TOUGHNESS ASSESSMENT OF CERAMIC MATRIX COMPOSITES

The favorable increase in toughness which can be achieved through the adjustment of fiber-matrix composites (CMC) has revealed the need for defining methods of toughness evaluation. Most of the methods which have been proposed are based on the determination of the compliances of notched specimens under loading/unloading cycles. The use of such methods is required to overcome the difficulties related to significant non-linearities and residual displacements observed on the load/displacement curves derived from testing of notched specimens. When tested under tension, unnotched specimens of the various CMC which were investigated show damageable, quasi-linear elastic behaviors. The discrepancy between the two types of tests is explained by the presence of debris which invalidate methods based on loading/unloading cycles. Moreover, the approximation which considers tested CMC as damageable linear elastic materials leads to very small errors in the determination of the crack growth resistance curves. Although these curves are not able to give intrinsic characteristics of toughness, they do allow a precise comparison between CMCs. © 1989.