Evaluation of fracture toughness of ceramic matrix composites using small specimens

Ceramic matrix composites (CMC) are being developed for high temperature utilization in aerospace and other industrial application. They offer several attractive features including high strength and modulus, improved fracture toughness, light weight, low thermal expansion coefficient and high thermal conductivity which contribute to good thermal shock resistance and high temperature stability in chemical and oxidative environments. Though the enhancement of fracture toughness is one of key issues for the development of these materials, the standardized evaluation method for the characterization of fracture toughness has not been established yet. Miniaturization of the test specimen for these tests has been also considered to be one of the most necessary issues because of the high cost of materials and the restriction of experimental environments. In this study, the influences of specimen size on the fracture resistance properties of plain-woven (P/W) Carbon/Carbon and Tyranno-LoxM/SiC composite were investigated. The fracture toughness tests with the unloading-reloading sequences were conducted with the compact tension specimens of different thicknesses and widths. The initiation fracture toughness J_Q of both materials increased with increasing of specimen thickness. It seems that interlayer frictional sliding between layered fabrics is one of energy dissipating mechanism in these materials. The initiation fracture toughness, J_Q, of both materials also increased with increasing of specimen width. The specimen width and the fabric condition affect the size of fracture process zone in 2D woven CMC.