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COMPACT TENSION MODE I FRACTURE OF CARBON/EPOXY AND CARBON/BISMALEIMIDE COMPOSITES AT CRYOGENIC TEMPERATURES

Steven L. Donaldson

AFRL/MLBC Wright-Patterson AFB OH 45433-7750

Ran Y. Kim

University of Dayton Research Institute 300 College Park

Dayton OH 45469

Abstract

The compact tension test was used to characterize the critical fracture toughness and energy release rates of three composite systems: two toughened epoxy systems (IM7/977-2 and IM7/977-3) and a toughened bismaleimide (IM7/5250-4). The tests were conducted at elevated-to-cryogenic temperatures of 149°C, 22°C, -129°C, -196°C, and -269°C. In general, the KQ fracture toughness (as measured by the compact tension test method) rose slightly as temperature decreased, except to drop back slightly at -269°C. At all temperatures, the IM7/977-2 exhibited the highest values.

Background and Objectives

The use of polymer matrix composite materials has been considered for use in launch vehicle components to reduce weight. Of particular interest, the components of interest here are lines, ducts, valves, and tanks subjected to extremely low cryogenic temperatures. For example, as shown in Figure 1, the potential oxidizer liquid oxygen has a temperature of -183°C; and liquid hydrogen, a potential fuel, has a temperature of -253°C. There is limited data for the performance of polymer matrix composites at these temperatures. In particular is the study of the potential leakage or permeation of cryogenic liquids through the composite laminates due to matrix microcracking and the subsequent linkage of these microcracks to form continuous crack paths through the laminate thickness. A key numerical value required for these calculations is the transverse (with the crack running parallel to the fibers) Mode I fracture toughness as a function of temperature, hence, the subject for the present study. Due to the hazardous nature of testing in both liquid oxygen and liquid hydrogen, testing was done in alternate cryogenic liquids. As shown in Figure 1, liquid nitrogen has a temperature 13

degrees below liquid oxygen, and liquid helium is 16 degrees colder than liquid hydrogen. Therefore, testing was conducted in liquid nitrogen to simulate the effects of liquid oxygen temperature on the composite. Similarly, testing was conducted in liquid helium to generate data at temperatures slightly colder than liquid hydrogen. Three materials were chosen for this study: Cytec IM7/977-2 and IM7/977-3, both carbon fiber-reinforced toughened epoxies, and IM7/5250-4, a carbon fiber-reinforced bismaleimide.

Test Specimen and Test Method

The specimen used was based on ASTM E399. This specimen was selected due to its small size, which would fit within the limited test chamber of the cryostat used to test at liquid helium temperatures. The specimen configuration is shown in Figure 2. The specimen dimensions chosen were w = 2.0 cm, h = 2.0 cm, and t = 0.31 cm (24 plies thick). Specimens were fabricated from unidirectional panels and laid up and cured according to the manufacturer’s instructions. Next, the specimens were cut from the panels, and pin-loading holes and a saw-cut precrack were added. The saw-cut precrack was extended to form a natural crack by first putting a C-clamp across the specimen (in the “h” direction as shown in Figure 1) at the end of the specimen opposite the pin loading holes and applying slight clamping pressure. Next, a razor blade was inserted into the saw cut and tapped slightly. The resulting crack lengths (combined saw cut and natural crack extension), “a” in Figure 1, was typically 4.5 mm. It should also be noted that the fiber direction for the specimens was parallel to the crack growth direction. The specimens were then pin-loaded in a servohydraulic load frame at 0.00076 cm/sec until failure occurred. Typically five specimens were tested at each temperature. The specimens tested at 150°C and -129°C were tested in a standard

44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Confere7-10 April 2003, Norfolk, Virginia

AIAA 2003-1431

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commercial environmental test chamber (not shown). The 23°C specimens were tested in the regular laboratory environment. The specimens tested at -196°C were tested in a custom stainless steel tub and filled with liquid nitrogen as shown in Figure 3. The -269°C data was collected in a custom MTS/Andonian cryostat as shown in Figure 4. This cryostat has an outer vacuum wall chamber, a middle wall chamber cooled with liquid nitrogen, and a central core cooled with liquid helium. The specimens were allowed to equilibrate at each temperature for at least 10 minutes before testing.

Test Results

Typical load-displacement curves are shown in Figure 5. The elevated and room temperature tests led to steady crack growth, whereas the tests conducted at cryogenic temperatures resulted in very rapid crack growth and corresponding rapid load drop. For each test, the peak load, P, was used to calculate the critical fracture toughness using:

KQ = )( ba/fbt

P(1)

Where f(a/b) is given in ASTM 399 as:

f(a/b) =

]3/2(a/b)[1

)4/b45.6(a)3/b314.72(a)2/b213.32(a4.64(a/b)a/b)[0.886(2

−

−+−++

(2) The critical energy release rate was then calculated using:

GIc = 21

LT2G

LELT

21

TE

LE21

TEL2E

12IK

+−

(3)

The temperature-dependent stiffness values used in Equation 3 are given in Table 1. The results for KQ and GIc as functions of temperature and material type are given in Figures 6 and 7, respectively. The IM7/977-2 KQ and GIc values exceed both IM7/977-3 and IM7/5250-4 over the entire temperature range. In addition, as shown in Figure 6, the trend for all materials is to increase KQ fracture toughness slightly with

decreasing temperatures, except at -269°C, where a small drop is observed. The trend in GIc as a function of temperature (Figure 7) is fairly flat (within the scatter) for the epoxies IM7/977-2 and -3. IM7/5250-4, however, shows a slight increase in GIc with decreasing temperature, except to drop at -269°C. In addition, the IM7/5250-4 GIc values appear to be lower than those of IM7/977-3 at the higher temperatures, but exceed IM7/977-3 at lower temperatures.

Conclusions

The compact tension test specimen was chosen (due to test chamber space limits) to characterize the fracture toughness of two toughened epoxies and one toughened bismaleimide matrix composite, IM7/977-2, IM7/977-3, and IM7/5250-4, respectively. The specimens were tested over a temperature range of 150°C maximum down to -269°C in a specialized liquid helium cryostat. Overall, the IM7/977-2 showed the greatest values of both KQ and GIc over all temperatures tested. Also observed were slight increases in KQ with decreasing temperature, except at -269°C, where a toughness decrease was observed. Less temperature sensitivity was observed in GIc. Future work, via further testing and analytical modeling, will examine the specimen thickness effects on the results.

Figure 1. Cryogenic Temperature Scale.

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(a)

(b)

Figure 2. Compact Tension Test Specimen.

Figure 3. Testing in Liquid Nitrogen.

(a)

(b)

Figure 4. Testing in Liquid Helium Using Custom Cryostat.

Figure 5. Load-Displacement Curves Showing Both Steady and Rapid Crack Growth.

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Figure 6. KQ Fracture Toughness vs. Test Temperature and Material Type.

Figure 7. GIc Energy Release Rate Fracture Toughness vs. Test Temperature and

Material Type.

Table 1. Stiffness Values Used in the GIc Calculation.

149°C 22°C -129°C -196°C -269°C IM7/977-2 Ex (GPa) 165 165 165* 165* 165*

Ey (GPa) 9.1 9.45 11.86 13.3 13.3* Q 0.35 0.33 0.33* 0.33 0.33*

Gxy (GPa) 4.62 5.86 7.65 8.07 7.58(1) IM7/977-3 Ex (GPa) 172 172 172* 172 172*

Ey (GPa) 9.1 9.72 12.9 11.7 11.7* Q 0.35 0.33 0.33* 0.33 0.33*

Gxy (GPa) 5.52 6.07 8.48 9.17 9.17* IM7/5250-4 Ex (GPa) 172 172 172* 174 174*

Ey (GPa) 9.86 10.5 10.7 12.1 12.4(1) Q 0.36 0.36 0.36* 0.36 0.36*

Gxy (GPa) 4.83 5.52 6.00 9.03 6.9(1) average of five specimens, except as noted in parentheses *estimated value