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

Abstract

In situ observations of crack propagation in applied-moment double cantilever beam specimens have been used to obtain the R-curve behaviour of Si₃N₄/50% BN–50% Al₂O₃ laminated composites, in which the BN–Al₂O₃ layers function as weak interphases. The crack plane and the crack direction were, respectively, normal and parallel to the plane of the laminated layers. During crack propagation, both delamination and crack deviation from the centreline of the specimen occurred. A deviated crack resulted in an uneven moment of inertia in the two beams of the specimen. For a non-laminated material, a deviated crack would become unstable, such that the crack would propagate towards the beam with the smaller moment of inertia. It was found in the present study that delamination in a laminated composite can stabilize the propagation of a deviated crack. The stabilization of a deviated crack with delamination was due to a decrease in the inequality in the moment of inertia of the two beams compared to that without delamination.