Crack closure stresses in fiber reinforced brittle matrix composites

Abstract

The fracture toughness of an alumina ceramic and a continuous SiC fiber reinforced alumina composite processed by pressureless sintering was studied in situ in a Scanning Electron Microscope (SEM). The applied stress intensity factor was obtained as a function of both applied load and crack extension. Closure stresses across crack surfaces imposed by grain-bridging and fiber-bridging, and hence fracture resistance from bridging were studied by both stress intensity factor and J-integral considerations. Theoretical calculations agree with experimental results. An average fracture resistance of ≈40 J m −2 per fiber and a corresponding toughness of ≈1·6 MPam 1/2 per fiber was obtained for fiber elastic bridging before fiber failure. Fiber-matrix interfacial properties were examined and a technique for evaluating interfacial frictional shear stress was developed.