A Detailed Investigation of the Micromechanisms of Compressive Failure in Open Hole Composite Laminates

Abstract

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing com pressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling into the edges of the hole. The length or size of the damage zone increases with increasing load, propagating stably until it reaches a critical size. Then, unstable growth begins, and the zone completely traverses the specimen. Subsequently, a more cata strophic failure occurs giving brooming and/or delamination. Several tests of 2.54 cm wide AS4/PEEK containing a 0.16 cm diameter hole were inter rupted prior to catastrophic damage growth to allow for a careful examination of the fiber shear crippling and/or microbuckling. These observations were accomplished using scan ning electron microscopy. Direct observation of the specimen revealed the surface size of the shear crippling zone. To determine the extent of this damage through the thickness of the laminate, systematic sectioning techniques were employed. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the damage zone was developed.