Mechanical Properties of T650-35/AFR-PE-4 at Elevated Temperatures for Lightweight Aeroshell Designs
Karen S. Whitley∗ and Timothy J. Collins†
NASA Langley Rearch Center, Hampton, VA 23681
Considerable efforts have been underway to develop multidisciplinary technologies for aeroshell structures that will significantly increa the allowable working temperature for
the aeroshell components, and enable the system to operate at higher temperatures while
sustaining performance and durability. As part of the efforts, high temperature polymer-
matrix composites and fabrication technologies are being developed for the primary load-
bearing structure (heat shield) of the spacecraft. New high-temperature resins and
composite material manufacturing techniques are available that have the potential to
significantly improve current aeroshell design. In order to qualify a polymer matrix
composite (PMC) material as a candidate aeroshell structural material, its performance
must be evaluated under realistic environments. Thus, verification testing of lightweight
PMC’s at aeroshell entry temperatures is needed to ensure that they will perform
successfully in high-temperature environments. Towards this end, a test program was
developed to characterize the mechanical properties of two candidate material systems,
T650-35/AFR-PE-4 and T650-35/RP46. The two candidate high-temperature polyimide
肾反射区
resins, AFR-PE-4 and RP46, were developed at the Air Force Rearch Laboratory and
NASA Langley Rearch Center, respectively. This paper prents experimental methods,
strength, and stiffness data of the T650-35/AFR-PE-4 material as a function of elevated
temperatures. The properties determined during the rearch test program herein, included
tensile strength, tensile stiffness, Poisson’s ratio, compressive strength, compressive stiffness,
shear modulus, and shear strength. Unidirectional laminates, a cross-ply laminate and two
eight-harness satin (8HS)-weave laminates (4-ply and 10-ply) were tested according to
ASTM standard methods at room and elevated temperatures (23, 316, and 343°C). All of
the relevant test methods and data reduction schemes are outlined along with mechanical
data. The data contribute to a databa of material properties for high-temperature
polyimide composites that will be ud to identify the material characteristics of potential
candidate materials for aeroshell structure applications.
I.Introduction
NASA’s In-Space Propulsion Technology Program1 is developing multiple technologies that will enable future robotic science and exploration missions. One of the goals of the program is to reduce mission cost for aerocapture systems. Aerocapture is an example of an aeroassist technology that us a planet’s atmospheric forces to decelerate a spacecraft during entry. The spacecraft is captur
ed in its proper orbit in one pass through the atmosphere, without the u of on-board propulsion. Elimination of an on-board propulsion system saves mass, allowing a greater scientific payload to be delivered to the planet. Since weight-savings is directly related to mission cost, another technology needed includes lightweight structures for aeroshell designs.
An aeroshell system is a protective shell on a spacecraft that acts as a shielding surface and provides protection from the inten heating experienced during high-speed atmospheric entry. A typical aeroshell consists of three main parts including the external thermal protection material; adhesives ud to bond the thermal protection system (TPS) to the aeroshell; and an underlying structure to which the spacecraft back-shell is attached, and the external thermal protection material is bonded. The aeroshell primary structure is typically fabricated from conventional composite materials. The amount of TPS, and thus the mass of the TPS layer, depends directly on the maximum ∗ Aerospace Engineer, Mechanics of Structures and Materials Branch, AIAA Member.
† Aerospace Engineer, Mechanics of Structures and Materials Branch.
allowable temperature for the primary structure and the adhesive bond that attaches the TPS material. Conquently, the u of advanced high-temperature composite materials and adhesives
can significantly increa the allowable working temperature for the aeroshell structure, and in so doing produce a significant TPS mass savings for many mission scenarios.
In order to qualify a polymer matrix composite (PMC) material as a candidate aeroshell structural material, its performance must be evaluated under realistic environments. Thus, verification testing of lightweight PMC’s at aeroshell entry temperatures is needed to ensure that they will perform successfully in high-temperature environments. Towards this end, a test program was developed that includes testing and characterization of two PMC’s and veral high-temperature adhesives2 whereas the objective of this paper focus on the material property characterization of the T650-35/AFR-PE-4 composite material as a function of elevated temperatures. Towards this objective, a description of the T650-35/AFR-PE-4 composite material is prented first. Then the experimental methods and testing procedures that are ud in the prent study are discusd along with the results, which will include tensile modulus and strength, compressive modulus and strength, and interlaminar shear strength.
II.Material Description
The polymer matrix composite material, T650-35/AFR-PE-4 consists of a high-strength continuous c
arbon fiber (T650-35), and a high-temperature polyimide matrix (AFR-PE-4). The glass transition temperature (T g) of the resin material is 360°C. Composite panels measuring 30.5 x 30.5 cm were fabricated by hand lay-up at the NASA Langley Rearch Center. The consisted of unidirectional, angle-ply, and 8HS-woven-fabric laminates.
The panels were vacuum bagged in a press and cured according to the process shown in figure 1. A full vacuum was applied to the bagged panel during the entire cure cycle. The cycle started with a ramp to 240°C at 2.2°C/minute. After a 2-hour hold at 240°C, the temperature was ramped to 280°C at 1.3°C/minute, and the temperature was held at 280°C for 1 hour. The temperature was then raid to 335°C at 1.3°C/minute, and 1379-kPa (200-psi) pressure was applied. The temperature was held at 335°C for 2 hours, and was then raid to 371°C at 1.3°C/minute and held at 371°F for 2 hours. The pressure was relead when the temperature reached 66°C. The panels were then post-cured in air for 4hours at 400°C and cooled to room temperature in 4 hours (the post-cure is not shown in figure 1). The panels were then cut into appropriate size test coupons for each ASTM standard.
III. Material Property Characterization Methods and Results The complete test plan for this material is given in table 1 which includes the laminate configuration ud for each ASTM standard test type,
the experimental test temperatures, and the properties determined by each test.
A. Tensile Modulus and Strength
Tensile modulus of elasticity and ultimate tensile strength were determined from tension tests performed according to the ASTM standard D30393. Longitudinal strain (εx) in all test specimens was measured using an extensometer (MTS model 633.11E-15). Transver strain (εy) was measured using electrical resistance strain gages bonded to the specimen. Measurements Group, Inc. strain gages CEA-06-250UW-350 were ud for room-temperature tests, and WK-06-250BG-350 gages were ud for elevated-temperature tests. A schematic of the tensile specimens is given in figure 2, which also depicts the placement of the strain gages and extensometers. Specimens were mounted in a rvo-hydraulic test machine with surfalloy wedge grips. Tests were performed using a constant displacement rate of 1.27 mm/min. Elevated temperature tests were performed in a forced-convection-heat oven mounted to the test stand. A type-K thermocouple positioned in clo proximity to the specimen was ud to monitor the temperature. When the temperature reached the desired t point, the specimen was soaked at that temperature for ten minutes prior to testing.
The lamina in-plane moduli values, E1 and E2 were calculated directly from the initial slope of the tangent to the initial linear portion of the stress-strain curve of [0]8 and [90]16 unidirectional laminates, respectively. In most cas, the linear region of the stress-strain curve was defined as being between 1000µε and 3000µε for all specimen lay-ups and test temperatures. The laminate modulus value (E x) was calculated from the laminate stress-strain behavior of the 4-ply-fabric laminate. The lamina in-plane shear modulus (G12) was calculated indirectly from the stress-strain behavior of [±45]5s laminates subjected to axial loading according to the ASTM standard D35184 by using equation 1 where E x is the laminate longitudinal modulus (equation 2) and νxy is the laminate Poisson’s ratio (equation 3). Longitudinal failure strength (σ1) and transver failure strength (σ2) are defined as the maximum load divided by the nominal cross-ctional area of the [0]8 and [90]16 laminate specimens.
G
12=
E
x
21+!
xy
()(1)
E
x =
!
x
"
一五一十的意思x
(2)
月经前褐色分泌物
!
xy ="
鹦鹉前头不敢言#
y
#
x
(3)
The average tensile properties are listed in table 2. The stress versus strain curves (corresponding to table 2) from the [0]8, [90]16, [±45]5s, and the 4-ply-fabric laminate coupons are shown in figures 3 – 6, respectively. For clarity in the figures 3 – 6, only one reprentative curve is shown for each test temperature that reprents the qualitative average for the number of replicates. The number of replicates at each test temperature is shown in parenthesis in the legend of the figure. For example, the blue curve in figure 3 reprents the general data trend for the five replicates tested at room temperature.
It can be en in figure 3 that temperature has little effect on the longitudinal modulus (E1). The effects of elevated temperature on the transver tensile (E2) and shear moduli (G12) were considerably stronger (figures 4 and 5). The average transver tensile modulus (E2) decread from 7.80 GPa at room temperature to 4.87 GPa at 316°C (600°F), which is a 37% reduction with respect to room temperature, and decread further to 4.21 GPa at 343°C (650°F), resulting in a 46% reduction with respect to room temperature. The average shear modulus (G12) decread from 4.68 GPa at room temperature to 2.14 GPa at 316°C (600°F), a 54% reduction with respect to room temperature, and further decread to 4.21 GPa at 343°C (650°F), a 65% reduction with respect to room temperature. Elevated temperatures significantly reduced longitudinal (σ1) and transver (σ2) tensile strengths. The transver strength (σ2) was reduced by 41% at 316°C, and 53% at 343°C, with respect to room temperature. The longitudinal strength (σ1) was reduced approximately 20% at the elevated temperatures. Despite considerable scatter in the strength data of the 4-ply-fabric, the average values indicate relatively little change in strength, σx, at the elevated temperatures compared to room temperature. Similarly, modulus E x of the 4-ply-fabric laminate also showed no significant effect due to temperature.
Regarding Poisson’s ratio (ν12), no clear evidence of an effect of temperature is obrved. Therefor
e, it is assumed that ν12 is independent of temperature. The average value of ν12 is 0.32, 0.31, 0.32 for room temperature, 316°C, and 343°C, respectively.
B. Compressive Modulus and Strength
The compressive modulus of elasticity and strength were determined by performing compression tests using the method described in the ASTM standard D695-965. The dog-bone shaped test specimen, depicted in figure 7, was mounted in a support fixture as shown in figure 8. This end-loaded compression fixture supported the specimen to prevent buckling, and allowed compression failure in the mid-ction. The support fixture and the specimen were placed between upper and lower compression platens. The tests were performed in a rvo-hydraulic test machine using a constant displacement rate of 1.27 mm/min. (0.05 in/min). Strain was measured using an extensometer mounted onto the expod edge of the compression specimen. The compressive strength and modulus were calculated according to the method described in ASTM D695, by calculating the slope of the tangent to the initial linear portion of the stress-strain curve. The linear region of the stress-strain curve from the compression test was defined as being between 1000µε and 3000µε for all test conditions and specimen ply lay-ups. Failure was defined as the point when the specimen broke, resulting in a complete loss of load-carrying capability during the test.
The compressive properties at room temperature, 316°C, and 343°C obtained from the dog-bone compression (DBC) tests are given in Table 3. The stress versus strain curves from the DBC tests are shown in figures 9 – 11. As previously discusd in the tension ction each curve in the figures 9 – 11 is reprentative of the replicate tests that were performed at each test temperature.
It can be en in figure 9 that the effect of temperature on the longitudinal compressive modulus at 316°C compared to room temperature is rather weak. There is a significant effect of temperature on the transver compressive modulus as shown in figure 10, which decread from 8.3 GPa at room temperature to 5.47 GPa and 4.34 GPa at 316°C, and 343°C, respectively. There was very little decrea in the compressive modulus of the 10-
ply-fabric, when going from room temperature to the elevated temperatures, especially considering that the decrea in modulus values were within the standard deviation. The longitudinal and 10-ply-fabric compressive strength decread about 50% at the elevated temperatures. The lowest strength value occurred in the [90]20 specimens. At room temperature, the longitudinal strength was 249 MPa. However, the [90]20specimens tested at 316°C and 343°C did not exhibit a failure mode, they simply compresd in the test fixture until there was no visible end-loading remaining on the specimen, and the test was stopped.
C. Interlaminar Shear Strength
维生素b1的用法用量The interlaminar shear strengths were determined using the ASTM standard D38466 as a guide. The specimens were mounted in the same support fixture that was also ud for the dog-bone-compression tests, and a compressive load is applied to a double-notched specimen as described in the standard. The notch depth was one-half the specimen thickness, t, and the notch width was 1.27 mm (0.05 inches). A schematic of the specimen is shown in figure 12. This test was performed with a constant displacement rate of 1.27 mm/min. (0.05 in/min). The interlaminar shear strength was calculated by dividing the maximum shear load by the product of the specimen width and the length of the failed area between the two notches. The effect of elevated temperatures on the interlaminar shear strengths of the [0]20, [90]20, and the 10-ply 8HS-fabric laminates is clearly demonstrated in figure 13. The results prented in figure 13 are also listed in table 4. The interlaminar shear strength decread significantly as the temperature incread for all the laminates tested.
IV. Discussion
The effect of elevated temperatures on the mechanical behavior of carbon/AFR-PE-4 needs to be thoroughly understood for the design of more advanced composite systems in structural applications
such as lightweight aeroshell designs. The elastic properties of T650-35/AFRPE-4 were experimentally determined as a function of temperature using ASTM standard test methods. This was accomplished by performing tension, compression, and shear tests on [0], [90], [±45], and 8HS-fabric composite materials at room temperature, 316°, and 343°C.
It was found that the effect of elevated temperatures on the tensile and compressive moduli, E1 (obtained from [0]-ply laminates), and E x(obtained from 8HS-fabric laminates) was not major. However, the tensile and compressive transver moduli were reduced by more than 30% at the elevated temperatures. The tensile strength, (σx) of the 8HS-fabric laminates was not affected by temperature. The longitudinal tensile strengths σ1, and σ2, and the compressive strengths σ1, σ2, and σx were reduced at elevated temperatures. Interlaminar shear strength of T650-35/AFRPE-4 composite were measured by the double-notch shear test method between room temperature and 343°C. Interlaminar shear strength decread substantially with temperature for the laminates tested ([0], [90], 8HS-fabric).
In general, as the temperature of the composite incread, the polymer matrix was more compliant, therefore the composite elastic stiffness constants decread, especially the matrix-dominated constants (e.g. shear modulus, transver Young's modulus). Another important factor associated wi
th the effect of temperature on the mechanical behavior of T650-35/AFR-PE-4 composite system was the significant reduction in strength properties, especially tho dominated by the strength properties of the polymer matrix. There was a tendency toward lower shear and transver tension strength values as the temperature was incread.
Acknowledgements
The authors wish to thank everyone who has contributed to this work. Clarence Stanfield for fabrication of the panels, Louis Simmons and Thomas Skalski for cutting the panels into test coupons, Stewart Walker, Doriann Muller, and Diane Griffin for testing the coupons.
References
1Johnson, L., et al., "NASA In-Space Propulsion Technology Program: Overview and Update," NTRS: 2005-01-11; 20050000112, NASA Center for Aerospace Information, 2004.
2Collins, T.J., et al., "High Temperature Structures, Adhesives, and Advanced Thermal Protection Materials for Next Generation Aeroshell Design," 53rd Joint Army-NAVY-Air Force Propulsion Meeting, Monterey, CA, 2005.
3ASTM, "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials," D3039-95, American Society for Testing and Materials, 1995.一笔画虾
4ASTM, "Standard Practice for In-plane Sear Stress-Strain Respon of Undirectional Reinforced Plastics," D3518-76, American Society for Testing and Materials, 1976.
5ASTM, "Standard Test Method for Compressive Properties of Rigid Plastics," D695-96, American Society for Testing and Materials, 1996.
6ASTM, "Standard Test Method for In-Plane Shear Strength of Reinforced Plastics," D3846-94, American Society for Testing and Materials, 1994.关于人的作文
Table 1. Test plan
Test Type
(ASTM Standard)
Laminate Property
Tension (D3039)[0]8Longitudinal tensile modulus, E1 Longitudinal tensile strength, σ1 Poisson’s ratio, ν12
Tension (D3039)[90]16Transver tensile modulus, E2 Transver tensile strength, σ2
Tension (D3039)4-ply-fabric Tensile modulus E x Tensile strength, σx
Tension (D3518)[±45]5s In-plane shear modulus G12
Compression (D695)[0]20Longitudinal compressive modulus, E1 Longitudinal compressive strength, σ1
Compression (D695)[90]20Transver compressive modulus, E2 Transver compressive strength, σ2
Compression (D695)10-ply-fabric Compressive modulus E x Compressive strength, σx
排卵期出血是什么原因Shear strength (D3846)[0]20Interlaminar shear strength Shear strength (D3846)[90]20Interlaminar shear strength Shear strength (D3846)10-ply-fabric Interlaminar shear strength Note 1. Test temperatures include 23, 316, and 343°C for each test type.
Note 2. All fabric laminates are 8HS-weave material.
