THE FORMATION AND PROPAGATION OF MATRIX MICROCRACKS IN CROSS-PLY LAMINATES DURING STATIC LOADING

Recently, a variational mechanics analysis approach has been used to determine the thermoelastic stress state in cracked, cross-ply laminates (Nairn, J. A. 1989. J. Comp. Mat., 23:1106). The analysis included a calculation of the energy release rate due to the formation of a microcrack in the 90-degrees plies. This paper describes a series of experimental results on a wide variety of composite material systems and of cross-ply layups of generic type [0m/90n]s. The variational mechanics energy release rate analysis can be used to predict all features of the experimental results and to draw some new conclusions about the progression of damage in cross-ply laminates. The recommended experiments are to measure the density of microcracks as a function of applied stress. Such results can be fit with the energy release rate expression and used to measure the microcracking or intralaminar fracture toughness. The measured microcracking fracture toughnesses are: Hercules AS4/3501-6-240 J/m2, Fiberite 934/T300-690 J/m2, DuPont Avimid(R) K Polymer/IM6-960 J/m2, Fiberite 977-2/T300-1800 to 2400 J/m2, and ICI PEEK/AS4-3000 J/m2. Experiments that measure only the stress to initiate microcracking are specifically not recommended because they do not give an accurate measure of the microcracking fracture toughness.