The Ultimate Tensile Strength of SiC/SiC Composites: Multiscale Approach

The present paper tackles the important issue of tensile ultimate strength of ceramic matrix composites, using a multiscale approach. The ultimate strength is investigated at the successive increasing length scales inherent to 2D woven SiC/SiC composites, i.e., single filaments, fibre tow, unidirectional composite (minicomposites), and 2D woven composite. First, experimental results on tensile behavior under strain-controlled conditions are summarized for tows, minicomposites, and composites. Then, models of tow ultimate failures under controlled force and strain are presented. The exact criterion of tow failure is developed for filament fracture initiation and then propagation based on applied stress and on filament strength gradient. The model of the ultimate failure of the composite under strain-controlled conditions is based on the strength of filaments in the presence of matrix cracks and the overstress induced by interactions of broken filaments and the matrix. The variability of ultimate strengths of filaments, minicomposites, and composites at various gauge lengths is described by linear p-quantile diagrams, which indicates that the data follow a normal distribution function. The contribution of structural effects to the variability of composite and minicomposite strength under strain-controlled loading is analyzed. Their dependence on specimen size is related to the reproducibility of critical flaw population and structural effects.