Effect of pre-fatigue on bending behavior of 2.5D C/C composites

This study investigates the influence of pre-fatigue on the bending behavior of needle-punctured 2.5D C/C composites with density between 1.75 and 1.80 g/cm3 prepared by chemical vapor deposition (CVD). Three-point bending pre-fatigue tests are carried out under applied stress/static flexural strength ratio of 60%, 70%, 80% and 90% up to one million cycles. The results show that except for the 60% sample, flexural bending strength of most samples subjected to pre-fatigue is improved significantly compared with that without pre-fatigue. Detailed investigation indicates that the subsequent bending behavior of sample is closely related to both stress level in pre-fatigue and the resultant microstructure. Under low stress level (60-70%) during pre-fatigue, the composite is characterized by a plastic deformation and a slight enhancement of bending strength ascribed to the weakened fiber/matrix interface after fatigue. After pre-fatigue with high stress level (80-90%), the composite demonstrates brittle failure mode but much higher residual strengths during subsequent bending test. The reason for this is ascribed to the variation of the microstructure in fiber/matrix interface during the pre-fatigue, which strongly affects the subsequent mechanical responses.