Effect of Thermal Residual Stress on Matrix Cracking Strain and Fracture Behavior of the Sintered SiC Fiber Reinforced SiO₂-Mullite Composites

The sintered SiC fiber reinforced SiO₂-mullite composites with matrix compositions, i.e., SiO₂-3.7, 30, and 50 mol%Al ₂O₃, had already been developed. In these composites, the thermal residual stress field changed with an increase in the coefficient of thermal expansion of the matrix accompanied by an increasing Al ₂O₃ content (mullite volume fraction) in the matrices. A three-point flexural test was conducted for the composites at room temperature. The strain gauge method and AE monitoring were used to detect the matrix cracking strain during the test. The effect of the thermal residual stress on the matrix cracking strain and fracture behavior were investigated. The results are as follows. The matrix cracking strain observed in the composites with a higher tensile thermal residual stress in the matrix region parallel to the fiber axis was well predicted by the BHE theory. For the SiC fiber/SiO ₂-50 mol%Al₂O₃ composite, the first acoustic emission signal was detected just after the initial proportional limit of the stress-strain curve. For the other composites, the signals were detected below this limit. When the residual stress in the matrix region parallel to the fiber axis was compressive, the linear fracture behavior was found. On the contrary, the bi-linear fracture behavior was enhanced by the tensile residual stress in the matrix. This tendency agreed with the prediction method by Luh and Evans. The magnitude of the fracture energy obtained in each composite was qualitatively explained by the conventional theoretical formula.