Fracture Properties on SiC Based Particulate Composites

Abstract

In order to evaluate the role of residual stresses in fracture toughening, a SiC-based particulate composite has been studied under uniform stressing conditions and in the near tip stress field of a pre-cracked specimen. First, residual stresses in a SiC-TiB_2 composite before and after stressing have been measured using x-ray diffraction. Tensile residual stresses in the TiB_2 drop by 50% after bending stresses of 250 MPa were applied. Likewise, the compressive residual stresses in the SiC phase decrease accordingly. Second, in the near tip stress field, a process zone of microcracks has been measured using transmission electron microscopy of thin foils taken from various locations from a fracture surface of a fracture mechanics specimen. Microcrack zones greater than 150 μm in height have been measured. Crack bridging sites of TiB_2 particles operate more than a few millimeters behind a propagating crack. Hence, the toughening in this system is comprised of both stress-induced microcracking and crack bridging. The various contributions to the toughening are discussed.