxy composites: (a) calendering of
MWCNT/epoxy; (b) calendering of DWCNT/epoxy [15] ....................................................... 19
Figure 18: (a) three-roll calender used for the effective dispersion of CNTs in an
epoxy matrix (b) the flow conditions in the roller clearance are schematically shown
[15] ............................................................................................................................................................... 19
Figure 20: scheme of the functionalization process of CNTs showing the whole cycle
from the oxidation to the composite manufacturing. In a first step the CNTs are
oxidised (1) then functionalized (2) and finally processed to the nanocomposite (3)
[15] ............................................................................................................................................................... 20
Figure 21: the apparent viscosity of aqueous CNTs suspensions as a function of
concentration [28] ................................................................................................................................. 22
Figure 22: illustration of composite cross section: (a) CNT/polymer in both intertow and intra-tow space of fiber; (b) resin deficiency due to the dual scale effect in
the filtering process [26] ..................................................................................................................... 22
xiv
Figure 23: fracture energy of (b) crack initiation and (c) crack propagation for UD
carbon fiber laminates with 0.5 wt% of CNTs [27] .................................................................. 23
Figure 24: a) comparison of accumulation behaviour of matrix cracks in cross-ply
laminates containing different CSCNT contents and b) matrix crack densities as a
function of laminate strain [30] ........................................................................................................ 24
Figure 25: applied cyclic stress versus the number of cycles to failure of glass
fiber/epoxy laminates with and without the addition of 1 wt% of CNTs [34] ............. 26
Figure 26: cyclic mode-I crack propagation data for glass fiber/epoxy laminates with
and without the addition of 1 wt% of CNTs. Values shown here are the constant C
and m in the equation da/dN=C(ΔG)m. [36] ................................................................................ 26
Figure 27: weave pattern of a Twill 2/2 woven fabric a) obtained from WiseTex and
b) from [37]............................................................................................................................................... 30
Figure 28: a) schematic representation of the RTM process used for the production
of carbon fiber composite plates in this study [38] b) RTM mould at the department
MTM ............................................................................................................................................................. 31
Figure 29: characteristic strain levels of damage development: (a) scheme 1 –
initiation of a transversal crack, 2 – initiation of micro-delamination between the
yarns, 3 – micro-delamination caused by a transversal crack, 4 – delamination
causes fiber breakage; (b) X-ray (below) and SEM (above) images, corresponding to
the case 3, triaxial braided composite: 1 – transversal cracks, 2 – micro-delamination
[39] ............................................................................................................................................................... 35
Figure 30: test sample used to for static tensile tests, with the black and white
speckle pattern for the full-field strain measurements .......................................................... 36
Figure 31: typical AE diagram (data for a woven fabric, tested in warp direction),
definition of the damage threshold strains: (a) logarithmic scale, full AE diagram;(b)
low energy events, the jump corresponds to ε1; (c) high energy events, the jump
corresponds to ε2 [43] .......................................................................................................................... 37
Figure 32: tensile test set-up with the attacged AE sensors and the camera in front of
the test sample ......................................................................................................................................... 38
Figure 33: matrix viscosity of the neat epoxy resin and of the epoxy resin with CNTs41
Figure 34: scanned pictures of the cross-sections of a high quality (upper picture)
and low quality (lower picture) virgin woven carbon/epoxy composite plate............ 42
Figure 35: stress-strain curves of virgin woven carbon/epoxy composite samples of
low (red curve) and high (blue curve) quality tested in the fiber direction .................. 42
xv
Figure 36: microscopical pictures of cross-sections of virgin (a, b and c) and nFRC (d,
e and f) woven carbon/epoxy composites ................................................................................... 43
Figure 37: distribution of thickness of virgin (open circles) and nFRC (dark circles)
woven carbon/epoxy composite plates ........................................................................................ 44
Figure 38: two woven carbon/epoxy composite tensile samples tested in the fiber
direction till failure ................................................................................................................................ 48
Figure 39: close-up of the fracture surfaces of woven carbon/epoxy tensile samples
tested in the fiber direction ................................................................................................................ 48
Figure 40: representative stress-strain curves for virgin and nFRC woven
carbon/epoxy composites tested in the fiber direction, normalised to Vf=55% ......... 49
Figure 41: Young’s modulus as a function of the strain for virgin and nFRC woven
carbon/epoxy composites tested in the 0°-direction, normalised to Vf=55% .............. 50
Figure 42: cumulative AE energy as a function of the strain for virgin and nFRC
woven carbon/epoxy composites tested in the fiber direction .......................................... 51
Figure 43: representative stress-strain curve (green) with AE events (blue dots) and
cumulative AE energy (red) plotted as function of the strain, for a virgin woven
carbon/epoxy composite tested in the fiber direction ........................................................... 53
Figure 44: representative stress-strain curve (green) with AE events (blue dots) and
cumulative AE energy (red) plotted as function of the strain, for a nFRC woven
carbon/epoxy composite tested in the fiber direction ........................................................... 53
Figure 45: a) value of the cumulative energy at the different damage threshold
strains and b) the number of events between the different damage threshold strains
for a virgin and nFRC woven carbon/epoxy composite tested in the fiber direction 54
Figure 46: close-up of the fracture surfaces of woven carbon/epoxy tensile samples
tested in the bias direction ................................................................................................................. 55
Figure 47: representative stress-strain curves for virgin and nFRC woven
carbon/epoxy composites tested in the bias direction........................................................... 55
Figure 48: cumulative AE energy as a function of the strain for virgin and nFRC
woven carbon/epoxy composites tested in the bias direction ............................................ 57
Figure 49: stress-strain curve (green) with AE events (blue dots) and cumulative AE
energy (red) plotted as function of the strain, for a virgin woven carbon/epoxy
composite tested in the bias direction ........................................................................................... 58
xvi
Figure 50: stress-strain curve (green) with AE events (blue dots) and cumulative AE
energy (red) plotted as function of the strain, for a nFRC woven carbon/epoxy
composite tested in the bias direction ........................................................................................... 58
Figure 51: a) value of the cumulative energy at the different damage threshold
strains and b) the number of events between the different damage threshold strains
for a nFRC woven carbon/epoxy composite tested in the bias direction ....................... 59
Figure 52: X-ray images of a virgin (left) and nFRC (right) sample loaded in the fiber
direction up strains just higher than ε1 (0.35% strain for the virgin composite, 0.40%
for the nFRC composite) ...................................................................................................................... 62
Figure 53: X-ray images of a virgin (left) and nFRC (right) sample loaded in the fiber
direction up to strains higher than ε2 (0.80% strain for both the virgin and nFRC
composite) ................................................................................................................................................. 63
Figure 54: X-ray images of a virgin (left) and nFRC (right) sample loaded up to
failure in the fiber direction (1.30% strain for the virgin composite, 1.41% strain for
the nFRC composite) ............................................................................................................................. 65
Figure 55: crack density in the virgin and nFRC composite as a function of the
applied strain in the fiber direction ................................................................................................ 66
Figure 56: X-ray images of a virgin (left) and nFRC (right) sample loaded in the bias
direction up strain higher than ε1 (2.27% strain for the virgin composite, 2.30%
strain for the nFRC composite) ......................................................................................................... 68
direction up to strain higher than ε2 (4.44% strain for the virgin composite, 4.16%
strain for the NFRC composite) ........................................................................................................ 69
direction up to failure (11.47% strain for the virgin composite, 11.63% strain for the
nFRC composite) ..................................................................................................................................... 70
Figure 59: X-ray images taken at the edge of a virgin (left) and nFRC (right) sample
loaded up to failure in the bias direction ...................................................................................... 71
Figure 60: cross-sections made in the woven carbon/epoxy composite samples for
SEM investigation ................................................................................................................................... 73
Figure 61: a) SEM image of a virgin woven carbon/epoxy composite loaded up to ε1
in the fiber direction, showing fiber/matrix debonding near the boundary of the
yarn, b) at the very and of the yarn and c) close-up of the area indicated by the red
square .......................................................................................................................................................... 75
xvii
Figure 62: SEM image of a nFRC composite loaded up to ε1 in the fiber direction,
showing fiber/matrix debonding a) at the end of the yarn and b) near the end of the
yarn............................................................................................................................................................... 76
in the fiber direction, showing a transversal crack through the yarn and in the matrix
b) close-up of the area indicated by the red square ................................................................. 77
Figure 64: SEM image of a nFRC composite loaded up to ε2 in the fiber direction,
showing a transversal crack through the yarn and in the matrix ...................................... 77
Figure 65: a) SEM image of possible crack bridging by CNTs at the crack tip in a
nFRC composite and b) at higher magnification ....................................................................... 78
Figure 66: SEM image of a virgin woven carbon/epoxy composite loaded up to
failure in the fiber direction, showing transversal cracks near the ends and in the
middle of the yarns. Also a large delamination between two plies is clearly visible . 79
Figure 67: SEM image of a nFRC composite loaded up to failure in the fiber direction,
showing transversal cracks near the ends of the yarns.......................................................... 79
in the bias direction, showing fiber/matrix debonding, b) close-up of the area
indicated by the red square ................................................................................................................ 80
Figure 69: a) SEM image of a nFRC composite loaded up to ε1 in the bias direction,
showing fiber/matrix debonding at the end of the yarns and b) close-up of the area
indicated by the red square ................................................................................................................ 81
Figure 70: a) SEM image of a virgin woven catbon/epoxy composite loaded up to ε2
in the bias direction, showing a transversal crack at the end of the yarns, b) close-up
of the area indicated by the red square and c) further close-up......................................... 82
Figure 71: a) SEM image of a nFRC composite loaded up to ε2 in the bias direction,
showing a transversal crack b) close-up of the area indicated by the red square ...... 83
Figure 72: a) SEM image of a virgin woven carbon/epoxy composite loaded up to
failure in the bias direction, showing transversal cracks in the yarns, b) a transversal
crack propagating at the yarn boundary and c) pulled out fibers...................................... 84
Figure 73: a) SEM image of a virgin nFRC composite loaded up to failure in the bias
direction, showing transversal cracks in the yarns, b) a transversal crack
propagating at the yarn boundary and c) pulled out fibers .................................................. 85
Figure 74: a) failed virgin and nFRC samples, fatigue tested at 600 MPa b) close-up
of the delaminations at the fracture surface ............................................................................... 88
xviii
Figure 75: number of cycles to failure in fatigue tests in the fiber direction of woven
carbon/epoxy composites at 600 MPa .......................................................................................... 89
List of tables
Table 1: parameters of the textile reinforcement ..................................................................... 30
Table 2: production parameters of the carbon fiber composite plates............................ 32
Table 3: thickness and fiber volume fraction of virgin and nFRC woven
carbon/epoxy composite plates ....................................................................................................... 45
Table 4: comparison of the mechanical properties of virgin and nFRC woven
carbon/epoxy composites tested in the fiber direction ......................................................... 49
Table 5: damage threshold strains obtained by AE measurements for virgin and
nFRC woven carbon/epoxy composites tested in the fiber direction .............................. 52
Table 6: comparison of the mechanical properties of virgin and nFRC woven
carbon/epoxy composites tested in the bias direction........................................................... 56
Table 7: damage threshold strains obtained by AE measurements for virgin and
nFRC woven carbon/epoxy composites tested in the bias direction................................ 57
Table 8: overview of the X-ray tested samples, with corresponding CNT content and
strain level at which tensile tests in the fiber direction were stopped ............................ 61
Table 9: overview of the X-ray tested samples, with their CNT content and strain
level at which the tensile tests in the bias direction were stopped ................................... 66
xix
List of abbreviations and symbols
AE
AFM
Cn(n,m)
CNT
CSCNT
CVD
d
dpi
DWCNT
E
MWCNT
nFRC
R
RTM
SEM
SWCNT
TEM
tex
VCNT
Vf
vol%
wt%
ε
ε1
ε2
εmin
εult
ς
ςult
acoustic emission
atomic force microscopy
chiral vector
carbon nanotube
cup-stacked carbon nanotube
chemical vapour deposition
diameter
dots per inch
double-wall carbon nanotube
Young’s modulus
multi-wall carbon nanotube
nano-engineered carbon fiber composite
stress ratio in tension-tension fatigue test (=ς min/ςmax)
resin transfer moulding
scanning electron microscopy
single-wall carbon nanotube
transmission electron microscopy
g/1000m
carbon nanotube volume fraction
fiber volume fraction
volume percentage
weight percentage
strain
first transition strain / first damage threshold / damage initiation
threshold
second transition strain / second damage threshold
acoustic emission threshold strain
strain-to-failure
stress
ultimate tensile strength
xxi
Chapter 1: Motivation
Discovery of carbon nanotubes (CNTs) with their exceptional mechanical
properties has lead to novel approaches of using them as reinforcing nanofillers in
polymer composite materials. The results obtained so far promise a unique level of
property enhancement through selective use of CNTs and processing conditions.
The focus of the present study will be to increase understanding of practical
aspects related to processing of fiber-reinforced composites with CNTs and
influence of the latter on mechanical performance, particularly damage initiation
and evolution (toughness).
Toughness is not yet well realized in the existing state-of-the-art polymer
composites such as epoxy-based carbon fiber composites. The latter possess
excellent stiffness and strength, but their toughness is limited by early damage
initiation. The strain-to-failure in cross-ply laminates is typically 1.3-1.8% (under
tension). However, first damage as detected by acoustic measurements and direct
observations (X-ray, scanning electron microscopy (SEM)) occurs at about 0.20.4% strain irrespective of the reinforcement architecture. It is commonly
manifested in the form of multiple microcracks that originate at high stress
concentration sites of matrix/fiber interfaces. A threshold for the first appearance
of microcracks puts tremendous design restrictions in applications involving
fatigue loading. Moving this damage threshold to higher strains could possibly be
achieved by the addition of CNTs to the epoxy matrix. To the author’s knowledge,
this effect has not yet been studied in the literature.
In the present study CNTs are integrated in an epoxy matrix in a limited amount
(0.25 wt%) and woven carbon/epoxy composites are produced using this nanoreinforced epoxy matrix. The damage initiation and evolution of this fiber
reinforced composite is evaluated and compared with those in the composite
without CNTs.
This master thesis report contains several chapters, 9 in total. Chapter 2 of this
report gives an overview of the most relevant articles found in the literature up to
now. This chapter contains a small introduction to CNTs with information on their
structure and exceptional properties, various methods to produce CNTs, etc. The
use of CNTs as nano-reinforcements in polymers and fiber-reinforced composites
will be discussed. The materials and methods used throughout this study are
1
Damage evolution in nano-reinforced
carbon fiber composites
highlighted in chapter 3. The experimental methodology is developed in-house at
the Department of Metallurgy and Materials Engineering (MTM) of the Katholieke
Universiteit Leuven. It consists of three main steps: tensile tests accompanied by
acoustic emission (AE) measurements and optical image correlation, post-mortem
X-ray investigation and post-mortem microscopy. The results of each step in this
methodology are presented in chapters 4, 5, 6 and 7. Finally, chapter 8 highlights
the results of preliminary fatigue tests. In the conclusion, the most important
findings are once again summarized and the influence of CNTs on the damage
initiation and evolution is described.
2
Chapter 2: State of the art
This chapter gives an overview of the most relevant articles found in the literature
up to now. First a small introduction to CNTs will be given with information on
their structure and exceptional properties, various methods to produce CNTs, etc.
Also the use of carbon CNTs as nano-reinforcements in polymers will be discussed.
Review on three-phase composites (matrix, fiber and CNTs) is of particular
relevance to the present study. The position of the present study in the world of
composites will be cleared out. Finally, the conclusion summarizes the most
relevant information.
2.1 Introduction to CNTs
2.1.1 What are CNTs?
CNTs have gained a lot of interest over the past 20 years due to their exceptional
properties. In most papers, the discovery of CNTs is dedicated to Sumio Iijima, a
science researcher at NEC Japan. However, tube-like carbon nanostructures were
first observed in 1952 by Radushkevish et al. [1]. In 1976 Endo and Oberlin [2]
also observed the presence of CNTs using an electron microscope. It was however
not until 1991, when Iijima reported the observation of multi-wall CNTs in Nature
[3], that CNTs created worldwide interest.
CNTs belong to the fullerene structural family, just like the spherical buckyballs [4].
In its simplest form, CNTs can be defined as allotropes of carbon with a cylindrical
nanostructure where the walls of the tubes are hexagonal carbon. Often the CNTs
are capped at each end.
The chemical bonding at the sidewall of a CNTs is composed entirely of sp2 bonds,
similar to those of graphite. This bonding structure, which is stronger than the sp3
bonds found in diamonds, provides the molecules with their unique strength. CNTs
naturally align themselves into ropes held together by Van der Waals forces. This
strongly depends on the diameter and also on the environment.
3
2.1.2 Types of CNTs
CNTs can have many structures, differing in length, thickness, and in the type of
chirality and number of layers. CNTs typically have diameters ranging from 1 nm
up to 100 nm. Their lengths are typically several microns, but recent advancements
have made the CNTs much longer and measured in centimetres [5].
CNTs can be categorized by their chirality [6]:
 Armchair type
 Zig-zag type
 Chiral type
A single-wall CNT can be visualized as a one-atom-thick layer of a graphite sheet,
called graphene, rolled-up into a seamless cylinder. The direction in which the
graphene layer is wrapped, is represented by the chiral vector C n(n,m). n and m are
the number of unit vectors along the two directions of the hexagonal graphene
sheet (Figure 1). If n=m, the CNT is called ‘armchair’ (Figure 2a). If m=0, the CNT is
called ‘zig-zag’ (Figure 2b). All other combinations of n and m result in CNTs of the
chiral type. The chirality has important consequences for the electrical properties
of CNTs: armchair nanotubes are conductors, zigzag nanotubes are
semiconductors and chiral nanotubes behave as a diode.
Besides chirality, CNTs can be also categorized by the number of layers:
 Single-wall nanotubes (SWCNT)
 Double-wall nanotubes (DWCNT)
 Multi-wall nanotubes (MWCNT)
As mentioned before, a SWCNT can be visualized as a one-atom-thick layer of a
graphite sheet, called graphene, rolled-up into a seamless cylinder. SWCNT have a
diameter close to 1 nm, depending on (n,m), and a length in the range of microns.
In the case of SWCNTs, functionalization (adding functional groups to the carbon
atoms to add some new specific properties) will lead to breakage of some C=C
bonds and thus imperfections in the CNT. These imperfections modify both the
electrical and mechanical properties.
MWCNTs are essentially an assembly of concentric SWCNTs, where each nanotube
can have its own chirality. The concentric nanotubes are held together by
secondary Van der Waals forces. The interlayer distance in MWCNTs is close to the
distance between graphene layers in graphite, 0.34 nm. The diameters of MWCNTs
are typically in the range of 5 nm to 100 nm.
SWCNTs are more difficult to produce on a large scale but they have a better
mechanical performance than MWCNTs. However, the difficult production is
reflected in its higher price.
4
DWCNTs are an important sub-segment of MWCNTs. DWCNTs combine similar
morphology and other properties of SWCNTs, while significantly improving their
resistance to chemicals. This property is especially important when functionality is
required to add new properties to the nanotube. In contrast to SWCNTs, with
DWCNTs only the outer wall is modified, thereby preserving the intrinsic
properties.
Figure 1: schematic diagram showing how a hexagonal sheet of graphene sheet is rolled to
form a CNT [6]
Figure 2: illustration of the atomic structure of (a) an armchair type and (b) a zig-zag CNT [6]
5
2.1.3 Properties of CNTs
The interest in CNTs is justified by their exceptional properties. Earlier studies
illustrated their superior thermal and electrical properties: thermally stable up to
2800°C, thermal conductivity about twice as high as diamond, electric-currentcarrying capacity 1000 times higher than copper [7]. Of course, for use in polymer
composite materials the mechanical properties are of main interest.
Important experimental studies on the mechanical performance of CNTs are done
by Yu et al. [8, 9]. Both SWCNTs and MWCNTs are subjected to a tensile load with a
nanostressing stage located within a SEM. Individual nanotubes are attached at
each end onto opposing ends of a atomic force microscope (AFM) cantilever
probes. This can be seen in Figure 3. Each nanotube is then stress-loaded and
observed in-situ in the SEM. The results obtained by Yu et al. show a strain-tofailure as high as 12%, a Young’s modulus ranging from 270 to 950 GPa and tensile
strengths ranging from 11 to 63 GPa for the outer layer of MWCNTs. Also higher
values for the tensile strength have been reported, up to 260 GPa [10]. For SWCNTs
the Young’s modulus ranges from 320 to 1470 GPa and the tensile strength from
13 to 52 GPa1.
Yu et al. [9] also investigated the failure mechanism of MWCNTs. First the outer
layer fails under the applied tensile load, followed by pullout of the inner
nanotubes. This failure mechanism is called the ‘sword-in-sheath’ telescoping
failure mechanism, illustrated by Figure 4.
Figure 3: A) SEM image of two AFM tips holding a MWCNT, which is attached at both ends on
the AFM silicon tip surface by electron beam deposition of carbonaceous material. B) highmagnification SEM image of the indicated region in (A), showing the MWCNT between the AFM
tips. [9]
For the calculation of the Young’s moduli and the tensile strengths, the force is first
converted to stress. The stress is calculated as ς=F/A with F the force, A the cross-sectional
area and ς the stress. The cross-sectional area for SWCNTs is A=π.diameter.d with d the
interlayer distance of graphite (0.34nm). For MWCNTs, it is assumed that the load is only
applied to the outer layer and hence the cross-sectional area of the outermost layer is taken.
1
6
Figure 4: fracture of an individually loaded MWCNT captured in SEM images. A) a MWCNT
having a section length of 6.9 µm under tensile load just before breaking. B) after breaking, one
fragment of the same MWCNT was attached on the upper AFM tip and had a length of 6.6 µm.
C) the other fragment of the same MWCNT was attached on the lower AFM tip and had a
length of 5.9 µm. The increase of the total can be explained by the ‘sword-in-sheath’
telescoping failure mechanism of MWCNTs. [9]
2.1.4 Synthesis of CNTs
CNTs can be synthesized following different routes. The first CNTs, discovered by
Iijima in 1991 [3], were produced by the carbon-arc discharge method. In 1995,
Smalley et al. [11] proposed a new method: laser ablation of carbon. Nowadays,
catalytic chemical vapour deposition is the most common way for the production
of CNTs.
The large-scale synthesis of CNTs was first reported by Ebbesen and Ajayan [12],
which used the carbon-arc discharge method. Figure 5 gives a schematic
representation of the used apparatus. The carbon-arc discharge method uses two
high-purity graphite rods as anode and cathode. Under an inert atmosphere (He or
Ar), a voltage is applied between the two electrodes until an electric arc discharge
is generated. This allows sublimation of the carbon anode. The carbon then
deposits on the cathode to form MWCNTs and other carbon materials. As the anode
is consumed, a constant gap between the anode and the cathode is maintained by
moving the anode towards the cathode. To produce SWCNTs, the graphite
electrodes are first doped with a small amount of metallic catalyst particles [13].
A second method to produce CNTs is the laser ablation method, illustrated in
Figure 6. Laser ablation was first used for the synthesis of fullerenes. In this
technique, a pulsed laser is used to vaporize a graphite target in a hightemperature reactor at 1200°C in the presence of an inert gas. The vaporized
carbon forms CNTs together with carbonaceous materials and the condensed
7
material is then collected on a water-cooled target. To produce SWCNTs, the
graphite target is doped with cobalt and nickel catalyst [11].
Both the arc-discharge and the laser-ablation techniques have some limitations.
First, the produced volume of nanotubes is rather limited and thus both methods
are not applicable for industrial scale production. Secondly, subsequent
purification steps are required in order to separate them from undesirable byproducts. On top of that, the laser ablation method is quite expensive.
These limitations are overcome by gas-phase techniques, such as chemical vapour
deposition (CVD). In this technique, CNTs are grown from nucleation sites of
catalyst in carbon-based gas environments at high temperatures. The CVD setup is
shown in Figure 7. The substrate is covered with a layer of the metal catalyst
nanoparticles (iron, nickel, cobalt,…) and placed inside a heated quartz tube at
600-1000°C. A gas mixture consisting of a carrier gas (N 2, NH4, He, Ar) and a
carbon-containing gas (mostly hydrocarbons) is injected into the quartz tube. At
the surface of the catalyst particles, the carbon-containing gas is decomposed into
carbon and the carbon starts to form nanotubes. Both SWCNTs and MWCNTs can
be produced with this technique. The final purity of the as-produced nanotubes
can be high, minimizing subsequent purification steps.
Figure 5: schematic representation of the carbon-arc discharge method for the production of
CNTs [14]
8
Figure 6: schematic representation of the laser-ablation method for the production of CNTs
[14]
Support
Figure 7: schematic representation of the CVD method for the production of CNTs [14]
9
2.2 Two-phase composites: CNT-reinforced polymers
As mentioned in section 2.1, CNTs possess excellent electrical, thermal and
mechanical properties. Especially the mechanical properties, combined with their
low density, can be of large interest for the world of polymer composites, using
CNTs as nano-reinforcements. Both in two-phase (matrix and CNTs) as in threephase (matrix, fibers and CNTs) composites, studies have been performed showing
a unique level of property enhancement.
This section gives an overview of the most recent work done in the field of twophase composites using CNTs as nano-reinforcements, also called nanocomposites.
Besides these results, the limitations as well as the challenges linked to processing
with CNTs are discussed.
2.2.1 Limitations of CNTs as reinforcements
Looking at the mechanical properties, CNTs could potentially replace conventional
carbon fibers as a polymer reinforcement. This could drastically improve strength
and stiffness of polymer composites. However, there are some limitations related
to CNTs.
Limited volume fraction
State-of-the-art carbon fiber/epoxy composites contain up to 60 vol% of carbon
fibers. CNTs on the other hand cannot be added to the epoxy resin in this amount.
Their maximum achievable volume fraction is limited to about 10 vol%. Figure 8
gives more insight in this problem. It illustrates the surface separation between
particles as a function of the filler volume content and the particle size. It can be
seen that for an hexagon arrangement of SWCNTs with a diameter of 2 nm, the
separation distance between the nanotubes is 17 nm at 1 vol% and only 0.7 nm at
50 vol%. These separations are very narrow, especially when compared to the
crystal size in semi-crystalline polymers (15 µm) or the radius of gyration of an
amorphous polymer chain (10 nm). Even the size of a non-cured epoxy chain is in
the order of nanometers. It is obvious that above a critical filler volume content it
is impossible to insert a polymer chain between the nanotubes. This critical
volume content is about 10 vol% for CNTs. Above this value the separation
becomes too narrow, leading to the formation of agglomerates.
Short fiber nature / waviness
A second problem is the short fiber nature of CNTs. From micro-mechanics of
composite materials, it is known that the mechanical properties of composites
reach a maximum value above a critical fiber aspect ratio. Both continuous carbon
fibers and CNTs have a length several orders of magnitude larger than their
diameters. Therefore they can be both considered as long fibers. The problem with
CNTs lies however in the fact that CNTs show serious waviness. CNTs can hence be
represented as an assembly of small straight segments with a low aspect ratio with
each segment having another orientation. This explains their short fiber nature. As
10
a consequence, the load cannot be transferred completely from the matrix to the
nanotubes. This means that only a part of the full potential of the nanotubes is
used.
Orientation
Another problem is the orientation of the CNTs. Orientation is strongly related to
waviness. In the ideal case, nanotubes should be oriented in the loading direction
of the composite. However, it is very difficult to orient CNTs because of their
waviness. In fact, nanotubes are most of the time randomly oriented.
Dispersion
The last difficulty for processing with CNTs is the dispersion quality. Good
dispersion of the CNTs in the matrix is required. Nanotubes tend to entangle and to
form agglomerates because of the strong Van der Waals attraction forces between
the graphene layers of neighbouring nanotubes. The dispersion methods will be
discussed later.
From this discussion, it can be concluded that CNTs cannot compete with
continuous carbon fibers as reinforcement for polymer matrices. Another
approach is using CNTs as additional reinforcement in fiber-reinforced composites.
The next paragraphs give an overview of nano-reinforced polymers and their
properties.
Figure 8: influence of particle volume content on separation between fibers of the same
diameter in hexagon arrangement [15]
11
Figure 9: ratio of particle surface and volume for spherical and fibrous particles as a function of
the particle diameter (FS=fumed silica, CB=carbon black, CF=carbon fiber, GF= glass fiber,
BG=glass balls, l/d= aspect ratio) [15]
2.2.2 Synergistic effect
CNTs can be used as reinforcement for both thermosets and thermoplastics, in
order to improve their mechanical properties such as stiffness, strength and
toughness. The key point is to transfer the potential mechanical properties of
CNTs to the polymer.
The use of CNTs as a reinforcement in polymers causes a so-called synergistic
effect. First the polymer is reinforced with nanotubes which have very high
mechanical properties. Secondly, the polymer morphology is improved, leading to
better mechanical properties of the polymer itself.
The change of the polymer morphology is caused by the large surface areas
provided by the presence of CNTs. Figure 9 shows the ratio of the particle surface
and volume for spherical and fibrous particles as a function of particle diameter.
On a logarithmic scale, this ratio is linearly decreasing with increasing particle
diameter. Since CNTs have very small diameters, ranging from 1 nm to about 100
nm, their surface-to-volume ratio is among the largest known. This means that for
only a small volume content of CNTs, a very large surface area is provided.
12
This surface area can enhance the nucleation of polymer crystals in thermoplastic
polymers, where CNTs act as nucleation spots. In thermoset polymers (e.g. epoxy),
the cross-linking density can be enhanced. Both mechanism lead to better
mechanical properties by changing the polymer morphology [15].
2.2.3 Mechanical properties of nanocomposites
Stiffness
Data on the stiffness of nanocomposites can be found in [16, 17]. Figure 10 and
Figure 11 give experimental data for the stiffness of two different CNT-reinforced
epoxy systems, as a function of the CNT weight fraction. The experimental data are
compared with the model of Krenchel, which is in fact a simple rule of mixture:
E  .ECNT .VCNT  Em .(1  VCNT )
with:
η
=
ECNT =
Em =
VCNT =
(2.1)
efficiency factor taking into account the aspect ratio and the
orientation distribution (for randomly oriented CNTs: η = 0.2)
stiffness of CNTs (1000 GPa)
stiffness of the matrix (3 GPa for epoxy)
volume fraction of the CNTs
For SWCNTs (Figure 10), the experimental data follow the model of Krenchel
reasonably. On the other hand, the experimental data for MWCNTs are much lower
than expected with the model of Krenchel (Figure 11). This large difference
between experiment and theory is explained by Fidelus [16] as followed:
“However, at 0.5 wt% MWCNTs, the modulus is lower than the rule of mixture
prediction, perhaps due to less than ideal properties of the MWCNTs and to the
possible relative slippage (telescopic behavior) of the shells in MWCNTs which
would lead to poor load transfer in tension”.
Figure 10: comparison between the experimental
Figure 11: comparison between the
data for the normalized Young’s modulus
experimental data for the normalized Young’s
(EN=EComposite/EResin) and the model of Krenchel for modulus (EN=EComposite/EResin) and the model of
SWCNT-reinforced epoxy [16]
Krenchel for MWCNT-reinforced epoxy [16]
13
In general, CNT-reinforced epoxy systems underperform due to several reasons, as
mentioned in the previous paragraph. First Krenchel’s model assumes straight
nanotubes while in reality nanotubes show some waviness. Secondly, perfect
dispersion of the nanotubes in the matrix is assumed. A last reason is the
interaction with the polymer: to efficiently transfer the load from the matrix to the
nanotubes, a good adhesion/interaction between the nanotubes and the polymer
matrix is required. More detailed discussion on the dispersion in and on the
adhesion of CNTs with the polymer matrix can be found in 2.2.4.
Strength
The tensile strength of an epoxy resin is also influenced by the presence of CNTs.
Figure 12 gives experimental data of the tensile strength of an epoxy resin
reinforced with SWCNTs, DWCNTs and MWCNTs. The addition of SWCNTs results
in a clear improvement of the tensile strength, while addition of MWCNTs results
in a decrease of the tensile strength. Gojny [17, 18] links this underperformance to
the absence of stress transfer from the outer layer to the internal layers in
MWCNTs. Also the formation of improper impregnated CNT agglomerates at high
CNT content, leading to imperfections in the composite, can result in a lower
strength increase. In the case of a high MWCNT content, the high matrix viscosity
also disabled degassing, resulting in numerous voids, inducing early failure.
Figure 12: Ultimate tensile strength of epoxy-based composites containing non-functionalized
nanoparticles [17]
Fracture toughness
Fracture toughness is maybe the most important material property to improve
since neat epoxy resins are very brittle. From Figure 13, it can be seen that the
fracture toughness of epoxy-based composites is already significantly increased at
low CNT contents. Increasing the filler content further increases the fracture
toughness. Similar data can be found in [15, 17, 18].
14
The most important micro-mechanical mechanisms leading to an increase in
fracture toughness are [15, 18]:
 localised inelastic matrix deformation and void nucleation
 particle/fiber debonding (CNT debonding)
 crack deflection
 crack pinning
 fiber pull-out (nanotube pull-out)
 crack tip blunting (or crack tip deformation)
 particle/fiber deformation or breaking at the crack tip
The most important mechanism for CNT-reinforced epoxies is illustrated in Figure
14 and Figure 15. A matrix crack is bridged by several CNTs, resulting in a closing
force acting on the crack. For the crack to propagate, extra energy is required for
nanotube pull-out. This energy dissipation significantly improves the fracture
toughness. The energy required for nanotube pull-out depends on the interfacial
strength and the interfacial area. It can be expressed with the following formula:
G pull out 
With:
Vf =
L =
τi =
r =
V f .l 2 . i
3.r
(2.2)
volume fraction of the CNTs
length of the CNTs
interfacial strength of CNT/matrix interface
radius of the CNTs
For CNTs, the ratio l/r is in the order of 109. Hence the pull-out energy is
exceptionally high and therefore the nanotube pull-out mechanism contributes a
lot to the improved fracture toughness.
Figure 13: experimentally obtained fracture toughness of epoxy-based composites containing
nano-particles, as a function of the weight fraction [17]
15
Figure 14: SEM micrograph of a DWCNT/epoxy
composite. A surface crack, induced by etching, is
bridged by the nanotubes [16]
Figure 15: TEM micrograph of a MWCNT
bridging a matrix crack [16]
Not only fiber-like nano-particles can enhance fracture toughness, but also
spherical nano-particles (e.g. carbon black). From this, one can conclude that the
main fracture mechanical mechanism is related to the enormous surface area of
nano-particles in general. The crack pinning theory developed by Lange gives more
insight is this phenomena. He showed that the bowing of a crack, due to the
presence of nano-particles, would lead to an increase in fracture energy. Lange
proposed a simple equation, relating the fracture energy GIc of the composite to the
separation of the obstacles/particles S’:
TL
S'
GIc  GIcM 
With
GIcM=
TL =
S’ =
(2.3)
fracture energy of the matrix
line tension of the crack tip
separation of the particles
For randomly arranged particles, the particle separation S’ can be calculated with
the next formula:
S'
With
d =
Vp =
2.d .(1  Vp )
3.Vp
(2.4)
particle diameter
volume fraction of particles
Since CNTs have diameters in the range of several nanometers, the particle
separation S’ is very narrow and thus the fracture energy significantly increases.
16
Fatigue properties
Zhang et al. [19, 20] reported that the mechanisms of crack bridging and nanotube
pull-out can reduce the crack growth rate with a factor 20 in fatigue loading with
only 0.25wt% of CNTs added to the epoxy matrix. This is shown in Figure 16.
Theoretical modelling also showed that a high aspect ratio (small diameter and
high length) of the CNTs and a good dispersion quality are beneficial for the fatigue
behaviour epoxy nanocomposites.
Figure 16: characterization of the fatigue properties of the nanocomposite and baseline epoxy.
Fatigue crack propagation tests for showing crack growth rate (da/dN) plotted as a function of
the stress intensity factor amplitude.
2.2.4 Challenges for processing
Dispersion
A prerequisite for the complete exploitation of the potential of CNTs is a
homogeneous dispersion and distribution in the matrix. As already mentioned
before, CNTs tend to form agglomerates as a result of strong Van der Waals
interaction forces between the large surface areas of neighbouring nanotubes2. To
overcome these Van der Waals forces and thus break up the agglomerates, shear
forces have to be introduced in the CNT suspension. Several methods are
developed to obtain a good dispersion quality [15]:
 Sonication
 Mechanical stirring
 Calendering
2
Besides the strong Van der Waals interaction forces, also the waviness of the CNTs plays an
important role in the formation of agglomerates. This waviness can cause mechanical
entanglement of neighboring CNTs, which leads to agglomerates.
17
The sonication process uses ultrasonic waves to break up the nanotube
agglomerates. Ultrasonic waves introduce locally a high impact of energy but only
low shear forces are generated between individual nanotubes. Hence sonication is
not very effective to disperse CNTs, as can be seen in Figure 17: only few CNTs are
properly dispersed, large agglomerates remain. Because of the low shear forces,
this method is only suitable for small volumes and for low viscosity matrices.
Another disadvantage is a consequence of the high impact of energy. Ultrasonic
treatment of CNTs can lead to rupture and damage in the carbon structure,
resulting in smaller effective lengths [21].
A more traditional method to separate particles in a liquid phase is mechanical
stirring. This method can also be applied to disperse CNTs. After intensive stirring
a relatively fine dispersion can be obtained. However, one has to be cautious to
avoid re-agglomeration while processing the dispersed nanotube-epoxy system.
Re-agglomeration can be minimized by optimizing the processing parameters but
especially by keeping the time-to-curing as low as possible [22].
The most promising dispersion method is calendering [18]. The CNT-epoxy system
is lead through a very narrow gap between ceramic rollers (Figure 18), creating
enormous shear forces. A first pre-dispersion takes place in the knead-vortex,
while the final dispersion takes place between the rolls. This process can be
repeated several times, each time with a smaller gap size and/or a higher roll
speed. Comparing Figure 17 and Figure 19 clearly shows the better dispersion
quality obtained by calendering than by sonication. Besides the better dispersion,
no damage is introduced and the CNT length is thus preserved. A major advantage
of the calendering method is the up-scalability to industrial volumes.
Agglomerates are formed as a consequence of the strong Van der Waals attraction
forces combined with the large surface areas of the CNTs. The formation of
agglomerates can thus be partially prevented by reducing the Van der Waals forces
by functionalizing the CNTs. Functionalization of the CNTs also leads to a better
adhesion with the polymer matrix. Next paragraph goes more into detail in the
concept of functionalization.
The most used techniques to study the dispersion of the CNTs are scanning
electron microscopy (SEM) and transmission electron microscopy (TEM). The
large magnifications provided by these techniques make it possible to visualise the
CNTs, as shown in Figure 17 and Figure 19.
18
Figure 17: typical TEM micrographs of CNT/epoxy composites: (a) sonication of MWCNT/epoxy;
(b) sonication of DWCNT/epoxy [15]
Figure 18: (a) three-roll calender used for the effective dispersion of CNTs in an epoxy
matrix (b) the flow conditions in the roller clearance are schematically shown [15]
Figure 19: typical TEM micrographs of CNT/epoxy composites: (a) calendering of
MWCNT/epoxy; (b) calendering of DWCNT/epoxy [15]
19
Functionalization
Functionalizing CNTs means attaching functional groups on the surface of the
CNTs. Different types of functionalization exist but the most promising one is
chemical functionalization with multifunctional amines. A possible process scheme
is shown in Figure 20. In a first step the CNTs are oxidized to form carboxylic
groups. The second step is the actual functionalization step in which the carboxylic
groups chemically react with multifunctional amines to form covalent bonds. The
amine groups on the CNT surface can then react with the epoxy matrix to form a
strong nanotube/epoxy matrix bonding.
Chemical functionalization causes structural changes in the graphene layer which
can degrade the mechanical properties. Therefore, other functionalization types
have been developed (e.g. the use of surfactants and conjugated polymers) in
which no chemical reaction occurs and the integrity of the nanotube is preserved.
However, these types of functionalization introduce only a physical bonding
between nanotubes and matrix, which is not that strong as the covalent bonding in
chemical functionalization.
The chemical bonding between the functional groups on the CNT surface and the
epoxy matrix leads to a stronger interfacial adhesion, which enables a better stress
transfer. This is a primary prerequisite to make use of the full potential of CNTs in
nanocomposites. Via numerical simulation, Frankland et al. [23] showed that an
improvement of the mechanical properties of nanocomposites can already be
achieved with less than 1% functionalization (1% of the carbon atoms are bonded
with functional groups and form bridges with the matrix), without decreasing the
CNTs strength significantly.
Another very important advantage of functionalization is the improved dispersion
quality. The functional groups prevent the CNTs to form agglomerates by steric
hinder and electrostatic repulsion between the functional groups, leading to a
better dispersion [24].
Figure 20: scheme of the functionalization process of CNTs showing the whole cycle from the
oxidation to the composite manufacturing. In a first step the CNTs are oxidised (1) then
functionalized (2) and finally processed to the nanocomposite (3) [15]
20
2.3 Three-phase composites: Nano-engineered fiber-reinforced
composites
Previous paragraph showed that CNTs cannot compete with conventional carbon
fibers as reinforcement for polymer composites. This is mainly due to their short
fiber nature and the lack of orientation. However, CNTs have a potential to improve
the damage resistance of polymeric matrices and their composites..
This leads to a new approach in which CNTs are used in combination with carbon
fibers. Composites, which use CNTs together with carbon fibers, are called nanoengineered fiber-reinforced composites (nFRC). Another term which is frequently
used is multiscale or hybrid composites. CNTs can be introduced in a composite in
several ways:
 CNTs grown on the fibers/fabric (CNT-grafted fibers)
 CNTs introduced as a special ply in a laminate with CNTs grown on a
substrate well aligned in the thickness direction
 CNTs dispersed in the fiber sizing
 CNTs dispersed in the matrix
In the present work, the latter approach is followed. First challenges related to
addition of CNTs are briefly discussed, followed by the most important results
obtained so far.
2.3.1 Challenges for processing
In the present work, CNTs will be integrated in the epoxy matrix. As already
mentioned in the previous chapter, proper dispersion is required to exploit the full
potential of the CNTs. On top of that, some other problems might occur during
processing.
Matrix viscosity
The first problem is related to the matrix viscosity. The composite plates in the
present work are produced with Resin Transfer Moulding (RTM). RTM can only be
performed with thermoset polymers (in this study epoxy resin) because of their
low viscosity. The thermoset polymer cures in the mold and forms a threedimensional solid structure. A major problem with CNT-epoxy systems is the
increase in viscosity as function of the volume percent of CNTs (Figure 21). Beyond
a critical value, the viscosity increases significantly. This may lead to improper
impregnation and porosities in the final composite part.
Filtering of CNTs
In addition, filtering of CNTs by the textile reinforcement can be problematic in
RTM, as reported in [25] and [26]. The filtering process may occur when CNT
agglomerates are larger than the inter-yarn and/or intra-yarn gaps. The CNTs can
eventually block these gaps, leading to bad impregnation and porosities as
illustrated in Figure 22. Even when impregnation is not hindered, the CNT content
21
can gradually decrease from the inlet to the outlet, causing an inhomogeneous
microstructure. Especially at high fiber volume fraction and non-unidirectional
textile reinforcements, filtering may be problematic.
Thickness change
Another phenomenon that has been reported is the increase in thickness of
composite plates after addition of CNTs in the matrix. The increase of thickness of
a carbon/epoxy plate after addition of the CNTs in the matrix was noted in [27].
The plates (unidirectional composite) in the latter work were made in autoclave,
under pressure of 1 bar, without a spacer. The increase of thickness reached 23%
for the 0.5 wt% CNTs in the matrix. Because the reinforcement as such is not
affected by the presence of CNTs, the compaction of the reinforcement under the
same pressure of 1 bar should be the same for virgin plate and nFRC. Hence the
change of thickness in this case also should be attributed to the changes in the cure
and thermal processes inside the matrix caused by the presence of CNTs.
Figure 21: the apparent viscosity of aqueous CNTs suspensions as a function of concentration
[28]
Figure 22: illustration of composite cross section: (a) CNT/polymer in both inter-tow and intratow space of fiber; (b) resin deficiency due to the dual scale effect in the filtering process [26]
22
2.3.2 Mechanical properties of three-phase composites
Koissin et al. [29] recently made an overview of the mechanical properties of fiberreinforced composites improved with CNTs. The most important and relevant
topics of that paper are briefly discussed below.
Stiffness, strength and strain-to-failure
The addition of CNTs to the epoxy matrix has only a significant influence on the
matrix dominated properties of the three-phase composites. The in-plane stiffness
and strength are not or only slightly influenced since these properties are fiber
dominated. Also the strain-to-failure does not differ significantly. Only in
unidirectional composites, transverse stiffness and strength can be improved [2931].
Toughness
Most potential of CNTs is seen in improving toughness of fiber-reinforced
composites. Godara et al. [27] produced unidirectional carbon fiber laminates with
different types of CNTs integrated in the epoxy matrix. For the tensile properties,
only a slight difference was noticed compared to the virgin laminates. The most
promising results were obtained for the interlaminar fracture toughness: for both
the crack initiation and crack propagation, the energy release rate was increased
respectively with up to 75 and 83% (Figure 23). Microscopical post-mortem
examination of the samples revealed that again crack-bridging and nanotube pullout were responsible for this increase.
Another study by Yokozeki et al. [31] confirmed these results: a clear improvement
of mode-I and mode-II interlaminar fracture toughness of three-phase carbon fiber
composites.
Figure 23: fracture energy of (b) crack initiation and (c) crack propagation for UD carbon fiber
laminates with 0.5 wt% of CNTs [27]
23
For glass fiber composite, the results are less beneficial. In the case of Wichmann
et al. [32], the fracture toughness of glass fiber composites was not affected
significantly, despite a pronounced increase in matrix toughness. According to
Wichmann, the improved matrix properties could not be transferred to the fiberreinforced composites due to a very weak glass fiber/matrix interfacial adhesion,
leading to a nearly entire interfacial failure in all mechanical test performed.
Warrier et al. [33] produced glass fiber composites with CNTs in the fiber sizing
and/or dispersed in the epoxy matrix. A clear improvement of the fracture
toughness for crack initiation is reported, while the fracture toughness for crack
propagation significantly decreased. This can be attributed to bundling of the glass
fibers, which hindered crack bridging by individual fibers.
Matrix cracking behaviour
The most relevant article for the present study is Yokozeki et al. [30] that
investigated the matrix cracking behaviour in carbon fiber/epoxy laminates filled
with cup-stacked CNTs (CSCNTs). In the article tensile tests were performed on
cross-ply laminates till predetermined strain levels, followed by X-ray radiography
to inspect the cracking patterns.
As can be seen in Figure 24a, the onset strain of matrix cracks is shifted towards
higher values for the laminates containing CNTs. From this graph, it is also noticed
that at a certain strain level the crack density is decreased. Figure 24b clearly
illustrates this: for the laminates without CNTs, a much higher crack density is see
at 0.5% strain.
From this, it can be concluded that CNTs cause a clear retardation of matrix crack
onset and accumulation in the CSCNT-dispersed carbon fiber laminates compared
to that in the laminates without CSCNT.
a)
b)
Figure 24: a) comparison of accumulation behaviour of matrix cracks in cross-ply laminates
containing different CSCNT contents and b) matrix crack densities as a function of laminate
strain [30]
24
Fatigue properties
In the literature only few articles are found that combine CNTs and fatigue life of
fiber-reinforced composites. Recently three articles were published, focused on the
influence of CNTs on the fatigue life of glass fiber-reinforced composites. It is
important to emphasise that all three articles handle glass fiber-reinforced
composites. No articles were found during this literature search on fatigue
properties of carbon fiber composites reinforced with CNTs
Grimmer et al. [34] added 1 wt% of MWCNTs to the matrix in a woven glass/epoxy
composite. Fatigue tests were done at 70, 60, 45 and 30% of the tensile strength. It
was found that the addition of CNTs improved the fatigue life, especially in the
high-cycle fatigue regime. For the lowest stress levels, the improvement in number
of cycles went up to a factor 2.5 (Figure 25). Post-mortem SEM investigation of the
fatigue samples showed CNTs that were either pulled out of the matrix or
fractured, resulting in energy-absorbing mechanisms responsible for the improved
fatigue life.
Böger et al. [35] added 0.3 wt% MWCNTs in an epoxy matrix which was then used
to produce glass fiber/epoxy laminates. Also here a clear improvement in fatigue
life was found, again more pronounced in the high-cycle fatigue regime. This is due
to the fact that the creation and growth of inter-fiber failure is delayed to higher
load levels (or to higher cycles numbers) by the CNTs. In the low-cycle fatigue
regime, failure is a fiber-dominated process and the improvement of the matrix
properties is of minor influence.
Further recent work by Grimmer et al. [36] in 2010 focused on the delamination
fatigue resistance of glass fiber composites with CNTs to explain the more
pronounced fatigue life improvement in the high-cycle fatigue regime seen in their
previous study. The results of their work showed that the cyclic delamination crack
propagation rates were significantly reduced by the addition of 1 wt% MWCNTs to
the matrix (Figure 26). The presence of the nanotubes at the delamination crack
front slows the crack propagation by mechanisms such as crack bridging, CNT pullout and CNT fracture. Important is that, at higher levels of the applied strain
energy, the crack propagation rate in the CNT-containing composite converges
towards that of the composite without CNTs. This suggests that the energy
absorbing mechanisms are diminished at higher levels of the applied strain energy,
explaining why the improvement in fatigue life is less pronounced in the low-cycle
fatigue regime (high stress levels).
25
Figure 25: applied cyclic stress versus the number of cycles to failure of glass fiber/epoxy
laminates with and without the addition of 1 wt% of CNTs [34]
Figure 26: cyclic mode-I crack propagation data for glass fiber/epoxy laminates with and
without the addition of 1 wt% of CNTs. Values shown here are the constant C and m in the
m
equation da/dN=C(ΔG) . [36]
26
2.4 Concluding remarks
CNTs possess excellent mechanical properties (with a stiffness up to 1 TPa and a
tensile strength up to 260 GPa) combined with a low density. This makes CNTs
extremely interesting for the use in polymer composites. However, CNTs form no
alternative for conventional carbon fibers. Another approach is CNTs as nanoreinforcements in the polymer matrix in-between the fibers or at fiber/matrix
interface. This leads to improved matrix toughness and reduces the stiffness
mismatch between fibers and matrix.
It has been shown that CNTs, indeed, improve the mechanical properties of epoxy
matrices. Especially the influence of CNTs on the fracture toughness is interesting.
This increase is due to the micromechanical mechanism of crack bridging,
nanotube pull-out and nanotube fracture. Also the very large surface areas of the
CNTs contribute to the increase. Two prerequisites to make use of the full potential
of the CNTs are their proper dispersion and good interfacial adhesion to the
matrix. A good dispersion can be achieved by sonication, stirring or calendaring.
The adhesion can be improved by adding functional groups to the surface of CNTs,
which can then connect to the epoxy matrix.
For three-phase composites, the stiffness and strength in the fiber direction
remains more or less uninfluenced because these properties are controlled by
properties of the fibers. The fracture toughness, on the other hand, is significantly
improved. Maybe the most relevant fact for the present study is that CNTs can
cause retardation of the matrix crack onset and accumulation. So far, no fatigue
tests are done on carbon fiber composites containing CNTs. The fatigue tests done
on CNT-containing glass fiber composites indicate that the fatigue life is improved
by a factor two to three in the high-cycle fatigue regime. Again, the
micromechanical mechanism of crack bridging, CNT pull-out and CNT fracture are
responsible for this.
However, little is known about the influence of CNTs on the damage initiation and
evolution, and on the fatigue properties of carbon fiber-reinforced composites. It
has never been investigated whether CNTs can delay the damage initiation towards
higher strains, using acoustic emission measurements. This study is the first one to
examine this effect. It is also investigated whether CNTs can improve the fatigue
life of carbon fiber composites, another aspect which is not yet explored by the
scientific community.
27
Chapter 3: Materials and methods
In this chapter, materials used for this study are first briefly introduced. Most
attention goes to the experimental methodology used to study the damage
evolution in polymer composite material and to characterize the influence of CNTs
hereon. The experimental methodology is developed in-house in the Department
of Metallurgy and Materials Engineering (MTM) of the Katholieke Universiteit
Leuven.
3.1 Materials
In the present study, woven carbon/epoxy composite plates with and without 0.25
wt% of CNTs in the matrix are produced and used for testing. The materials are
briefly described below. More information can be found in the datasheets in
Appendix A.
Reinforcement
The used textile reinforcement is a twill 2/2 woven fabric from Hexcel (G0986
injectex), as shown in Figure 27. Detailed information about the fabric can be
found in Table 1. The fabric is balanced since the ends and picks are equal. The
AS4C GP carbon fibers have a Young’s modulus of 231 GPa, a tensile strength of
4385 MPa and a strain-to-failure of about 1.8%.
Matrix
Epikote 828LVEL is used as epoxy resin to impregnate the fabric, with a 1,2diaminocyclohexane (Dytek DCH-99) as hardener.
CNTs
The CNTs were produced by Nanocyl by CVD and already incorporated in a
Bisphenol-A epoxy resin (EpoCyl NC R128-02). This master batch epoxy resin
contains 3 wt% of non-functionalized MWCNTs, with an average diameter around
9 nm and a length of several microns , a specific surface of 250–300 m2/g and
carbon purity >90%.
29
The cost of CNTs produced using the CVD process is about € 120/kg. The EpoCyl
resin with 3 wt% CNTs costs € 43/kg. Considering that the composite plates in this
study contain only 0.25 wt% of CNTs in the epoxy resin, only a small amount of the
EpoCyl resin is used. The increase in cost of the final composite due to the addition
of CNTs can be neglected, compared to the high cost of the textile reinforcement
(about 50 €/m²).
a)
b)
Figure 27: weave pattern of a Twill 2/2 woven fabric a) obtained from WiseTex and b) from [37]
Table 1: parameters of the textile reinforcement
Fabric type
Areal density, g/m²
Fibers
Yarns
Hexcel G0986 injectex
Twill 2/2 woven
300
Carbon AS4C GP
6K
Linear density, tex
400
Ends, yarns/cm
3.5
Picks, yarns/cm
3.5
Unit cell size, mm
11.4 x 11.4
30
3.2 Production of carbon fiber composite plates
Before the actual methodology can start, test samples have to be produced. The
woven carbon/epoxy composite is produced with the Resin Transfer Moulding
(RTM) technique. In this technique the fibrous reinforcement is first placed in the
mould, the mould is closed and the mixture of resin and hardener is injected into
the mould under pressure. After the reinforcement is completely impregnated with
the resin, the mould is heated to the curing temperature of the resin. This
temperature is maintained till the resin is fully cured. After cooling down, the
composite structure can be removed from the mould. A schematic representation
of this process is shown below.
The RTM equipment at the department MTM uses two flat mould halves that form
a flat plate when put together. To control the thickness of the produced plate, a
spacer with the desired thickness is put between the two mould halves, as shown
in Figure 28.
Upper mould half
Spacer
Lower mould half
a)
b)
Figure 28: a) schematic representation of the RTM process used for the production of carbon
fiber composite plates in this study [38] b) RTM mould at the department MTM
Before injection, the neat epoxy resin is mixed with the EpoCyl resin containing the
CNTs. To obtain a CNT content of 0.25 wt%, 1 part of EpoCyl is mixed with 11 parts
of neat epoxy. The mixture is thoroughly mixed for 5 minutes with an mechanical
mixer and then the hardener is added in a ratio of 100g resin/15.2g hardener. With
the hardener added, the final resin contains 0.227 wt% CNTs. For the virgin
composite, the hardener is directly added to the neat epoxy resin in the same ratio.
After mixing it again for 5 minutes, the resin is degassed for 10 minutes in a
31
vacuum oven3. The resin is then transferred to the resin tank, degassed in the resin
tank for 10 minutes and injected into the mould under pressure. At the exit of the
mould (where the excess resin comes out of the mould), a vacuum is applied to
assist the injection.
When the injection phase is completed, the exit tube is clamped off and pressure is
increased to remove all remaining air bubbles from the mould. The injection tube
is then also clamped off and temperature is increased to the curing temperature of
the resin. In this study, the post-curing happens also inside the mould.
The production parameters are summarised in Table 2
Table 2: production parameters of the carbon fiber composite plates
Number of fabric plies
Spacer thickness, mm
Degassing time, min
Applied vacuum during
degassing, mbar
Injection temperature, °C
Injection pressure, bar
RTM technique
7
2.1
10 in vacuum oven
10 in resin tank
10-20
40 (41.8 measured)
1
Applied vacuum during
injection, mbar
Curing temperature, °C
10-20
Curing pressure, bar
4
Curing time, hours
1 (heating up not included)
Post-curing temperature, °C
150 (148.5 measured)
Post-curing pressure, bar
4
Post-curing temperature, hours
1 (heating up not included)
70 (71.8 measured)
3
During the degassing phase, a foam comes up from the resin. When this foam disappears, the
degassing of the resin is completed. This takes only few minutes but 10 minutes is used to make
sure that all air bubbles are removed from the resin since air bubbles can cause pores and voids
in the final composite.
32
3.3 Quality control
Prior to production, the matrix viscosity is measured to check if the good
impregnation is possible. After the production, a quality control is performed on
the woven/carbon epoxy composites. This control is necessary to be able to
compare the samples before and after the tensile tests.
Matrix viscosity
As already mentioned before, the addition of CNTs leads to an increase in matrix
viscosity, which can cause difficulties during injection and. therefore, the viscosity
of the neat epoxy resin is measured, as well as the epoxy resin containing different
CNT contents. The viscosity measurements are done with an AR2000 EX
Rheometer (TA Instruments) at a frequency of 10 rad/s. A temperature sweep is
applied ranging from 35°C to 60°C, with steps of 2.5°C. The tests are done in the
Department of Chemical Engineering (CIT) of the Katholieke Universiteit Leuven.
Optical microscopy
In a second step the plates are cross-sectioned at different places and the crosssections are scanned at high resolution (4800 dpi) with a simple computer
scanner. On these picture large pores and voids can be visibly, if present.
To check the impregnation quality of the composite, the cross-sections are
embedded in a polymer matrix, grinded and polished before examined with an
optical microscope at large magnification (up to 100x). Small pores and voids, as
well as cracks, can be seen at this magnifications.
Thickness measurements
A last quality control is done by measuring the thickness of the composite plates,
since CNTs can influence the thickness of the composite plates (as explained in
section 2.3.1). This thickness is determined by the thickness of the spacer. Based
on the thickness of the plates, the fiber volume fraction is calculated as:
Vf 
# plies. A
h. f
With
# plies
ρA
h
ρf
=
=
=
=
(3.1)
number of plies of the fabric
areal density of the fabric
thickness of the spacer
density of the fibers (for carbon fibers: 1.78x10 6 g/m³)
The fiber volume fractions are not measured using the epoxy burn-off test due to
extra precausions for the release of CNTs during this test.
The results of the quality control will be discussed in the next chapter.
33
3.4 Methods
The goal of the present study is to produce and to examine the damage initiation
and development in carbon fiber composites, reinforced with CNTs in the epoxy
resin (further referred to as nano-engineered carbon fiber composite, nFRC) and to
compare it with carbon fiber composites without CNTs (further referred to as
‘virgin’ composite). It is reported that the damage pattern in textile-reinforced
composites is always similar, regardless of the used textile reinforcement. The
following damage phenomena can be identified in textile composites during tensile
loading [39], as can be seen in Figure 29:
 onset of transverse cracks (inter-fiber, intra-yarn failure);
 onset of delamination on the boundaries of the fiber bundles;
 onset of fiber failure, starting at delamination, the ultimate failure of the
sample.
Damage does not necessarily start with transverse cracking, but it can also start
with localized cracks on the boundary of the fiber bundles (Figure 29 a2).
Lomov et al. [39] developed a special experimental methodology, which makes it
possible to study the different damage phenomena. This procedure consists of
three main steps:
 Tensile tests in the characteristic direction of the textile reinforcement,
accompanied with acoustic emission (AE) registration and full-field
strain mapping (SM)
 X-ray examination of the samples after tension up to certain strain levels,
identified by AE and SM results
 Cross-sectioning and microscopical examination of the samples in the
places defined by the X-ray examination
In the next sections, each of these steps will be explained in more detail.
34
Figure 29: characteristic strain levels of damage development: (a) scheme 1 – initiation of a
transversal crack, 2 – initiation of micro-delamination between the yarns, 3 – microdelamination caused by a transversal crack, 4 – delamination causes fiber breakage; (b) X-ray
(below) and SEM (above) images, corresponding to the case 3, triaxial braided composite: 1 –
transversal cracks, 2 – micro-delamination [39]
3.4.1 Static tensile test
The tensile tests (according to the ASTM D3039 standard) are performed in the
characteristic directions of the used textile reinforcement: the machine direction
or warp direction (0°), the cross-direction or weft direction (90°) and the bias
direction (45°). By performing tensile tests in different directions, the damage
pattern of the whole composite is studied. Performing tensile test in only one
direction would not give the full picture. Since the textile reinforcement used in
this study is a balanced twill 2/2 woven fabric, only tensile tests are done in the
warp and the bias direction. Tests done in the weft direction should give the same
results as for the warp direction. The tensile tests are done on an Instron 4505
tensile machine with a load cell of 100 kN. The test speed was 2 mm/min. Tensile
samples of 250 mm long and 25 mm wide are used. Fiberglass end-tabs are glued
onto the tensile samples with a two-component epoxy glue (Araldite 2011,
Huntsmann). The glue is cured at 100°C for one hour. The length between the
fiberglass end tabs is around 165 mm (Figure 30).
35
165 mm
25 mm
40 mm
60 mm
Figure 30: test sample used to for static tensile tests, with the black and white speckle pattern
for the full-field strain measurements
The tensile tests are assisted by full-field strain measurements, also called strain
mapping. This technique uses a camera to take subsequent digital images during
tensile loading. The gauge region of the test samples (about 60 mm) are first
painted with a black and white speckle pattern, which allows the Vic2D software
(LIMESS Messtechnik und Software GmbH) to quantify the deformation of the
sample. The strain mapping techniques can be used as an optical extensometer
with a precision of about 0.01%. This technique can also be used to image the
complete displacement field over the whole sample, and hence the complete strain
field. This is of large interest for textile-reinforced composites since the strain field
can be very inhomogeneous over the textile pattern. Averaging the strain field over
the sample allows deriving the average strain. This average strain is used to plot
the stress-strains curves of the tested samples. More information on full-field
strain measurements can be found in the literature [40, 41].
By applying a tensile load on the test specimen, damage is created inside the
composite material. This first damage (local fiber debonding, transverse cracks) is
not visible on the stress-strain diagram but can be observed with AE
measurements during the tensile test. During the tensile tests, the AE signals were
registered and processed by AE system AMSY-5 (Vallen Systems Gmbh). AE is
defined by Wevers et al. [42] as transient stress waves propagating in a material
that are the result of a fast release of strain energy in that material. In practice, two
AE sensors with a diameter of 2 cm are placed at the boundaries of the gauge
length (about 11.5 cm in this study). During the tensile test, these two sensors
record all signals (resulting from initiated damage) occurring between the sensors.
All other signals are filtered out by performing a calibration test before the tests.
At a certain strain level, the sensors are removed to avoid their damage. The
energy of the AE events is recorded and can afterwards converted to a cumulative
energy curve plotted as function of the applied strain (Figure 31). Based on this
cumulative energy curve, different damage threshold strains can be identified [39,
43]:
 εmin:
AE threshold strain
 ε 1:
first transition strain
 ε 2:
second transition strain
At a certain strain level, low energy AE events start to occur with low frequency.
This threshold is the AE threshold strain ε min and is believed to be caused by micro-
36
debonding of individual fibers. The first transition strain ε 1 is typically situated at
0.2-0.4% strain and is often referred as the first damage threshold or the damage
initiation threshold. It can be identified in the cumulative energy curve by a
increase of the slope, which was negligible before. This increase can be linked to
thr onset of fiber/matrix debonding. At higher strain levels, a second knee or jump
appears in the cumulative energy curve, corresponding to the second damage
threshold strain ε2. This transition strain refers to the onset of transverse intrayarn cracks, connecting debonded fibers. Just before ultimate failure, very high
energy AE events develop indicating delamination and subsequent fiber failure.
Sometimes the corresponding strain is referred to as ε 3.
Figure 31: typical AE diagram (data for a woven fabric, tested in warp direction), definition of
the damage threshold strains: (a) logarithmic scale, full AE diagram;(b) low energy events, the
jump corresponds to ε1; (c) high energy events, the jump corresponds to ε2 [43]
Figure 32 shows the complete tensile test set-up with the tensile sample clammed
in the upper and lower grip, the AE sensors attached at the boundaries of the
gauge length and the camera placed in front of the sample.
37
camera
Tensile sample
AE sensors
Figure 32: tensile test set-up with the attacged AE sensors and the camera in front of the test
sample
3.4.2 Post-mortem X-ray investigation
After the tensile test in the characteristic directions, the samples are examined
with X-rays. X-ray examination or radiography allows detecting very fine matrix
cracks within the fiber yarns.
To visualize the cracks, the samples are first penetrated with an X-ray penetrant
(diiodomethane) to fill the cracks in order to improve the contrast. Only the cracks
which are connected to the surface can be filled with the penetrant and are thus
visible. The tested tensile samples are put in the penetrant for 4 hours. The
samples are then thoroughly cleaned with a dry and a wet tissue. To prevent
evaporation of the penetrant, the samples are wrapped in a plastic foil.
The wrapped samples are placed inside to X-ray machine (AEA Tomohawk
system). A beam energy of 65 kV and a current of 0.54 mA were used to investigate
the samples. These parameters were optimized. To improve the image quality, 64
images are taken and averaged.
The X-ray pictures are analysed in a quantitative manner by estimating the cracks
density ( # cracks / mm²). The number of cracks is manually counted and divided
by the area to obtain the crack density. The area (length x width) can be calculated
based on the pixel size. The pixel size, in turn, is known by taking an image of a
known-sized object at the same magnification as the sample images.
38
3.4.3 Post-mortem microscopical investigation
The X-ray examination allows identifying the positions of the cracks. The next step
is cutting cross-sections of the samples at the defined positions and polishing
them. The polished samples are then examined with SEM to study the damage
patterns at the different applied strain levels. Prior to SEM, a gold layer is
sputtered on the polished samples to make the samples conductive and hence
prevent charging. The samples are imaged with a Philips XL30 FEG microscope at
an acceleration voltage of 15 kV. Sometimes it needs to be adjusted depending on
the material and magnification.
3.4.4 Fatigue tests
Dynamic fatigue tests (according to the ASTM D3479 standard) form the last step
in this study. Since the fatigue life of a polymer composite is dominated by the
formation of cracks, it is interesting to look at the influence of CNTs.
Fatigue tests are done on a MTS (Eden Prairie, Minnesota, USA) 100 kN servohydraulic testing machine, at a frequency of 3 Hz. The stress ratio R (=ςmin/ςmax)
was 0.1. Samples of 250 mm long and 24 mm wide were used. Aluminium end-tabs
of 24 mm by 40 mm are glued onto the tensile samples with a two-component
epoxy glue (Araldite 2011, Huntsmann). The length between the aluminium endtabs was around 165 mm. The glue is cured at 100°C for one hour.
3.5 Conclusion
The proposed methodology used throughout this study consists of tensile tests up
to certain strain levels followed by X-ray examination and a microscopical study of
the tested samples. This methodology allows to study the damage evolution of the
produced composite material and to understand which role CNTs play. It is
expected that CNTs do play a role in the formation of cracks and therefore
influence the damage evolution of the produced woven carbon/epoxy composite.
The influence of CNTs on the damage evolution should be reflected in the fatigue
life. This is investigated by doing preliminary fatigue tests at different load levels
on both virgin and nFRC composites. The full study of fatigue behaviour of the
composites is not in the scope of the present work.
The results of all the methods explained in this chapter are given in the next five
chapters, starting with the results of the quality control.
39
Chapter 4: Quality control results
In an industrial company quality control of the produced parts is of major
importance. This is also valid for the composite plates in this study. The quality
control consists of viscosity measurements of the matrix, optical microscopy of
cross-sections of the composite plates and thickness measurements to determine
the fiber volume fraction.
4.1 Matrix viscosity
The matrix viscosity of the neat epoxy resin is measured as a function of
temperature, as well as for epoxy resins containing 0.25 wt%, 0.50 wt% and the
master batch containing 3 wt% CNTs. The results are shown in Figure 33. At low
CNT content, the viscosity does not increase spectacularly compared to the neat
epoxy resin. At higher CNT content, the matrix becomes more viscous which could
give problems for impregnation.
1000
Viscosity, Pa.s
100
epoxy + 3 wt%
CNTs
epoxy + 0.50 wt%
CNTs
epoxy + 0.25 wt%
CNTs
neat epoxy
10
1
0.1
35
40
45
50
Temperature, °C
55
60
Figure 33: matrix viscosity of the neat epoxy resin and of the epoxy resin with CNTs
In the present study, the resin (containing 0.25 wt% CNTs) is injected in the mould
at 40°C. Under this condition, the matrix viscosity is increased by only 7%
compared to the neat epoxy resin, which should give no problems concerning
impregnation.
41
4.2 Optical microscopy
The cross-sections in Figure 34 are a perfect illustration of the importance of
production parameters on the final quality of the product. The virgin composite in
Figure 34a is produced with the parameters given in table 3.2. For the virgin
composite in Figure 34b produced in the first trials, the production parameters are
different: the resin is not degassed in the vacuum oven after mixing, the applied
vacuum during injection is only 300-400 mbar and finally the curing and postcuring pressure is only 3 bar. It is already clear that the above mentioned
parameters give rise to air bubbles in the epoxy resin. These air bubbles remain
present during curing and post-curing. As a result the composite plate contains
large voids (visible in Figure 34b) that act as stress concentration sites where
damage starts to develop.
Figure 34: scanned pictures of the cross-sections of a high quality (upper picture) and low
quality (lower picture) virgin woven carbon/epoxy composite plate
For this study it is very important to use composite plates with as less
manufacturing defects as possible since damage phenomena are studied. If the
plate contains pores and voids, the damage evolution is controlled by these voids
and not by the material itself. Figure 35 compares the stress-strain curve of a low
quality virgin plate with that of a high quality virgin plate. It is clear that the
tensile strength (-35%) and the strain-to-failure (-20%) drastically decrease by the
presence of voids and pores. The voids also affect the stiffness (-10%).
900
800
Stress, MPa
700
600
500
High quality virgin
400
Low quality virgin
300
200
100
0
0
0.005
Strain, -
0.01
0.015
Figure 35: stress-strain curves of virgin woven carbon/epoxy composite samples of low (red
curve) and high (blue curve) quality tested in the fiber direction
42
Chapter 4:Quality control results
To inspect the impregnation quality and the porosity content of the composite
plates after optimalisation of the production parameters, optical microscopy is
done on cross-sections coming from different regions in the virgin and nFRC
composite plates. As can be seen in Figure 36, no pores are present and the fabric
is nicely impregnated. The scratches present in some pictures are due to grinding
and polishing of the samples.
0.5 mm
a
0.5 mm
d
100 µm
100 µm
e
b
50 µm
c
50 µm
f
Figure 36: microscopical pictures of cross-sections of virgin (a, b and c) and nFRC (d, e and f)
woven carbon/epoxy composites
43
4.3 Thickness measurements
Figure 37 shows the results of thickness measurements on the composite plates.
The virgin plates are just 0.027 mm thicker than the spacer (2.10 mm). The
increase of thickness for them is 1.3% in comparison with the spacer thickness.
The nFRC plates with 0.25wt% CNTs in the matrix are thicker than the spacer by
0.258 mm (or by 12.2%). The analysis of variation (ANOVA) leaves no doubts on
the statistical significance of the difference in the plate thickness caused by the
presence of CNTs in the matrix. The final thickness of the plate is defined by the
spacer thickness and by relaxation of the plate after the mould is open, which, in
its turn, depends on cure shrinkage of the matrix and CTE, which is affected by the
presence of CNTs.
2.100
2.200
2.300
2.400
2.500
thickness, mm
Figure 37: distribution of thickness of virgin (open circles) and nFRC (dark circles) woven
carbon/epoxy composite plates
The increase of thickness of the carbon/epoxy plate after addition of the same
CNTs in the matrix was noted in [27]. The plates (UD composite) in the latter work
were made in autoclave, under pressure of 1 bar, without a spacer. The increase of
thickness reached 23% for the CNT content in the matrix of 0.5wt%. Because the
reinforcement as such is not affected by the presence of CNTs, the compaction of
the reinforcement under the same pressure of 1 bar should be the same for virgin
plate and nFRC. Hence the change of thickness in this case also should be
attributed to the changes in the cure and thermal processes inside the matrix
caused by the presence of CNTs. The thickness increase seems to be proportional
to the CNT content in the matrix (12% thickness increase for load 0.25wt%, RTM
and 23% thickness increase for load 0.5wt%, autoclave), in spite of the difference
between the manufacturing processes.
The increase in thickness also has its effect on the fiber volume fraction V f. Since
the same amount of textile reinforcement is used for both virgin plates and nFRC
plates, an increase in thickness will cause a decrease of V f (according to formula
3.1). The thickness and Vf of the tensile samples are given in Table 3. In order to
compare the tensile test results, the results have to be normalized to the same V f.
This will be further discussed in chapter 5.
44
Chapter 4:Quality control results
Table 3: thickness and fiber volume fraction of virgin and nFRC woven carbon/epoxy composite
plates
Thickness, mm
Virgin
nFRC
average
2.137
2.358
std
0.025
0.064
Fiber volume
fraction Vf, % (*)
average
std
55.2
0.7
50.2
1.3
(*) calculated according to formula (3.1)
4.4 Conclusion
The addition of 0.25 wt% CNTs to the neat epoxy matrix makes the viscosity
increase by 7% at 40°C. The increase should give no problems for impregnation,
which is confirmed by the quality control.
The quality control performed on the virgin and nFRC composite ensures good
impregnation without any pores or voids. Also no cracks caused by thermal
stresses are revealed. This means that all the cracks observed by post-mortem
investigation will be the result of the tensile loading and not of the production.
Another phenomenon is noticed by the quality control and that is the increase in
plate thickness by the presence of only 0.25wt% of CNTs in the matrix. This leads
to a lower fiber volume fraction for the nFRC composite. The increase in thickness
is due to more relaxation of the nFRC plates, which is attributed to changes in the
cure and thermal processes inside the matrix caused by the presence of CNTs. At
first sight, the increase in thickness is proportional to the CNT content.
45
Chapter 5: Static tensile tests
results
This chapter contains results of the static tensile tests, done on virgin and nFRC
tensile samples. Besides the stress-strain curves and mechanical properties of the
materials, also the AE results are highlighted. The test set-up can be found in
chapter 3.
In total, twelve tensile tests are performed in each direction (0° and 45°) on both
the virgin and nFRC woven carbon/epoxy composite:
 3 tests till failure without AE measurement
 3 tests till failure with AE measurement
 3 tests till after ε2
 3 tests till after ε1
5.1 Tensile tests in the fiber direction
The most important test direction for fiber-reinforced composites is the fiber
direction, since composites are mostly loaded in this direction. Since the used twill
2/2 woven fabric is balanced, it doesn’t matter if the tests are performed in the
warp or weft direction. In this study, all tests are done in the warp direction (0°direction). The mechanical properties measured in this direction are fiber
dominated.
The virgin and nFRC fail in the same way. The final fracture happens near the endtabs, while for some samples two fracture surfaces are noticed. The fracture
surface is perpendicular to the loading direction (Figure 38). No difference in
fracture behaviour is observed between the virgin and nFRC samples (Figure 39).
47
virgin
nFRC
Figure 38: two woven carbon/epoxy composite tensile samples tested in the fiber direction till
failure
virgin
nFRC
Figure 39: close-up of the fracture surfaces of woven carbon/epoxy tensile samples tested in
the fiber direction
5.1.1 Mechanical properties
The mechanical properties obtained from the tensile test are the Young’s modulus
E, tensile strength ς ult and strain-to-failure εult (more infromation on the data
processing can be found in Appendix B). Since the increase in thickness has lead to
different fiber volume fractions for the virgin and nFRC samples, the Young’s
modulus and tensile strength are normalised to V f=55%. The Young’s modulus is
taken as the derivative of the stress-strain curve between 0.1 and 0.3% of strain.
The Young’s modulus is calculated for all twelve samples. The strength and strainto-failure is – of course – only calculated for the six samples tested till failure.
Figure 40 gives a representative stress-strain curve of the virgin and nFRC
composite tested in the fiber direction. It can be seen that CNTs in the matrix have
no influence on the Young’s modulus of the composite, which corresponds to data
found in the literature (see section 2.3.2). The composite stiffness is controlled by
stiffness of the carbons fibers, not by the stiffness of the matrix. The increase in
Young’s modulus of the CNT-reinforced epoxy matrix is hence not reflected in a
48
Chapter 5: Static tensile tests results
higher nFRC composite stiffness. The tensile strength is also not significantly
improved because of the same reason.
Strain-to-failure is slightly improved. The improved strain-to-failure could be the
results of a delayed damage pattern due to micromechanical mechanism such as
crack bridging by CNTs and CNT pull-out. This will be further investigated and
studied in chapters 6 and 7.
Stress, MPa
The absolute values and relative differences in mechanical properties are
summarised in Table 4.
1000
900
800
700
600
500
400
300
200
100
0
virgin
nFRC
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
Strain, Figure 40: representative stress-strain curves for virgin and nFRC woven carbon/epoxy
composites tested in the fiber direction, normalised to Vf=55%
Table 4: comparison of the mechanical properties of virgin and nFRC woven carbon/epoxy
composites tested in the fiber direction
virgin
nFRC
absolute difference
relative difference
E, GPa
average
60.9
62.1
+1.2
+1.97%
std
2.0
2.2
ςult, MPa
average
847
873
+26
+3.07%
std
16
24
εult, %
average
1.32
1.38
+0.06
+4.55%
std
0.02
0.04
As can be seen in Figure 40, the stress-strain curves seem to become steeper as a
function of the applied strain. In other words, the derivative of the stress-strain
curves becomes higher and thus the Young’s modulus becomes higher. In Figure 41
the Young’s modulus is given as function of the applied strain. This clearly
49
illustrates the stiffening behaviour of the woven/carbon epoxy composite. This
behaviour is typical for carbon fiber composites loaded in the fiber direction and
can be attributed to the inherent stiffening of carbon fibers. The stiffening
behaviour of carbon fibers was first observed by Curtis et al. [44] and is caused by
improvement in orientation of imperfectly aligned crystallites in the carbon fibers
by an increasing tensile load.
Besides the inherent stiffening of the carbon fibers, decrimping of the fiber yarns
also attributes to the stiffness increase. As in every textile, the fiber yarns in the
twill 2/2 woven fabric are crimped. During the tensile tests, the fiber yarns are
first decrimped before the fibers are actually loaded and strechted. Decrimping the
yarns requires less energy than stretching the carbon fibers and this reflected in a
lower Young’s modulus in the beginning of the stress-strain curve. Together with
the inherent stiffening behaviour of the carbon fibers, this explains the stiffening
behaviour of the woven carbon/epoxy composite observed in the figure below.
80
Young's modulus, GPa
70
60
50
40
virgin
30
nFRC
20
10
0
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
Strain, Figure 41: Young’s modulus as a function of the strain for virgin and nFRC woven carbon/epoxy
composites tested in the 0°-direction, normalised to Vf=55%
5.1.2 AE measurements
The tensile tests are accompanied by AE measurements to detect damage inside
the tensile sample. The damage thresholds εmin, ε1, and ε2 are determined (as
explained in chapter 3) and compared for the virgin and nFRC composite. The
average acoustic wave velocity is 5750 m/s in the virgin composite and 6245 m/s
in the nFRC composite.
50
The strain-to-failure in cross-ply laminates is typically 1.3-1.8% (under tension in
the fiber direction). However, first damage as detected by acoustic measurements
occurs at about 0.2-0.4% strain irrespective of the reinforcement architecture. Also
the virgin and nFRC composite show signs of first damage around these values.
Figure 42 presents the cumulative AE energy curves of all samples tested in the
fiber direction. The AE sensors are removed at about 1.0% strain to avoid their
damage. At first sight, it is already clear that the nFRC composite show a delayed
damage pattern.
A more in-depth analysis of these results reveals a clear improvement due to te
presence of CNTs. The first damage detected in the virgin composite is situated
around 0.20% strain, while this value for the nFRC is increased to 0.26% strain.
Also the first transition strain ε1 is significantly shifted towards higher strains:
0.26% strain for the virgin composite compared to 0.37% strain for the nFRC
composite, an increase of more than 40%. The same trend can be recognised for
the second transition strain ε2: 0.34% strain for the virgin composite versus 0.53%
strain for the nFRC composite. Here the increase is even more than 50%. All
differences are tested with ANOVA, which indicates a significant difference for all
the transition strains.
1.0E+10
Cumulative AE energy
1.0E+09
1.0E+08
1.0E+07
1.0E+06
1.0E+05
virgin
1.0E+04
nFRC
1.0E+03
1.0E+02
1.0E+01
1.0E+00
0
0.002
0.004
0.006
0.008
0.01
Strain, Figure 42: cumulative AE energy as a function of the strain for virgin and nFRC woven
carbon/epoxy composites tested in the fiber direction
In general, it can be said that the damage initiation and evolution in woven
carbon/epoxy composites, tested in the fiber direction, is delayed by the presence
of only 0.25 wt% CNTs in the epoxy matrix. Table 5 summarizes the results. The
51
values of εmin and ε1 are determined for nine samples, the value of ε 2 for six
samples.
Table 5: damage threshold strains obtained by AE measurements for virgin and nFRC woven
carbon/epoxy composites tested in the fiber direction
virgin
nFRC
absolute difference
relative difference
εmin, %
average
0.20
0.26
+0.06
+30%
std
0.05
0.03
ε 1, %
average
0.26
0.37
+0.10
+42%
std
0.04
0.04
ε 2, %
average
0.34
0.53
+0.18
+56%
std
0.04
0.06
Analysing the cumulative energy curves together with the individual events gives
more insight in the damage evolution. In Figure 43 and Figure 44, three types of
events can be considered: events with low energy (1E+01 to about 1E+03), the
middle energy events (between 1E+03 and 1E+06) and the high energy events. In
the beginning, only few events occur with low energy. These events are believed to
be due to micro-debonding of individual carbon fibers. At the second transition
strain, higher energy events start to occur which could refer to the formation of
transverse intra-yarn cracks, connecting debonded fibers. The transverse intrayarn cracks then further develop into inter-yarns cracks causing microdelamination at the yarn boundaries. This is believed to happen at the second
transition strain. X-ray and SEM investigation will go more into detail on this topic.
Comparing the two figures, it can be said that the low energy events (micro
debonding) are delayed by the presence of the CNTs in the matrix, but not that
much. These low energy events start at low strains and keep developing till failure.
For the virgin composite, the middle and high energy events come almost
immediately after the micro-debonding. The cumulative energy curve of the virgin
composite rises fast to a high energy level. This abrupt jump is also noticed in
Figure 42 for all the cumulative energy curves of the virgin composite (not for the
nFRC composite). This means that once the fibers have debonded, there is no
mechanism stopping the cracks from growing. In the case of the nFRC, the middle
and high energy events only start to occur at higher strains which indicates that
after the micro-debonding of individual fibers, the growth of the cracks into intrayarn cracks is hindered by some kind of mechanism. The CNTs hinder the crack
growth. Which micromechanical mechanism is provided by the presence of the
CNTs will be further discussed in the next chapters.
52
1.E+10
800
1.E+09
ε2
700
1.E+08
Energy [eu]
1.E+06
500
ε1
1.E+05
1.E+04
400
300
εmin
1.E+03
200
1.E+02
1.E+01
100
1.E+00
0
0
0.002
0.004
0.006
0.008
0.01
Stress [MPa]
600
1.E+07
0.012
Strain, Figure 43: representative stress-strain curve (green) with AE events (blue dots) and cumulative
AE energy (red) plotted as function of the strain, for a virgin woven carbon/epoxy composite
tested in the fiber direction
1.E+10
800
1.E+09
700
1.E+08
Energy [eu]
1.E+06
500
1.E+05
400
1.E+04
300
1.E+03
Stress [MPa]
600
1.E+07
200
1.E+02
1.E+01
100
1.E+00
0
0
0.002
0.004
0.006
0.008
0.01
0.012
Strain, Figure 44: representative stress-strain curve (green) with AE events (blue dots) and cumulative
AE energy (red) plotted as function of the strain, for a nFRC woven carbon/epoxy composite
tested in the fiber direction
53
1.E+10
1.E+09
10000
virgin
virgin
nFRC
nFRC
1.E+08
Cumulative energy, eu
1000
Number of events, -
1.E+07
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
100
10
1
1.E+00
at ε_min at ε_1
at ε_2 at 1,0%
strain
between between ε_1 between ε_2
ε_min and
and ε_2
and 1,0%
ε_1
strain
a)
b)
Figure 45: a) value of the cumulative energy at the different damage threshold strains and b)
the number of events between the different damage threshold strains for a virgin and nFRC
woven carbon/epoxy composite tested in the fiber direction
In Figure 45a, the average value of the cumulative energy at the different damage
threshold strains is given. The cumulative energy is higher for the virgin composite
at all stages (except at 1.0% strain). This corresponds to the observation that the
energy level takes an abrupt jump to higher values in the virgin composite. At 1.0%
strain the nFRC composite has reached the same energy level, it has gone more
gradually to this level. The data in Figure 45b shows that the number of events
remains however approximately the same for both types. From these two findings,
it can be concluded that the average energy for a single event at low strains is
higher for the virgin composite than for the nFRC composite.
5.2 Tensile tests in bias direction
In the bias direction, the mechanical properties are no longer fiber dominated but
properties of the matrix become more important. Compared to fiber-reinforced
composites tested in the fiber direction, composites tested in the bias direction
exhibit a much lower Young’s modulus, a lower tensile strength and much higher
strain-to-failure.
The fracture behaviour is again similar for the virgin and nFRC samples. The final
fracture occurs more or less in the middle of the sample, not near the end-tabs. All
samples have a V-shaped fracture surface, as shown in Figure 46.
54
virgin
nFRC
Figure 46: close-up of the fracture surfaces of woven carbon/epoxy tensile samples tested in
the bias direction
5.2.1 Mechanical properties
Figure 47 presents representative stress-strain curves for virgin and nFRC
composite samples tested in the bias direction. The Young’s modulus is again
comparable for both types. Most improvement is seen in the strain-to-failure. The
nFRC can be elongated up to almost 14% strain, which means an improvement
with almost 20% compared to the virgin composite. This significant improvement
is more pronounced than in the fiber direction, because the matrix becomes more
dominant in the bias direction. This would suggest that also the tensile strength of
the nFRC composite is improved. The average value has indeed increased (+9%)
but the large standard deviation does not allow consider this improvement as
significant. Table 6 summarizes the results.
250
Stress, MPa
200
150
virgin
100
nFRC
50
0
0
0.02
0.04
0.06
0.08
Strain, -
0.1
0.12
0.14
Figure 47: representative stress-strain curves for virgin and nFRC woven carbon/epoxy
composites tested in the bias direction
55
Table 6: comparison of the mechanical properties of virgin and nFRC woven carbon/epoxy
composites tested in the bias direction
virgin
nFRC
absolute difference
relative difference
E, GPa
average
13.2
13.3
+0.1
+0.32%
std
0.5
0.3
ςult, MPa
average
203
222
+19
+9.36%
std
8
23
εult, %
average
11.64
13.79
+2.15
+18.47%
std
0.29
0.93
5.2.2 AE measurements
The situation for the tensile tests in the bias direction is somewhat different. In
Figure 48, it can be seen that the cumulative AE energy curve of the nFRC samples
is shifted to lower strain values instead of higher strain values. The damage starts
to initiate earlier and also the other damage thresholds appear earlier. The data in
Table 7 reveals that all the damage thresholds for the nFRC decrease with more
than 30% compared to the virgin composite.
At first sight, these results are in contradiction with the idea that CNTs can delay
the damage initiation and evolution, like the results in the fiber direction showed.
The hypothesis here is that the CNTs hinder the deformation of the epoxy matrix
under shear, causing the first damage to appear at low strains. The epoxy matrix is
mainly shear loaded in tensile tests in the bias direction. Pure epoxy can be is
much more ductile in shear than in tension. It can be elongated up to 40% in shear
and it might be possible that CNTs hinder this high shear deformation. In the
department MTM, a study is in progress to investigate the shear behaviour of
CNT/epoxy nanocomposites. This will be further investigated with SEM.
Despite the early AE events, the ultimate tensile strength of the nFRC samples has
not dropped below the strength of the virgin samples (see section 5.1.2). Moreover,
the strain-to-failure of the nFRC has increased with almost 20%. Attention will be
given to the source of the first AE events in the X-ray and SEM investigation.
56
1.0E+09
Cumulative AE energy
1.0E+08
1.0E+07
1.0E+06
1.0E+05
1.0E+04
virgin
1.0E+03
nFRC
1.0E+02
1.0E+01
1.0E+00
0
0.02
0.04
0.06
Strain, Figure 48: cumulative AE energy as a function of the strain for virgin and nFRC woven
carbon/epoxy composites tested in the bias direction
Table 7: damage threshold strains obtained by AE measurements for virgin and nFRC woven
carbon/epoxy composites tested in the bias direction
virgin
nFRC
absolute difference
relative difference
εmin, %
average
1.34
0.89
-0.45
-34%
std
0.17
0.14
ε 1, %
average
2.02
1.37
-0.65
-32%
std
0.32
0.26
ε 2, %
average
2.90
2.01
-0.89
-31%
std
0.56
0.24
57
1.E+09
200
1.E+08
180
1.E+07
160
140
1.E+06
120
1.E+05
100
1.E+04
80
1.E+03
60
1.E+02
40
1.E+01
20
1.E+00
0
0
0.02
Strain, -
0.04
0.06
Stress [MPa]
Energy [eu]
0.08
Figure 49: stress-strain curve (green) with AE events (blue dots) and cumulative AE energy (red)
plotted as function of the strain, for a virgin woven carbon/epoxy composite tested in the bias
direction
1.E+09
200
1.E+08
180
1.E+07
160
140
120
1.E+05
Stress [MPa]
Energy [eu]
1.E+06
100
1.E+04
80
1.E+03
60
1.E+02
40
1.E+01
20
1.E+00
0
0
0.02
0.04
Strain, -
0.06
0.08
Figure 50: stress-strain curve (green) with AE events (blue dots) and cumulative AE energy (red)
plotted as function of the strain, for a nFRC woven carbon/epoxy composite tested in the bias
direction
58
1.E+09
1.E+08
10000
virgin
nFRC
virgin
nFRC
1.E+07
Cumulative energy , eu
1000
Number of events, -
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
100
10
1.E+01
1
1.E+00
at ε_min at ε_1
a)
at ε_2
at 6,0%
strain
between between ε_1 between ε_2
ε_min and
and ε_2
and 6,0%
ε_1
strain
b)
Figure 51: a) value of the cumulative energy at the different damage threshold strains and b)
the number of events between the different damage threshold strains for a nFRC woven
carbon/epoxy composite tested in the bias direction
If we look at individual AE events in Figure 49 and Figure 50, the difference in
number of events is striking. Not only does the damage seems to start earlier in the
nFRC composite but also much more AE events are detected after ε2. This is
confirmed by the data in Figure 51b: the number of events detected before ε2 is the
same for both types while the number of events detected after ε 2 is significantly
higher for the nFRC composite.
Also the value of the cumulative energy at ε2 is almost a factor 30 higher for the
nFRC composite. At failure, the nFRC composite has thus dissipated much more
energy than the virgin composite.
59
5.3 Conclusion
The tests in the fiber direction indicate a slight improvement of about 5% of the
strain-to-failure for the nFRC composite. Young’s modulus and tensile strength do
not change significant. Most important improvement is noticed in the AE
measurements. First AE events initiate at a higher strain (30% improvement) and
also the first and second transition strain are remarkably higher (respectively 42%
and 56% improvement). In general, it can be said that already a small amount of
CNTs (only 0.25 wt%) delays the damage initiation and evolution in the fiber
direction.
The Young’s modulus of the composite in the bias direction is hardly influenced by
the presence of CNTs. The tensile strength and especially the strain-to-failure, on
the other hand, are improved because these properties are more matrix dominated
than in the fiber direction. The AE measurements give rather unexpected results:
all damage threshold strains drop with more than 30% compared to the virgin
composite. It is hypothesized that the shear deformation of the epoxy matrix is
hindered by the CNTs, causing earlier damage initiation and evolution. This will be
investigated with SEM.
The next chapter handles the X-ray investigation in which damage patterns are
investigated inside the loaded samples.
60
Chapter 6: Post-mortem X-ray
investigation results
In the previous chapter it was shown that CNT positively influence the damage
thresholds in the fiber direction and negatively in the bias direction. This chapter
gives the results of the X-ray investigation done on failed samples and samples
loaded up to strains somewhat higher than the damage threshold strains ε1 and ε2.
The objective of this investigation is to get more insight in what happens in the
material at the different damage threshold strains and what role CNTs might play.
6.1 Samples tested in the fiber direction
For both virgin and nFRC six tensile samples (two loaded up to strains just above
ε1, two up to strains just above ε2 and two up till failure) are imaged with X-rays. An
overview of the samples and the strain at which the tensile tests are stopped, is
given in Table 8. The failed samples are looked at near the failed zone, near the
middle and in-between these two regions. The samples loaded up to ε 1 and ε2 are
looked at near the end-tabs.
Table 8: overview of the X-ray tested samples, with corresponding CNT content and strain level
at which tensile tests in the fiber direction were stopped
Sample
0_03
0_05
CNT_0_10
CNT_0_11
0_06
0_08
CNT_0_09
CNT_0_13
0_09
0_11
CNT_0_05
CNT_0_06
wt% CNTs
0
0
0.25
0.25
0
0
0.25
0.25
0
0
0.25
0.25
Stopped after
ε1
ε1
ε1
ε1
ε2
ε2
ε2
ε2
failure
failure
failure
failure
Stopped at strain, %
0.35
0.36
0.36
0.40
0.80
0.80
0.68
0.80
1.30
1.35
1.39
1.41
61
6.1.1 Samples loaded up to ε1
In Figure 52, X-ray images of the samples tested in the fiber direction up to strains
just above ε1 are given. No cracks are visible on the frontal images (not even at the
edge of the sample), neither on the side view images. The first damage detected by
AE can thus not be detected by X-ray investigation. These small cracks are
probably situated inside the yarns (micro-debonding at the fiber/matrix interface
and intra-yarn cracks) and are not connected to the surface. Because of the small
dimensions and the lack of penetrant in these small cracks, the cracks are not
visible with X-rays. These cracks can however be detected by SEM (see chapter 7).
5 mm
5 mm
Figure 52: X-ray images of a virgin (left) and nFRC (right) sample loaded in the fiber direction up
strains just higher than ε1 (0.35% strain for the virgin composite, 0.40% for the nFRC composite)
The X-ray images of the samples tested in the fiber direction up to strains just
above ε2 are given in Figure 53. In contrast with the samples loaded up to ε1, clear
cracks are visible on the frontal images and also on the sideview images. The crack
density is however still rather low. This can be linked with the relatively small
number of events detected by AE measurements up to ε 2. Up to ε2, about 100 events
were detected in the entire sample. With X-ray, only certain parts are imaged and
hence the number of cracks is even lower.
Damage is still very localised and not at all regular. No periodicity is noticed in the
crack pattern. The images in sideview show no signs of delamination.
The crack length is also limited: most cracks have a length of about 3 mm, which
corresponds to the width of a yarn (2.9 mm). The cracks are stopped where the
yarn goes under another yarn. Some larger cracks seem to be present. It is also
possible that this is just optical illusion were several small cracks are present at
the same height, but at a different place through the thickness. SEM investigation
will clarify this.
62
Chapter 6: Post-mortem X-ray investigation results
5 mm
5 mm
Near the end-tabs
5 mm
5 mm
In-between end-tabs and middle of the sample
5 mm
5 mm
Middle of the sample
Figure 53: X-ray images of a virgin (left) and nFRC (right) sample loaded in the fiber direction up
to strains higher than ε2 (0.80% strain for both the virgin and nFRC composite)
63
6.1.3 Failed samples
In Figure 54 X-ray images of the samples tested up to failure in the fiber direction
are given. On the frontal images, transversal cracks are clearly visible. Compared to
the samples loaded up to ε2, the cracks density has increased a lot. The sample is
saturated with cracks. This is consistent with the large number of events detected
with AE after ε2. The crack pattern is also more periodic, the distance between two
cracks is more or less constant.
The crack length has also increased. While in the samples loaded up to strains just
above ε2 the crack length was limited to about 3 mm, the crack length is now
higher. The small cracks seem to be connected and to form one large crack
together. This is however not proven. It might as well be possible that the small
cracks are situated at another place through the thickness, as explained in the
previous section.
Not only transversal cracks are present, but also large delaminations. This can be
seen on the sideview images. The closer to the failed zone, the more severe these
delaminations become. These delaminations are responsible for the final failure of
the composite samples.
Near the failed zone, the phenomenon of “splitting” or “kinking” is observed:
transversal cracks can change direction and turn into longitudinal cracks. This
phenomenon is more pronounced in the nFRC composite.
6.1.4 Crack density
It is difficult to compare the X-ray images of the virgin and nFRC samples by just
looking at them. Therefore, a semi-quantitative analysis is done on all the images
as explained in chapter 3.
The results are given in Figure 55. At strains just after ε1, no transversal cracks are
seen in the virgin composite, neither in the nFRC composite. After strain higher
than ε2 (at 0.80%), the crack density is about 10% higher in the virgin composite:
6.2 cracks/cm² in the virgin composite against 5.0 cracks in the nFRC composite.
Generally spoken, the cracks density is lower in the nFRC composite compared to
the virgin composite. In other words, the same level of cracks/damage is only
reached at higher strains: the damage evolution is delayed.
64
5 mm
5 mm
Failed zone
5 mm
5 mm
In-between failed zone and middle of the sample
5 mm
5 mm
Figure 54: X-ray images of a virgin (left) and nFRC (right) sample loaded up to failure in the fiber
direction (1.30% strain for the virgin composite, 1.41% strain for the nFRC composite)
65
50
Crack density, /cm²
45
40
35
30
25
virgin
20
nFRC
15
10
5
0
0.00%
0.50%
Strain, %
1.00%
1.50%
Figure 55: crack density in the virgin and nFRC composite as a function of the applied strain in
the fiber direction
6.2 Samples tested in the bias direction
Just as for the samples tested in the fiber direction, six tensile samples (two loaded
up to strains just above ε1, two up to strains just above ε 2 and two up till failure)
are imaged with X-rays for both the virgin and nFRC composite. An overview of the
samples and the strain at which the tensile tests are stopped, is given in Table 9.
Again, the failed samples are looked at near the failed zone, near the middle and inbetween these two regions. The samples loaded up to ε 1 and ε2 are investigated
near the end-tabs.
Table 9: overview of the X-ray tested samples, with their CNT content and strain level at which
the tensile tests in the bias direction were stopped
Sample
45_09
45_11
CNT_45_07
CNT_45_08
45_06
45_08
CNT_45_13
45_15
45_17
CNT_45_02
CNT_45_05
wt% CNTs
0
0
0.25
0.25
0
0
0.25
0
0
0.25
0.25
Stopped after
ε1
ε1
ε1
ε1
ε2
ε2
ε2
failure
failure
failure
failure
Stopped at strain, %
2.27
2.21
2.35
2.30
3.44
4.44
4.16
11.63
11.47
11.63
14.17
66
Figure 56 gives the X-ray images of the samples tested in the bias direction up to
strains just above ε1. For the virgin sample, some cracks are visible. The nFRC
composite sample shows no signs of damage. This contradicts with the fact that AE
measurements indicated an earlier damage initiation for nFRC samples. SEM
investigation will bring more insight.
The X-ray images of samples loaded up to strains just above ε2 clearly show an
increase of the crack density (Figure 57). Especially the virgin sample contains a
lot of cracks, while the nFRC sample contains only a few cracks. Again, this is not as
expected based on the results of the AE measurements. This could possibly mean
that the events detected by AE in the nFRC samples are not due to cracks, but due
to another damage mechanism which cannot be detected by X-ray investigation.
6.2.3 Failed samples
X-ray images of the samples tested up to failure in the bias direction are given in
Figure 58. The number of cracks has spectacularly increased. Besides the large
number of cracks, also starting delaminations has occurred between the fabric
plies. This has lead to failure of the samples.
All the frontal images in Figure 58 are taken in the centre of the sample. It is also
interesting to compare these images with images taken at the edge of the sample.
From the images in Figure 59, it is clear that the crack density at the edge of the
sample is much higher than in the centre. It is found that cracks often initiate at the
edge.
67
5 mm
5 mm
Near the end-tabs
5 mm
5 mm
In-between the end-tabs and middle of the sample
5 mm
5 mm
Figure 56: X-ray images of a virgin (left) and nFRC (right) sample loaded in the bias direction up
strain higher than ε1 (2.27% strain for the virgin composite, 2.30% strain for the nFRC
composite)
68
5 mm
5 mm
Near the end-tabs
5 mm
5 mm
In-between the end-tabs and middle of the sample
5 mm
5 mm
to strain higher than ε2 (4.44% strain for the virgin composite, 4.16% strain for the NFRC
composite)
69
5 mm
5 mm
Failed zone
5 mm
5 mm
In-between the failed zone and middle of the sample
5 mm
5 mm
to failure (11.47% strain for the virgin composite, 11.63% strain for the nFRC composite)
70
5 mm
5 mm
Figure 59: X-ray images taken at the edge of a virgin (left) and nFRC (right) sample loaded up to
failure in the bias direction
6.3 Conclusion
The results of the X-ray investigation has given more insight in the damage
evolution of both the virgin and nFRC composite.
For the samples tested in the fiber direction, no cracks can be seen just after ε1: the
cracks are probably too small and not connected to the surface. It is believed that
these cracks are situated at the fiber/matrix interface and inside the yarns. At ε2,
clear cracks are visible. These cracks have a limited length and their appearance is
not periodic. At failure, the number of cracks has spectacularly increased. Not only
transversal cracks are present but also large delaminations, causing the failure of
the sample. An important difference between the virgin and nFRC composite is
found in the crack density. It has also been shown that the crack density at a given
strain is lower in the nFRC composite or, in other words, that the same level of
damage is only reached at a higher strain level.
For the samples tested in the bias direction, the results of the X-ray investigation
are in contradiction with the results of the AE measurements, at least, in the scope
of what is known about damage development in textile composites without CNTs.
After ε1, some cracks are seen in the virgin sample while the nFRC sample contains
no cracks. AE measurements, on the other hand, had indicated that damage starts
earlier in the nFRC composite. Also at ε2, more cracks are visible in the virgin
sample. This could mean that the many events detected by AE in the nFRC sample
are not due to cracks but due to another damage mechanism which cannot be seen
by X-ray radiography. SEM investigation should be able to clarify this.
71
Chapter 7: Post-mortem SEM
investigation results
In this chapter, the results of the SEM investigation are presented. After the X-ray
investigation, the loaded tensile samples are cross-sectioned and prepared for
SEM. The damage at the different damage threshold strains is carefully studied and
compared for the virgin and nFRC composite. Also micromechanical mechanisms
such as crack bridging by CNTs, CNT pull-out and CNT breakage are searched for.
7.1 Damage patterns in the fiber direction
The samples are cross-sectioned at three places, corresponding to the regions
imaged with X-rays. The cross-sections are made perpendicular to the transversal
cracks, as shown in Figure 60.
1
2
3
Figure 60: cross-sections made in the woven carbon/epoxy composite samples for SEM
investigation
7.1.1 Damage at ε1
X-ray images of the samples loaded up to strains just after ε 1 have shown no signs
of transversal cracks. The number of events detected by AE measurements is still
rather low and these events are probably linked to fiber/matrix debonding. This
first damage is too small to be seen by X-ray investigation.
However, SEM investigation is able to detect very fine fiber/matrix debonding. At
small magnifications, the debonded fibers are surrounded with a blur, white area.
73
At larger magnifications however, these white areas disappear and reveal the
fiber/matrix debonding. The debonded fibers are often not isolated, but are part of
a chain of debonded fibers (=several debonded fibers close together)
The virgin samples contain a lot more chains of debonded fibers. The debonded
fibers are usually situated at the boundaries and the very ends of the yarns (Figure
61). Not all yarns have debonded fibers at those places, only yarns which seem to
have an irregular shape at this places. At this stage, no transversal cracks are yet
created.
It is quite unusual to find chains of debonded fibers at the very ends of the yarns,
as transversal cracks at higher strains do not develop at this location but rather
away from the ends. This will be discussed in the next section.
a)
b)
74
Chapter 7: Post-mortem SEM investigation results
c)
Figure 61: a) SEM image of a virgin woven carbon/epoxy composite loaded up to ε1 in the fiber
direction, showing fiber/matrix debonding near the boundary of the yarn, b) at the very and of
the yarn and c) close-up of the area indicated by the red square
The nFRC samples also contain chains of debonded fibers, also near and at the
ends of the yarns as shown in Figure 62. The number of debonded fibers is
however less than in the virgin samples. It is believed that the CNTs in the matrix
resist debonding. A better fiber/matrix interfacial adhesion is suspected at the
fiber/matrix interface.
a)
75
b)
Figure 62: SEM image of a nFRC composite loaded up to ε1 in the fiber direction, showing
fiber/matrix debonding a) at the end of the yarn and b) near the end of the yarn
7.1.2 Damage at ε2
The second damage threshold strain is defined by a jump in the cumulative energy
curve. This means that high energy events start to occur and another damage
mechanism than fiber/matrix debonding becomes active. X-ray investigation has
shown that transversal cracks become visible after ε2. This is confirmed by SEM
investigation: besides the debonded fibers, also some transversal cracks are
present at this stage.
The transversal cracks in the virgin samples are situated near the end of the yarns,
where a chain of debonded fibers was formed at ε1. The transversal cracks have
connected the debonded fibers. After the crack has reached the edge of the yarn,
the crack goes further in the matrix. All this is shown in Figure 63.
a)
76
b)
Figure 63: a) SEM image of a virgin woven carbon/epoxy composite loaded up to ε2 in the fiber
direction, showing a transversal crack through the yarn and in the matrix b) close-up of the
area indicated by the red square
The transversal cracks is the nFRC composite are also situated near the end of the
yarns. At the boundary of the yarns, they grow further into the matrix (Figure 64).
The number of cracks is less than for the virgin composite. At closer look at the
crack tip shows some crack bridging by CNTs, which was not present in the virgin
samples. It is believed that CNTs (covered with resin) bridge the cracks. Also some
fractured or pulled-out objects that look like CNTs can be seen in Figure 65. It is
unfortunately not proven that this are really CNTs.
Figure 64: SEM image of a nFRC composite loaded up to ε2 in the fiber direction, showing a
transversal crack through the yarn and in the matrix
77
a)
b)
Figure 65: a) SEM image of possible crack bridging by CNTs at the crack tip in a nFRC composite
and b) at higher magnification
7.1.3 Damage at failure
The damage at failure has drastically increased, compared to the damage after ε2. A
lot of additional transversal cracks are created, which was already noticed based
on the AE measurements and the X-ray investigation.
The main difference between the damage in the virgin and nFRC samples is the
position of the transversal cracks. The transversal cracks in the virgin samples are
situated not only near the end of the yarns, but also in the middle of the yarns.
Almost every yarn contains four transversal cracks: two in the middle and two
near the end of the yarn (Figure 66). The average distance between the cracks is
around 700 µm.
In the nFRC samples, on the other hand, the number of yarns with transversal
cracks is remarkably lower. Yarns without cracks are regularly noticed, while in the
virgin sample almost every yarn shows cracks. A second difference with the virgin
78
samples is that the damaged yarns in the nFRC samples contain only transversal
cracks near the end of the yarns. This can be seen in Figure 67. Only occasionally a
damaged yarn with four cracks is noticed. The average distance between the cracks
is about 1.9 mm.
Besides the transversal cracks, also some delaminations are created. X-ray
investigation has already indicated this. Since these delaminations were not yet
present at ε2, this implies that they were created in the stages before failure and
that they have probably lead to final failure of the composite.
Figure 66: SEM image of a virgin woven carbon/epoxy composite loaded up to failure in the
fiber direction, showing transversal cracks near the ends and in the middle of the yarns. Also a
large delamination between two plies is clearly visible
Figure 67: SEM image of a nFRC composite loaded up to failure in the fiber direction, showing
transversal cracks near the ends of the yarns
79
7.2 Damage patterns in the bias direction
The samples in the bias direction are cross-sectioned at the same places and in the
same direction as the samples tested in the fiber direction (see Figure 60). The
images are now different since the fibers and yarns lay under an angle of 45° with
the cutting direction.
7.2.1 Damage at ε1
At this stage no transversal cracks are present in both materials. Only chains of
debonded fibers are noticed. Again, the debonded fibers are surrounded by blur,
white areas. A closer look proves that this are really debonded fibers. The chains of
debonded fibers occur at the end of the yarns and at the places where the yarns
have an irregular, unsmooth shape. The red square in Figure 68 indicates such a
place. No differences are seen between the virgin and nFRC samples.
Although the X-ray images of the virgin samples have shown a few cracks at this
stage, no cracks were found during the SEM investigation in the virgin material.
This is very likely, since SEM investigation is only done at three sections. The
probability of finding a crack, is hence not that big.
a)
b)
Figure 68: a) SEM image of a virgin woven carbon/epoxy composite loaded up to ε1 in the bias
direction, showing fiber/matrix debonding, b) close-up of the area indicated by the red square
80
a)
b)
Figure 69: a) SEM image of a nFRC composite loaded up to ε1 in the bias direction, showing
fiber/matrix debonding at the end of the yarns and b) close-up of the area indicated by the red
square
7.2.2 Damage at ε2
The X-ray images at ε2 have shown a lot of cracks in the virgin sample, while only a
few cracks were present in the nFRC sample. This observation is confirmed by the
SEM investigation.
The virgin samples contain more cracks in the yarns than the nFRC samples. At
first sight, these cracks like polishing lines but a closer look shows a rough cracks
running from one fiber to another. This can be seen in Figure 70. The cracks are
situated near the end of the yarns. The cracks do not further propagate into the
matrix but stop at the yarn boundaries.
81
a)
b)
c)
Figure 70: a) SEM image of a virgin woven catbon/epoxy composite loaded up to ε2 in the bias
direction, showing a transversal crack at the end of the yarns, b) close-up of the area indicated
by the red square and c) further close-up
82
The damage at ε2 is less pronounced in the nFRC samples. In the three crosssections, only one clear crack was found. No other cracks were found, only
debonded fibers. It can be said that the crack density is lower in the nFRC samples,
compared to the virgin samples. This observation is consistent with the results of
the X-ray investigation.
a)
b)
Figure 71: a) SEM image of a nFRC composite loaded up to ε2 in the bias direction, showing a
transversal crack b) close-up of the area indicated by the red square
7.2.3 Damage at failure
The damage at failure has become much more severe than at ε2. This was expected
from the AE measurements and the X-ray investigation. The number of cracks in
the virgin samples is enormous (Figure 72a). In one single yarn, a crack is present
every 200 µm. At ε2, the cracks stopped at the yarn boundary, they did not went
into the matrix. The SEM images at failure indicate that the cracks are deflected at
the yarn boundaries and that they further develop into longitudinal cracks causing
the yarn to delaminate. This can be seen in Figure 72b. Close to the failed zone,
fiber groups are pulled out of the yarns (Figure 72c).
83
a)
b)
c)
Figure 72: a) SEM image of a virgin woven carbon/epoxy composite loaded up to failure in the
bias direction, showing transversal cracks in the yarns, b) a transversal crack propagating at the
yarn boundary and c) pulled out fibers
84
In the nFRC sample, the crack pattern is similar. A lot of transversal cracks in the
yarn, that develop into longitudinal cracks at the yarn boundaries. The big
difference with the virgin samples is a lower crack density. Approximately every
400 µm, a crack is formed in the yarns against every 200 µm in the virgin sample.
The pulled-out fibers are also present near the failed zone.
a)
b)
c)
Figure 73: a) SEM image of a virgin nFRC composite loaded up to failure in the bias direction,
showing transversal cracks in the yarns, b) a transversal crack propagating at the yarn
boundary and c) pulled out fibers
85
When comparing the results of the AE measurements, the X-ray investigation and
the SEM investigation of the composite samples loaded in the bias direction, there
exist a clear inconsistency between them. AE measurements predict earlier
damage initiation and evolution in the nFRC composite. It was first thought that
CNTs could hinder the shear deformation of the epoxy matrix but SEM
investigation has found no evidence for this. The X-ray and SEM investigation have
indicated less damage in the nFRC composite at all stages. There can be no doubt
on the results of the X-ray and SEM investigation, so the reason of the
inconsistency should be searched for in the AE measurements.
The inconsistency might be related due to a different AE calibration. More
precisely, it is possible that CNTs change the damping properties of the matrix,
leading to a difference in the detection of the AE events. The inconsistency might
also be due to CNT-related events (e.g. debonding of the CNTs). Future research
should be able to clarify this.
7.3 Conclusion
The SEM investigation on the samples tested in the fiber direction showed a clear
crack pattern. At ε1, only chains of debonded fibers can be seen at the end and near
the end of the yarns. Less debonded fibers are present in the nFRC composite. The
transversal cracks in the virgin samples detected by X-ray investigation at ε1 are
not found by SEM investigation. The chains of debonded fibers then further
develop into transversal cracks in the yarns. This happens at ε 2. The number of
transversal cracks is again lower in the nFRC samples. At failure, the yarns contain
several cracks: four cracks per yarn in the virgin composite, only two cracks near
the end of the yarns in the nFRC composite.
In the bias direction, chains of debonded fibers also form the first phase of damage.
Cracks only start to form at ε 2. The cracks in the virgin composite are numerous
and very rough. In the nFRC composite only one clear crack is found. The cracks
stop at the yarn boundary and then further develop at this boundary. At failure the
crack density in the virgin composite (700 µm between two cracks) is higher than
in the nFRC composite (1.9 mm between two cracks).
Generally spoken, the nFRC composite is less sensitive to damage. At the same
strain level, it contains less debonded fibers and less transversal cracks compared
to the virgin composite. In other words, the same level of damage is only reached
at higher strains. CNTs can delay the damage initiation and evolution in carbon
fiber composites.
Closer inspection of the crack tip in the nFRC samples revealed bridging of the
crack, possibly by CNTs. This is unfortunately not proven.
86
Chapter 8: Fatigue tests results
This chapter presents the results of the preliminary fatigue tests. Only fatigue tests
in the fiber direction are performed at one load level. The objective of these fatigue
tests is to obtain preliminary data on whether CNTs can also extend the fatigue life
of carbon fiber-reinforced composites. The tests set-up can be found in chapter 3.
8.1 Motivation
During the literature search, no articles were found on the influence of CNTs on the
fatigue life of carbon fiber-reinforced composites. Some articles were found that
discuss the influence of CNTs on the fatigue life glass fiber-reinforced composites
(see section 2.3.2). The present study is thus the first one to touch this subject. The
full study of fatigue behaviour of the composites is not in the scope of the present
work.
The AE measurements and SEM investigation have shown that CNTs can delay the
damage initiation and evolution in woven carbon/epoxy composites in the fiber
direction. Since the fatigue life is also controlled by the damage evolution, it is
interesting to look at the fatigue properties of the composite.
8.2 Technical difficulties
On the MTS testing machine, it requires a good sample preparation to obtain good
results. The thickness of the samples, including end-tabs, should be exactly 6 mm ±
0.02 mm. If the thickness is too low, the samples slips from the grips. If the
thickness is too high, the clamps crush the sample and failure happens after only a
couple of cycles. This makes it extremely difficult to perform good and reliable
fatigue tests.
The fatigue tests are done at only one load level, 600 MPa. At this load level, the
samples fail due to extensive delamination. The samples that have slipped or have
been crushed, are not taken into account for the results.
87
Fatigue tests at a higher load are also done, but the above mentioned problems of
slippage and crushing made it impossible to obtain good results.
8.3 Fatigue tests at 600 MPa
As already mentioned in the previous section, both the virgin and nFRC samples
tested at 600 MPa fail due to delamination and subsequent fiber failure. This can
be seen in Figure 74.
a)
b)
Figure 74: a) failed virgin and nFRC samples, fatigue tested at 600 MPa b) close-up of the
delaminations at the fracture surface
The results of the fatigue tests at 600 MPa are given in Figure 75. The average
number of cycles to failure is slightly higher for the nFRC (460 000 cycles) than for
the virgin samples (434 000 cycles). However, because of the large standard
deviation, the results are inconclusive. More tests should be done to get more
accurate results.
The results for fatigue life of glass fiber composites, found in the literature, showed
an increase of the fatigue life by a factor 2-3 in the high cycle fatigue regime. In the
low-cycle-fatigue regime, the increase was less pronounced. The energy absorbing
mechanism of crack bridging, CNT pull-out and CNT fracture diminished at higher
load levels.
88
Chapter 8: Fatigue tests results
It might be possible that carbon fiber composites show the same trend. The results
obtained in the present study are at a high load level (almost 75% of the tensile
strength) and CNTs might play a minor role at this level. In the future, it should
also be checked how the nFRC composite behaves in the high-cycle fatigue regime.
Maximum stress, MPa
650
600
virgin
nFRC
550
0.E+00
2.E+05
4.E+05
6.E+05
8.E+05
Number of cycles
Figure 75: number of cycles to failure in fatigue tests in the fiber direction of woven
carbon/epoxy composites at 600 MPa
8.4 Conclusion
The results of the preliminary fatigue tests at 600 MPa show insignificant
improvement of the fatigue life of carbon fiber-reinforced composites by addition
of CNTs in the matrix. More testing can decrease the standard deviation and more
accurate conclusions can then be drawn.
This should be subject of a more extensive study on the fatigue behaviour of nFRC
composites, together with tests at lower load levels.
89
Chapter 9: General conclusion
In this study, it has been shown that it is possible to integrate CNTs in the matrix of
fiber-reinforced composites without hindering the production of a high quality
composite by RTM. The (small) increase in matrix viscosity gives no problems for
impregnation of the textile reinforcement. This might be the case at higher CNT
contents. A phenomenon related to the presence of CNTs in the matrix is an
increase of the thickness of the final composite plate, which leads to a lower fiber
volume fraction.
The results of static tensile tests in the fiber direction have shown a small increase
of the tensile strength and strain-to-failure for the nFRC composite. The Young’s
modulus remains the same as for the virgin composite. Most important results are
however obtained from the AE measurements: an increase of the damage initiation
strain εmin (+30%) and the damage threshold strains ε 1 (+42%) and ε2 (+56%) is
noticed in the nFRC composite. The X-ray investigation of loaded tensile samples
shows no damage at ε1. Transversal cracks are only found in specimens loaded just
above ε2 and their number is increased significantly in failed specimens. At failure,
delaminations occur which lead to subsequent fiber failure and hence failure of the
composite. The crack density at a given strain is lower in the nFRC composite. SEM
investigation of cross-sections of the loaded samples shows that ε1 corresponds to
the onset of fiber/matrix debonding. The chains of debonded fibers are situated at
the boundaries and at the very ends of the yarns. At the same strain level, less
debondings are present in the nFRC composite. The transversal cracks found at ε2
go through the entire yarn and further develop in the matrix. These transversal
cracks are situated inside the yarns near their tips. The number of transversal
cracks is lower in the nFRC composite than in the virgin composite. This is also
valid for the samples loaded up to failure. In the virgin samples almost every yarn
contains four transversal cracks, while in the nFRC composite the number of yarns
with transversal cracks is much less. Most of these damaged yarns only have two
cracks and only a few of them contain four cracks. The preliminary fatigue tests in
the fiber direction showed insignificant improvement of the fatigue life at high load
level for the nFRC composite, in comparison with the virgin composite.
The mechanical properties of the composite in the bias direction are hardly
influenced by the presence of CNTs, except for the strain-to-failure (+18%). First
AE events in the bias direction initiate, rather unexpected, at lower strains in the
91
nFRC composite. Also the other damage threshold strains seem to occur earlier. Xray investigation of the loaded samples, on the other hand, shows more damage in
the virgin composite than in the nFRC composite at the same strain level. SEM
investigation confirms the results of the X-ray investigation and gives more insight
in the damage pattern. At ε1, again only chains of debonded fibers are seen. The
transverse cracks are only found at ε2. The surfaces of the cracks are rough and
they do not further develop in the matrix but grow at the yarn boundaries. The
nFRC composite contains less cracks after ε2 and at failure the distance between
the cracks is also smaller than in the virgin composite. The inconsistency of the AE
results with the results of the X-ray and SEM investigation might be related due to
a different AE calibration. More precisely, it is possible that CNTs change the
damping properties of the matrix, leading to a difference in the detection of the AE
events. The inconsistency might also be due to CNT-related events (e.g. debonding
of the CNTs).
All this can be summarized: CNTs dispersed in the matrix of carbon fiber
composites results in a delayed damage initiation, less debonding and less cracks
at a given strain level. The same level of damage is only reached at higher strains.
As a general conclusion of this study, it can be said that CNTs can, indeed, improve
the damage initiation and evolution in carbon fiber-reinforced composites. As
mentioned in the motivation, the main drawback of state-of-the-art carbon fiber
composites is their limited toughness by early damage initiation. Dispersing CNTs
in the matrix can offer a simple and relatively cheap solution to this problem.
There has however still some research to be done in the field of nFRC composites.
First of all, the fatigue properties of CNT-modified composites are only briefly
studied in this master thesis. More extensive fatigue testing and post-mortem
investigation is absolutely necessary to understand the behaviour of CNTs and
their influence on the fatigue properties of nFRC composites.
Secondly, this study has given no attention to the influence of the CNT content.
Only 0.25 wt% CNTs was used in the matrix. The CNT content can be increased to
0.50 wt% or maybe even 1 wt%. It is believed that the positive effect of CNTs on
the damage evolution of carbon fiber composites can be even more pronounced at
a higher CNT content. Also the effect of other types of CNTs than the MWCNTs used
in this study (e.g. SWCNTs, funtionalised CNTs, ...) should be investigated. All this
can be the scope of future research.
In the present study, CNTs were used as an additional reinforcement for carbon
fiber composites by dispersing them in the epoxy matrix. Also other approaches
exist to combine carbon fibers and CNTs. For example, CNTs can be used in the
fiber sizing or can be grown on the carbon fibers. Two approaches can also be
combined. The damage evolution of nFRC using these approaches should given
attention to in future research.
92
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95
Appendices
Appendix A: Datasheets
The appendix contains the datasheets of the materials used for the present study.
This includes the textile reinforcement and the carbon fibers, the epoxy resin and
the hardener, and the CNTs.
A1
A.1 Textile reinforcement: Hexforce G0986 Injectex
A2
A.2 Carbon fibers: Hextow AS4C GP
A3
A4
A.3 Epoxy resin: Epikote resin 828LVEL
A5
A6
A.4 Hardener: Dytek DCH-99
A7
A.5 CNTs: Nanocyl NC7000 series
A8
Appendix B: Data processing of
tensile tests
In this appendix, the different steps in the data processing of the tensile tests is
explained. It is explained how to get the final stress-strain curve with AE events
and cumulative energy curve, starting from the raw data.
B.1 Raw data
The raw data is consists of three data series:
 load and displacement
 strain
 AE events
From the Instron 4505 tensile machine, the load and displacement is obtained. The
displacement of the crosshead can not be used to calculate the strain of the tensile
sample because the tensile apparatus is not infinitely rigid. This would lead to a
incorrect (higher) strain.
To overcome this 'problem', the strain is measured using strain mapping as
explained in chapter 3. The Vic2D software gives the strain in the loading direction
as a function of time. To avoid confusion in the next sections, the strain based on
the displacement of the crosshead is referred to as 'machine strain'. The correct
strain from the Vic2D software is referred to as 'Limess-strain'.
The AE data is given in the form of AE events with corresponding energy and load
at which they occurred.
The objective is now to combine all the raw data into one graph where AE events,
cumulative energy and stress are plotted as a function of strain. Based on the
stress-strain curve, the mechanical properties (Young's modulus, tensile strength
and strain to failure) can be derived. All calculations are done in Microsoft Office
Excel 2007.
B1
B.2 Steps in the data processing
B.2.1 Stress-strain curve
Machine strain versus Limess strain
The first step in the data processing is linking the Limess-strain to the stress, since
the Limess-strain is only known as a function of time. The machine strain however
is known as a function of the load and hence the stress. This means that, if a
relationship can be found between the two strains, the Limess-strain is
automatically linked to the stress.
To find this relationship, a theoretical machine strain corresponding to the real
Limess-strain is simulated. This is done by multiplying the time interval between
the Limess-pictures (0.5 s) with the crosshead speed (2 mm/min) and dividing the
result by the length between the end-tabs of the tensile sample. It is important to
understand that this machine strain is not the strain obtained from a real tensile
test, it is just a simulated one.
The Limess-strain is then plotted as a function of this theoretical machine strain
(see Figure B1). As can be seen in the figure, the Limess-strain is constant in the
beginning and at the end of the curve. The reason is very simple: the Limesssoftware is started before and stopped after the actual tensile test so that the
strain derived from the first and last pictures is constant. These points of constant
strain are first removed, the curve is shifted to the origin and afterwards a second
order polynomial is fitted onto the curve. The equation of this fitted curve is the
relationship between the Limess-strain and the machine strain.
If the tensile test is stopped to remove the AE sensors, the second part of the
Limess-strain is attached to the first part to obtain only one Limess-strain, which
can then be fitted.
0.016
Limess-strain, -
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
0.000
0.005
0.010
0.015
0.020
Simulated machin strain, Figure B1: plot of the Limess-strain versus the machine stress
B2
0.025
Appendix B: Data processing of the tensile tests
Limess-strain versus stress
Once the second order relationship between the simulated machine strain and the
real Limess-strain is known, it can be used to couple the Limess-strain to the
machine strain from a real tensile test. The real machine displacement (as a
function of the load), obtained from the Instron machine, is therefore converted to
a machine strain by dividing it by the length between the end-tabs of the tensile
sample. The second order polynomial is applied on this machine strain to obtain
the Limess-strain with the corresponding load. Load is simply converted to stress
by dividing the load by the thickness and width of the tensile samples. The Limessstrain is now known as a function of the stress (see Figure B2).
900
800
Stress, MPa
700
600
500
machine strain
400
Limess-strain
300
200
100
0
0
0.005
0.01
0.015
0.02
0.025
Strain, Figure B2: stress -strain curves based on the machine strain and the Limes-strain
B.2.2 Calculation of mechanical properties
The mechanical properties are derived from the stress-strain curve. The tensile
strength is obtained by simply looking for the maximum value of the stress. The
corresponding strain is taken as the strain-to-failure.
The Young's modulus requires a bit more action. The stress-strain curve is looked
at between 0.1 and 0.3% strain. This straight segment of the stress-strain curve is
isolated from the rest and approximated by a linear fit. The slope of this fit is taken
as the Young's modulus.
B3
B.2.3 AE events and cumulative energy curve
The last part of the data processing is adding the AE events and cumulative energy
curve to the stress-strain curve. The raw data give the energy of the AE events as a
function of load and hence stress. This data should be represented as a function of
the strain. This can be solved by inverting the stress-strain curve into a strainstress curve and fitting a polynomial onto this curve. In this way , the stress can be
converted into strain and the AE events can be plotted as a function of the strain.
The cumulative energy curve is constructed starting from the energy of single AE
events. The events are ranked according to the load/stress at which they have been
detected. The energy of the second AE event is then added to the energy of the first
one, the energy of the third one to the sum of the first two, etc.
B.3 Final result
The final results of all the data processing is a plot in which the stress-strain curve
is combined with the single AE events and the cumulative energy curve, as a
function of the strain. This is visualized in Figure B3.
1.E+09
1.E+08
Energy [eu]
1.E+07
800
Energy
700
Cumulative
600
Stress
1.E+06
500
1.E+05
400
1.E+04
300
1.E+03
200
1.E+02
1.E+01
100
1.E+00
0
0.000
Stress [MPa]
1.E+10
0.002
0.004
0.006
0.008
0.010
0.012
Strain, Figure B3: stress-strain curve (green) with AE events (blue dots) and cumulative AE energy (red)
plotted as function of the strain
B4

Damage evolution in nano- reinforced carbon fiber composites

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