Test research on fatigue damage failure of C/C composites

The longitudinal tensile-tensile fatigue characteristics and in-plane shear tensile-tensile fatigue characteristics of unidirectional C/C composites and the longitudinal tensile-tensile fatigue characteristics of three-dimensional and four-directional braided C/C composites were researched through tests. The interface residual strength between fiber bundle and matrix was measured. The residual stiffness degradation model and the residual strength degradation model under longitudinal and in-plane shear tensile-tensile fatigue loading of unidirectional C/C composites were fitted by the least square method, and the interface residual strength model between fiber bundle and matrix was established. The results show that the biggest stiffness degradation of the unidirectional C/C composites at the fatigue load stress level of 87.5% is only about 8.8%, whereas, the biggest in-plane shear stiffness degradation goes to about 30% at the fatigue load stress level of 70.0%. The strength and stiffness of three-dimensional and four-directional braided C/C composites are both improved after fatigue loading. The interface strength between fiber bundle and matrix of the three-dimensional and four-directional braided C/C composites decrease gradually with the increase of fatigue loading cycles.