The objective of this study was to investigate the effect of thermal cycles encountered by an SST in service on the cumulative frequency of microcracks and degradation of open-hole-compressive (OHC) strength in high-temperature polymer-matrix composite materials. One cycle of thermal cycling was designated as the sequence from room temperature (RT) to 54C, up to +177C, and back to RT. Thermal-cycling tests were conducted up to 10,000 cycles on two kinds of carbonfiber/thermoplastic polyimide composite material: IM7/PIXA, IM7/K3B, and up to 1000 cycles on G40-800/5260 carbon fiber/bismaleimide composite material. At scheduled thermal cycles, transverse microcracks initiated on the sectional surface of the laminates were observed and counted using an optical microscope. Static mechanical tests at RT provided OHC strength before and after thermal cycles. In addition, a simple and approximate finite element model (FEM) analysis using basic lamina data of the T800H/PMR-15 carbon fiber/polyimide composite was conducted to estimate the thermal stresses generated in the laminate. Major results obtained by the tests and FEM analysis are as follows: A fairly large number of microcracks were initiated, though the number as a function of thermal cycles varied according to the material; OHC strength before and after thermal cycles did not change during the course of this study; thermal cycles and transverse microcracks did not affect OHC strength; the calculated thermal-stress level in layers generated by one thermal-cycle of 231C temperature difference was under the limit for crack initiation of the T800H/PMR-15.

The objective of this experimental study was to evaluate the effect of iso-thermal aging on the ultimate strength of three kinds of carbon/high-temperature composite materials, i.e., G40-800/5260 and MR50K/MR2000N bismaleimide composites and T800H/PI-SP amorphous thermoplastic polyimide composite. These materials are current candidate structural materials for a supersonic transport of the next-generation. The hole-notched and unnotched panels, before being machined to specimens, were isothermally aged at 120°C and 180°C for up to 15,000 hours. Static tests at room and elevated temperatures before and after thermal aging provided the open-hole tensile, open-hole compressive, and short beam shear strengths. Moreover, the effects of five oxidation resistant treatments on open-hole compressive strength at 180°C were investigated after isothermal aging of 5,000 hours at 180°C. The test results clarified the effects of isothermal aging on ultimate strengths and oxidation resistant treatments on open-hole compressive strength. Moreover, the possibility of developing an accelerated aging test method is discussed using a modified Larson-Miller type equation.

This paper presents fatigue lifetime data of impact-damaged carbon fiber/toughened epoxy composites under compressive loading to elucidate the lifetime prediction methodology based on statistical approaches. Drop-weight impact damage was induced to a composite specimen with impact energy of 6.7 J/mm in accordance with ASTM D7136. Postimpact fatigue tests were conducted using a test fixture defined in ASTM D7137 under compression-compression loading (R = 10) at room temperature. The maximum compressive stress (S min ) was 200, 210, 220, 230, and 240 MPa, and the total number of specimens was 31. The compression-after-impact strength and fatigue lifetime show considerable scattering. This result is apparently derived from the variation in impact damage size. A simple statistical model based on the weakest link theory was proposed for predicting the lifetime of impact-damaged composite laminates. Results show that the experimentally obtained data were confirmed consistently using this model. The ratio between the endurance limit at 10 6 and 10 7 cycles and the initial static strength was estimated as 0.74 and 0.68 as B-basis allowable values.

The objective of this study is to statistically investigate the effect of moisture absorption on the hot/wet compressive strength of a T800H/PMR-15 carbon/polyimide composite with a quasi-isotropic stacking sequence. Compression tests on the moisture absorbed specimens were conducted at 260'C. These tests clarified the compression fracture mode, the effect of moisture absorption on hot/wet compressive strength, statistical properties and a design allowable of hot/wet compressive strength, the effectiveness of a traveler coupon to monitor moisture absorption, and the relationship between specimen thickness and hot/wet compressive strength. Important results are as follows: (1) hot/wet compressive strength was closely related with the residual moisture content measured immediately after the test, and (2) the coefficient of variation of hot/wet compressive strength was approximately twice as large as that of room temperature/dry compressive strength.

The objective of this study was to investigate the effect of thermal cycles, encountered by an Super Sonic Transport (SST) in service, on the cumulative frequency of microcracks and the degradation of compressive strength in carbon fiber/polyimide resin composite materials.One thermal cycle was designated as a sequence from room temperature finally back to RT. Transverse microcracks initiated on the sectional free edge surface of the laminates were observed and counted by using an optical microscope. Approximate 10 000 thermal cycles were given for five kinds of carbon fiber/polyimide composite material: IM7/PIRA, IM600/PIXA-M, IM600/PIXA-MT3, IM7/K3B, IM7/R1-16, as well as a Ti/Gr (polyimide CFRP) fiber metal laminate. For IM7/R1-16 thermal cycling tests were conducted up to 40 000 cycles. Transverse microcracks were observed not only on the sectional free edge surface but also on the inside cross section of specimens cut after thermal cycling tests for IM7/R1-16. Static open-hole-compressive (OHC) strength was measured at RT before and after thermal cycling tests on the five kinds of material except for IM7/PETI-5. Static nonhole-compressive (NHC) strength was measured at RT on IM600/PIXA-M and IM600/PIXA-MT3. In addition, in order to know the estimates of thermal stress generated by the thermal cycles, the thermal stresses were calculated by the classical lamination theory (CLT) using basic lamina data given in a reference.