Mechanisms of kink-band formation in graphite/epoxy composites: a micromechanical experimental study

Abstract

Graphite-fiber-reinforced epoxy composites often fail in compression by kink band formation. Micro-Raman spectroscopy (MRS), scanning electron and optical microscopy, and mechanical testing have been used to study kink-band initiation in Toray M40/DGEBA epoxy composites at the microscopic and macroscopic levels. The results show that kink bands initiate from a damage zone comprising crushed and broken fibers. The damage-zone forms from a nucleus that expands by failure of adjacent fibers because of local strain concentrations. Once the damage zone reaches a critical size the unsupported column of matrix causes a local matrix shear instability (or microbuckling). This local instability propagates as a kink band. The damage zone angle was found to depend on the interface properties, and to affect the strain to form kink bands. The damage angle was used to obtain a qualitative prediction of composite compression strength.