Some considerations of asymmetric cracking in an applied-moment double cantilever beam

In situ observations of crack propagation in applied-moment double cantilev er beam specimens have been used to obtain the R-curve behaviour of Si3N4/5 0% BN-50% Al2O3 laminated composites, in which the BN-Al2O3 layers function as weak interphases. The crack plane and the crack direction were, respect ively, normal and parallel to the plane of the laminated layers. During cra ck propagation, both delamination and crack deviation from the centreline o f the specimen occurred. A deviated crack resulted in an uneven moment of i nertia in the two beams of the specimen. For a non-laminated material, a de viated crack would become unstable, such that the crack would propagate tow ards the beam with the smaller moment of inertia. It was found in the prese nt study that delamination in a laminated composite can stabilize the propa gation of a deviated crack. The stabilization of a deviated crack with dela mination was due to a decrease in the inequality in the moment of inertia o f the two beams compared to that without delamination. (C) 1998 Kluwer Acad emic Publishers.