Micromechanics of Compression Failures in Open Hole Composite Laminates

Abstract

Compression failures in laminates with holes are often preceded by the development of a damage zone which grows with increasing compressive load. This damage zone is due to local fiber buckling and/or shear crippling. The local softening in the damage zone has been modeled using a Dugdale-type approach. This paper compares this analytical model to the actual development of the damage zone size as a function of load. Comparison of the model predictions to the experimental observations indicates that the Dugdale model does not accurately predict the load-damage zone size relationship in open hole composite specimens loaded in compression. Hole size and resin ductility were the two primary variables used in this investigation. Three hole diameters were used in specimens 2.54 cm (1 in.) wide containing 5.08 cm (2 in.) gauge length. Matrix ductility and stiffness were also varied by using matrices of polyetheretherketone (PEEK) and Hexcel F155. Very brittle resins were not studied because the damage zone development that precedes unstable damage extension is too small to record. Several coupon tests were interrupted prior to catastrophic damage growth to allow for a more careful examination of the fiber shear crippling and/or buckling. These observations were accomplished using nondestructive examination (C-scan and X-ray) and scanning electron microscopy. A preliminary sectioning study was conducted through the thickness of the damage zone of an AS4/PEEK specimen; each section was observed in the scanning electron microscope.