Post-Test Sample Analysis of Fiber Reinforced Plastic Using Arcjet

AIAA 2010-4657
10th AIAA/ASME Joint Thermophysics and Heat Transfer Conference
28 June - 1 July 2010, Chicago, Illinois
Post-Test Sample Analysis of Fiber Reinforced Plastic
Using Arcjet
Takeharu Sakai*, Masami Tomita†,
Nagoya University, Aichi, 464-8603, Japan
Toshiyuki Suzuki‡, Kazuhisa Fujita§
Japan Aerospace Exploration Agency, Tokyo, 182-0012, Japan
and Kenichi Hirai**
IHI AEROSPACE Co., Ltd., Gunma, 370-2398, Japan
Coking phenomenon for silica fiber reinforced plastic(SFRP) exposed to aerodynamic heating is examined
experimentally and numerically. The SFRP material with the nominal virgin density of 1.70 g/cm3 is heated in
an arcjet wind tunnel. The spatial distribution of in-depth gas permeability and material density is measured
for post-test arcjet sample. Thermal response analysis is made by evaluating thermophysical parameters for
the SFRP material. Comparison of the density and permeability distribution gives a fair agreement between
measurement and calculation, suggesting that coking effect could be negligibly small for the case of the high
density ablator examined in the present study.
Nomenclature
A
: cross sectional area of unsealed portion of test specimen for permeability measurement
Cp
: specific heat at constant pressure
m
: flow rate
: pressure
: universal gas constant
: temperature
: thickness of an ablator
: volume of chamber 1
: gas permeability
: void fraction
: cofficient of viscosity
: density
p
T
x
v1




Subscript
c
s
v
:char
:solid
:virgin
I.
Introduction
An ablator is a composite material made of organic or inorganic fiber and resin. A silica fiber reinforced plastic
(SFRP) has a better oxidation resistant characteristic than carbon fiber reinforced plastic. Currently in IHI
*
Associate Professor, Department of Aerospace Engineering, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi, 464-8603,
Senior Member AIAA.
†
Graduate student, Department of Aerospace Engineering, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi, 464-8603.
‡
Researcher, Institute of Aerospace Technology, 7-44-1, Jindaiji, Higashi-machi, Chofu-shi, Tokyo, 182-8522,
Member AIAA.
§
Senior Researcher, Institute of Aerospace Technology, 7-44-1, Jindaiji, Higashi-machi, Chofu-shi, Tokyo, 1828522, Member AIAA.
**
Manager, Technologies Development Dept., 900, Fujiki, Tomioka-shi, Gunma, 370-2398
1
Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
AEROSPACE in Japan, an SFRP ablator is expected to be used as a heatshield material for the combustion chamber
of a next generation rocket engine.1 When the SFRP ablator is applied to such a heating environment, it is expected
that surface recession should be avoided during the exposure of the combustible gases. Therefore, fundamental mass
loss characteristics in the ablative behavior of SFRP ablators need to be understood.
When the ablator is heated, the resin decomposes thermally to gas and is charred while a solid residue forms
porous material made of silica and carbon. The thermally decomposed gas is called pyrolysis gas and the layer of the
solid residue called char layer. Pyrolysis gas flows through the pores within the char layer and escapes from the
surface. Under certain conditions, the pyrolysis gas cokes out carbon which deposits on the char. This phenomenon
is called coking phenomenon.2The effect of the coking phenomenon is represented typically by a material density
increase in the char layer toward the surface of the material. In addition, it is reported that the coking phenomenon
reduces the recession of the char layer.3 So far, many efforts have been made to understand the thermal response of
SFRP ablators (see a review paper4, for example), but coking behavior is believed to be less understood among the
ablation characteristics.
Recently, the in-depth material density variation within the ablative material tested in an arcjet wind tunnel was
measured in order to examine coking process in the char layer 5. In their study, the core of the ablative material was
extracted by shaving the outside of the material and the density variation within the core was measured. The results
showed that the in-depth density tends to fluctuate for air arcjet testing: it was difficult to judge a possible coking
characteristic from the measured density data. It is believed that this result stems from the inherent difficulty in
measuring the material density through the methodology used in the study. Therefore, alternative approach might be
desirable to observe a possible coking effect.
When coking process occurs within the char layer of the ablative material, the solid carbon deposited therein
might occupy a relatively large cross sectional area of the material. As a result, the porosity or void fraction in the
deposited region might be reduced. This nature can be examined by measuring gas permeability in the char layer:
there could be a strong correlation between permeability and porosity. There is a standard experimental method to
determine the gas permeability value of the ablative material: a number of small and thin specimen are prepared; the
specimens are heated in an oven at a prescribed temperature to make a different state of porosity; the gas
permeability of the heated specimen is measured by flowing a gas into the specimen through the Darcy’s law6. The
permeability of the specimen with a high porosity value, which corresponds to a charred material, could be different
from that obtained in a gas-dynamic heating such as in an arcjet, if coking would be observed.
This approach has been applied in our independent effort7 very recently to the charred carbon-phenolic material
used in Ref.5. The results show that the gas permeability decreases toward the surface within the char layer, while
the material density in the char layer increases. This trend in the char layer is different from the thermal response
calculation without accounting for coking effect.
The purpose of the present study is to apply the same methodology to the SFRP ablators. Because it is difficult at
present to heat the ablator in a combustible gaseous environment simulated in the combustion chamber of a rocket
engine, the test piece of the SFRP ablator is tested in an arcjet wind tunnel. The permeability of the ablator heated
in the aerodynamic heating is compared with that one obtained through an oven heating via a thermal response
calculation within the ablative material used in the present study.
II.
Experimental
A silica fiber reinforced plastic is used in the present study. This ablative material is a cloth-layered type and the
phenolic resin is used for matrix. The virgin density is about 1.7g/cm3. The ply-angle of the material chosen in the
present study is 0.
2
A. Permeability measurement
The experimental setup for gas permeability measurement is given in Fig. 1. The setup used in the present study
is similar to the one used in Ref. 8. The test specimen of an ablative material is set inside the flange, which is
connected to chamber with different volume on both sides. The pressures upstream and downstream of the specimen
are measured using pressure sensors. The specimen is embedded in an aluminum ring, and the gap between the
specimen and the ring is sealed by a bond. The ring is pinched between the flanges. The effective diameter of the
unsealed area for the permeability measurement is 10mm.
Figure 1 Experimental setup for permeability measurement
The experimental procedure taken in the present study is similar to the one in Ref 8. A dry air is used for a test gas.
The measurement is done under room temperature. Initially, two valves are remained open and air flow is
reservoired until the pressure in the chamber 1 given in Fig. 1 is maintained at typically 1MPa, while the pressure in
the chamber 2 is kept atmospheric pressure. The pressure histories are recorded after the two valves are closed. The
typical time history of the pressure values are shown in Fig.2. Based on the Darcy’s law, the gas permeability is
obtained through the following relation6 using the time history of the measured pressures:
dp1
( p12 p22 )
dt
2 v1 x
Figure 2 Typical time history of the measure pressure
3
Figure 3 Evaluation of gas permeability
(1)
The relation between
( p12 p22 ) and dp1 in Eq. (1) is plotted in Fig.3 using the measured time history of the
dt
2 v1 x
pressure and the geometrical data of the test specimen. The slope of the regression line given in Fig. 3 is a deduced
gas permeability value. The measurement for each specimen was done three times. The experimental error is
estimated to be about 7%.
B. Oven heating
An oven at Japan Aerospace eXploration Agency (JAXA) is used to evaluate a correlation between the gas
permeability and the void fraction of the ablative material. The correlated formula is used in the thermal response
calculation to deduce the computed spatial distribution of the permeability, as will be shown later. The specimens
are heated at three different temperatures of 770, 970, and 1170 K, respectively for 1 hour in low pressure argon
environment. Two specimens are used for each of exposure temperature.
The specimen has a cylindrical geometry: its diameter and thickness is about 13mm, and about 1.5mm,
respectively. Before the oven heating tests, the moisture content of each specimen is removed by exposing the
specimens at 373 K for 1 hour: the dried density of the test specimen is lower typically by about 1% compared
with the non-dried density. The experimental data for test specimen after oven heating at three different
temperatures is also shown in Table 1. The void fraction is used to calculate the permeability value via material
densities. The void fraction is given by the following relation9:
s
c
(2)
max
p
In the oven heating tests, max, and the intrinsic resin density p are estimated to be 0.32, and 930kg/m3,
respectively.
Table 1 Summary of test conditions and material test results
Dry mass, g
Final mass, g
Exposure temp., K

0.341
0.305
770
0.21
0.325
0.290
770
0.22
0.336
0.285
970
0.30
0.331
0.282
970
0.29
0.341
0.285
1170
0.32
0.338
0.282
1170
0.32
C. Arcjet heating
A 750kW arcjet wind tunnel at JAXA is used. The arcjet is run for one operational condition: mass flow rate of
0.02 kg/s and electrical current of 700A, respectively. The measured cold wall heat flux and the impact pressure
values are 2.2MW/m2 and 4.6kPa, respectively. The heater thermal efficiency is measured and the obtained massaveraged enthalpy value is about 18.0MJ/kg. Centerline enthalpy values are evaluated by using a computer code to
calculate arcjet flows.10,11 The evaluated centerline enthalpy value is about 28.7MJ/kg, respectively. The testing time
for the experiment is set to be 300 seconds.
Figure 4 shows the schematic diagram of the test pieces used in the arcjet testing. Two models are used: Model A
is used for in-depth temperature measurements; Model B is used for material density measurements, respectively.
The diameter of both of the specimens is about 40 mm. As shown in the figure, the part of the test specimen is put
into a bakelite tube to hold the specimen. For Model A, type K thermocouple wires are mounted in radially inward
direction into the test piece to measure in-depth temperature. The in-depth temperatures are measured at the
4
positions of 7.75, 15, and 25 mm downstream from the surface. The surface temperature is measured by using onecolor and two-color optical pyrometers.
(a) Model A
(b) Model B
Figure 4 Schematic diagram of test specimens
D. Post-processing of SFRP test piece heated in arcjet
The test piece obtained by arcjet testing is processed to measure the spatial variation of the material density and
permeability values. In the present arcjet heating tests, surface recession is not observed for the operating condition
used in the present study. The diameter and the thickness of the test piece heated in the arcjet freestream are almost
unchanged compared with the original test piece: the difference of the diameter and the thickness between the pre
and post test piece is less than 1%. The cylindrical core portion is removed out from the heated test piece. The test
specimen is processed from the cylindrical core portion. The number of the test specimen is 9 at most from one test
piece. The diameter and thickness for each specimen is about 13mm, and 1.5mm, respectively. This geometry is
almost the same as the one used for oven heating. Each of the specimen is weighted with an electronic precision
balance.
III. Physical modeling
To calculate the thermal response within ablators in the arcjet testing, the Super Charring Material Ablation
(SCMA)9 code is used. Although the original SCMA code is developed to solve one-dimensional solid mass and
pyrolysis gas mass and momentum, and total energy conservation equations in a time-dependent manner, a so-called
steady ablation calculation can be also carried out: solid mass and total energy conservation equations are calculated
for this case. Thus, the steady-ablation portion of the SCMA code is analyzed in the present study. The methodology
used in the SCMA code is mostly retained, but the thermophysical material properties such as thermal conductivity,
specific heat, and the decomposition rate of resin are updated to be able to calculate the thermal response of the
SFRP material. Preliminary measurements to obtain the thermophysical parameter will be explained next.
Mass loss rate for the thermal decomposition of the ablative sample is measured using a thermogravimetric
analysis (TGA). The TGA measurement is carried out in a low pressure argon environment at different heating rates
of 2K/min, 10K/min, and 40K/min, respectively. The obtained mass loss data is plotted against the temperature in
Fig. 4. The expression for the pyrolysis rate is given in a following form:
nk
2
Pyrolysis rate
Ak
k 1
s
c
v
c
v
5
exp
Ek
T
(3)
Two different curves for pyrolysis rate are given in Fig. 3. The Curve 1 is determined based on the measured TGA
data. The Curve 2 is used to examine a possible effect of the resin decomposition in the oxidizing gaseous
environment, as will be shown later.
Figure 5 Comparison of mass loss between TGA measurement and models
A differential scanning calorimetry is used to evaluate the specific heat of the ablative material used in the
present study. The four samples are extracted from a bulk virgin ablative material. The data is measured for the
temperature range from about 300 to 700K. The result shows that the measured specific heat differs about 40%
between the minimum and maximum values: the obtained data is strongly dependent on the extracted sample.
Nevertheless, an averaged specific heat value is evaluated and the correlated formula is given as a function of
temperature as follows:
Cp
2.1T
2900
(4)
The specific heat obtained in this formula is compared with the specific heat evaluated for a silica-phenolic
composite given by Auerbach et al.12 The present specific heat value is higher than by a factor of three than the one
given by Auerbach et al.12 over the measured temperature range. Because the specific heat of the charred material is
unknown, the charred specific heat value is also given by Eq. (4)
Thermal conductivity is deduced by using a laser flash method. The measurements are made at five different
temperatures. The thermal conductivity value is estimated from the measured thermal diffusivity, and the specific
heat, and the material density. The material density is evaluated by accounting for the mass loss rate given by the
thermogravimetric analysis. The evaluated thermal conductivity values are 1.43, 4.97, 4.315, 3.95, and 3.828 W/(mK) at 300, 370, 470, 570, and 670 K, respectively. The obtained data is used in the calculation by interpolating or
extrapolating at a given temperature.
The number of grid used in the present calculation is 300 grid points. The grid is equally distributed for the test
piece thickness of 40mm. The boundary condition at the wall surface exposed to arcjet freestream is approximately
treated in the present study. A temporal history of convective heating rate is assumed: the heat flux value is reduced
from the cold heat flux value of 2.2MW/m2 to 1.1MW/m2. In addition, in order to roughly reproduce the measured
temporal history of the surface temperature, the wall emissivity value is taken to be 0.75 when the surface
temperature is lower than 1997K, and 1 for the case that the surface temperature is higher than that value. The
temperature value of 1997 K is a melting temperature1 of SiO2. It should be noted that a possible silica-carbon
reaction in char layer is ignored in the present study.
6
IV.
Results and discussions
A. General feature of computed solution within the test piece
Calculation is carried out to calculate the thermal response of the silica fiber reinforced ablator used in the present
study for the arcjet condition explained earlier. The thermophysical properties are implemented into the SCMA code.
Computed temporal variation of the material density and temperature inside the test piece is shown in Fig.6. The
results are presented until the time has elapsed up to 5000 seconds: calculation is made until a steady-state condition
is nearly satisfied. From the figure, the depth of char layer is about 10 mm during heating. After testing time, the
pyrolysis zone penetrates up to about 13mm from the surface. In addition, because the temperature increases up to
about 500 K in the region from 25 to 40mm from the frontal surface, the virgin material at the deep inside of the
material is pyrolyzed partially.
(a) Solid density
(b) In-depth temperature
Figure 6 Temporal variation of spatial distribution of material density and in-depth temperature
with TGA curve 2
B. Comparison of spatial profile of arcjet post-test sample density between experiment and calculation
The spatial profile of solid density within the ablative test piece is compared between post-test sample and
calculation. All the measured data is tested in the same arcjet heating condition. From the measured data, the depth
of the char layer is about 15mm at most. Even though the experimental scatter is relatively large, the char density is
nearly constant value. A roughly constant density distribution is seen in the measured profile from 25mm to 40 mm:
the averaged density value of about 1.55 g/cm3 is lower than the virgin density. This trend is believed to be a result
of thermal decomposition of the resin after testing.
The calculated spatial profile is given at t=5000s. The calculated results are given for the cases with two TGA
curves shown in Fig.5. The result with TGA curve 1 will be discussed at first. The calculated char density is higher
than the measured one. In addition, the calculated depth of the char layer is less than the measurement by about
30 %.
There will be many reasons for the quantitative difference of the spatial density distribution between measurement
and calculation. In order to examine a possible reason for the difference of the measured and calculated char layer
density, the effect of the pyrolysis rate is modified: the final mass loss value is approximately equal to the char
density obtained in the arcjet testing. In addition, the chemical species in the pyrolysis gas includes oxygencontaining one9: in such an oxidative gaseous environment, mass loss rate could be faster than the one in an inert
7
environment.13 The TGA curve 2 is chosen to represent such behaviors which likely occur within the material in the
arcjet heating environment. The calculated result with TGA curve 2 is shown also in Fig.7. Calculated solid density
profile gives a fair agreement with the measured one, even though the calculated density value is still higher than the
measured one in the region from 25mm to 40mm away from the frontal surface.
Figure 7 Comparison of solid density between measurement and calculations
C. Comparison of spatial profile of arcjet post-test sample permeability between experiment and calculation
The permeability value obtained through oven heating is plotted against void fraction. The permeability value is
expressed as a function of void fraction:
1.02 10
11 6.08
(5)
The void fraction is calculated via the material density s in Eq. (2).
The comparison of the spatial profile of gas permeability between post test sample and calculation is made in Fig.8.
The calculated results are given for the cases with two different TGA curves. From the spatial distribution of the
measured permeability, the depth of the char layer is found to be about 15mm. This data is consistent with the
measured density profile. The permeability value calculated by using the TGA curve 1 is lower than the measured
one not only in the char layer but also in the entire region. This trend for permeability is consistent with the one seen
in the density distribution shown in Fig.7.
Figure 8 Variation of permeability obtained by oven heating against void fraction
8
When calculation is made using the TGA curve 2, a reasonable agreement of the permeability value in the char
layer is obtained between experiment and calculation as expected: the permeability value in the char layer is nearly
constant. It should be noted that the void fraction in the char layer is higher than the max (0.32)value given by the
oven heating test specimen and is about 0.45. Because the char density is also in a fair agreement between
measurement and calculation, the extrapolated permeability value in the higher void fraction is believed to be valid
in the arcjet heating condition. In contrast, the curve fit of Eq. (2) could overestimate the permeability in the lower
void fraction: the calculated solid density in the region from 25 to 40mm from the frontal surface is higher than the
measurement while the calculated permeability gives a satisfactory agreement with the measurement.
Figure 9 Comparison of permeability between measurement and calculations
D. Comparison of surface and in-depth temperatures between experiment and calculation
The surface and in-depth temperatures are compared in Fig. 10 between measurement and calculation. The
calculated temperature histories are given for the case with TGA curve 1 and 2, respectively. From the surface
temperature Fig.10(a), the surface temperature is in a fair agreement between measurement and calculations: the
computed overall heat input into the ablative test piece during testing could be matched with measurement. One can
see that the present calculation predicts the measured in-depth temperature at 7.8mm from the frontal surface fairly
well, if the heat input is correctly simulated. The temperature increases up to 1300 K and the fully charred state is
believed to be established: this result is consistent with the measured density and permeability distributions shown in
Figs. 7 and 9. After the testing time (t=300s), calculation tends to underestimates the cooling process of the test
specimen at 7.8mm and the overall histories at 15, and 25mm.The difference of the surface and in-depth temperature
histories between the two different calculations is relatively small, although the calculation with the TGA curve 1
gives a better agreement with the measurement.
From the computed spatial profile of the material density shown in Fig.10 (a), the depth of the char layer is about
10mm during testing and the in-depth temperature in the char layer varies about 1000K to 2000K. The in-depth
temperature within the char layer and the depth of the char layer presumably established in the present arcjet
experiment study roughly covers the condition in which the coking effect was observed in a previous study. 2
Nevertheless, the typical trend observed in coking process within the char, that is, material density increase toward
the heated surface is not seen in the silica reinforced ablator used in the present study.
9
(a) Surface temperature
(b) In-depth temperature (7.8mm)
(c) In-depth temperature (15.0mm)
(d) In-depth temperature (25.0mm)
Figure 10 Comparison of surface and in-depth temperatures between measurement and calculations
V. Concluding Remarks
Typical characteristics seen in coking phenomenon was unable to be observed even if the condition for coking
seen in a past study is satisfied for high density silica fiber reinforced plastics. This trend is different from the one
seen in a low density carbon fiber reinforced plastic. The exact cause of the reason for the difference between the
high density and low density ablation is unclear. The effect of the virgin material density on coking characteristics
will be examined in the future. It is believed that such a task could help understanding of the condition whether
coking occurs or not.
VI. Acknowledgment
The first and second authors would appreciate the assistance of Drs. Morimoto and Ishida at JAXA for the
measurement of the thermophysical properties of the ablative material used in the present study, and Prof. Sasoh at
Nagoya university and Prof. Matsuda at Meijo university for their many helpful comments.
10
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13
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11