The Fatigue Behaviors And Damage Evolution Of 3D Braided C/C Composites

Carbon/carbon composites(C/C composites) are the only material that can maintain high strength and toughness up to high temperatures of more than 3000 K. Thus, it has been considered as the most promising candidate for high temperature structural material for various purposes. As preferred high-temperature structural materials, the potential applications for C/C composites are bound to involve fatigue loads. Therefore, a clear understanding of the behavior of these composites under fatigue loads is necessary. In the recent years, research studies on the fatigue behavior of C/C composites have been performed; however, the results of these studies were still insufficient to establish the design criteria for load-bearing structures. In particular, the influence of fatigue loading on mechanical properties of C/C composites is not well understood. Thus, the usage of C/C composites is still limited to heat resistant components of spacecraft, rocket nozzles and nuclear reactors, etc. Therefore, in this present study, attention was mainly focused on the fatigue behaviors and damage evolution of 3D integral braided C/C composites, and some new contributions are summarized as follows:In the 3D reinforcement performs, Carbon fibers were braided into a rectangular strip(with width of 6.5mm and thickness of 3.5mm) by a four-step method using 1K T-300 carbon fibers, in which the braided angle was 22°and the fiber volume fraction was 45％. In the 3D reinforcement performs, Carbon fibers were braided into a rectangular strip(with width of 6.5mm and thickness of 3.5mm) by a four-step method using 1K T-300 carbon fibers, in which the braided angle was 22°and the fiber volume fraction was 45％. 3D integral braided C/C composites have been prepared by an isothermal chemical vapor infiltration(CVI) process, and their flexural and tensile fatigue behaviors were examined under load control at a sinusoidal frequency of 10 Hz.In the present study, the stress-fracture cycles(S-N) relationships of these composites in bend and tensile fatigue mode were obtained. The load-displacement curves and the fracture surfaces of specimens at various fatigue cycles were observed in order to identify the micro-damage induced during fatigue loading. Finally, the mechanisms responsible for strength enhancement after fatigue loading was discussed based on the experimental evidence.For the flexural fatigue behavior study, attention was focused on the effect of applied stress on damage mode of 3D integral braided C/C composites under bend-bend fatigue loading. The flexural fatigue limit of the C/C was examined. The fracture surfaces and cross-sections of the original and fatigued C/C composites at various level of applied stress were observed. It is revealed that the interfacial sliding abrasion play an important role in the fatigue failure process, and the extent and speed of sliding abrasion were controlled by the level of applied stress; for the tensile fatigue behavior study, the results show that the strength of specimens was enhanced with increase in tensile fatigue cycles and applied stress, but the fatigue enhancement was not boundless and would decline gradually.With the help of modern nondestructive testing (NDT) technology, the fatigue damage and fatigue enhancement have been investigated deeply and entirely. Considering the structure character of 3D integral braided C/C composites, X-ray diffraction method (XRD) and electrical resistance monitoring, assisted with scanning electron microscopy (SEM), is selected from several available NDT means after practical trials. It is testified experimentally that X-ray diffraction method (XRD) and electrical resistance monitoring is suitable for non-microcosmic defect detection during fatigue loading process of C/C composites. With these NDT technology, it has found that the augment of interlaminar distance, the decline of graphitization degree and microcrystalline stack and the release of remain thermal stress have contributed to the enhancement of strength of C/C composites under fatigue loading.The flexural performances of un-notch and notch C/C composites have been experimentally studied before and after fatigue loading. The sensitivity of notch and the domino offect of notch have been analyzed.In addition, the effect of fatigue loading on the fracture toughness and fracture behaviors of a 2.5 dimensional tightly woven C/C composites (2.5D C/C composites) was investigated by using the compact tension (CT) test. The fracture surfaces of CT specimens were observed using a scanning electron microscope (SEM) and a digital camera respectively. The compliance curves and the crack growth resistance curves (R-curves) showed that the fracture toughness of C/C composites was increased evidently after fatigue loading. The fiber-matrix interface was weakened during fatigue loading by analyzing the fracture morphology of the specimen. The weakened interfaces relaxed the stress concentration of the tip of cracks, increased the resistances of crack expansion process effectively, improved the fracture energy remarkably, prolonged the destroying time, and reinforced the damage capacity of the material such that the fracture toughness of the composites was enhanced by fatigue loading.Meanwhile, the flexural properties and damage modes have been investigated at 1000℃, 1300℃, 1700℃and room temperature respectively, and the strength improvement mechanism at the high-temperature was discussed according to the character of the thermal expansion rules and load-displacement curves as well as the fracture morphology of C/C composites. The results show that flexural properties at high temperature are super!or greatly to these at room temperature. The enhancement of internal" friction is the main reason for the increasing strength of C/C at high temperature.