Fracture energy measurements of 2-D carbon/carbon composites

Abstract

Fracture toughness of ceramic-matrix composites cannot be provided by an intrinsic fracture parameter. The presence of a fracture process zone induces stress redistribution mechanisms that partially shield the crack tip area from the applied stress field. The shape and size of the process zone depends upon extrinsic factors such as specimen shape and thickness, load distribution on the propagation zone, etc. The present work deals with the determination of the crack growth resistance curve ( R-curve) of carbon/ carbon ( C C ) composites using the standard compact tension geometry. Two different material thicknesses were used and both monotonie and loading/unloading test patterns were applied. The determination of the R-curve in all cases was based on a compliance calibration technique. The results show that the monotonic loading leads to higher plateau values for the R-curve of the C C material compared to that calculated from cyclic loading and referred to the same material thickness. Furthermore, the thicker specimens appear to have slightly higher plateau values. A precise examination of damage initiation and propagation was attempted to explain the monitored differences.