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

Graphite-fiber-reinforced epoxy composites often fail in compression by kin k band formation. Micro-Raman spectroscopy (MRS), scanning electron and opt ical microscopy, and mechanical testing have been used to study kink-band i nitiation in Toray M40/DGEBA epoxy composites at the microscopic and macros copic 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 concentr ations. 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 fou nd 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. (C) 1999 Elsevier Science Ltd. All rights reserved.