Damage and fracture in monotonic and cyclic loading of cross-woven carbon/silicon carbide composites.

摘要

Textile composites such as cross-woven composites possess a far more complex structure than the well-studied unidirectional and cross-ply types of composite, since the making of the textile cross-woven composites involves interlacing of fiber bundles and laying-up of fabric laminates. It is of great importance to understand the influence of the complex structures on the mechanical response in cross-woven composites in terms of damage and damage development. In the present study, the microstructure of a cross-woven C/SiC ceramic composite was characterized. The damage and fracture of the composite subject to tension, compression and cyclic tension were investigated. Due to the weaving of the carbon fiber bundles, the SiC matrix was found to be distributed inhomogeneously in the fiber bundle. The chemical vapor infiltration (CVI) processing of the composite produced cracked SiC coatings in the composite. Under monotonic tensile loading, the composite showed six modes of damage, which include (1) matrix cracking, (2) transverse bundle cracking, (3) interfacial debonding/sliding, (4) fiber breaking, (5) ply delamination, and (6) bundle splitting. The development of those damage modes, especially matrix crack multiplication, led to a largely nonlinear stress-strain behavior and caused the composite modulus to decrease substantially during tensile loading. Associated with those damage modes and the complex composite structures, four types of bundle fracture surfaces were found in monotonic tensile loading. The damage modes in cyclic loading (pulsating tension) were similar to those in monotonic tension except that an additional wear mechanism of damage, produced by cycling, was identified in the internal structure of the composite. Under repeated tensile loading, those fatigue damage modes developed in a subtle manner, leading to gradual increase in cyclic elapsed strain and decrease in composite modulus. Fatigue failure of the composite occurred at high fatigue stress levels where continued fiber fracture took place in the composite. This study clearly demonstrated that the architecture and the microstructure of the composite have a profound effect on the damage and fracture mechanisms under the loading conditions performed in this study.