A CRITICAL-EVALUATION OF THEORIES FOR PREDICTING MICROCRACKING IN COMPOSITE LAMINATES

We present experimental results on 21 different layups of Hercules AS4 carbon fibre/3501-6 epoxy laminates. All laminates had 90-degrees pli es; some had them in the middle ([(S)/90n]s) while some had them on a free surface ([90n/(S)]s). The supporting sublaminates, (S), where [0n ], [+/- 15], or [+/- 30]. During tensile loading, the first form of da mage in all laminates was microcracking of the 90-degrees plies. For e ach laminate we recorded both the crack density and the complete distr ibution of crack spacings as a function of the applied load. By rearra nging various microcracking theories we developed a master-curve appro ach that permitted plotting the results from all laminates on a single plot. By comparing master-curve plots for different theories it was p ossible to critically evaluate the quality of those theories. We found that a critical-energy-release-rate criterion calculated using a two- dimensional variational stress analysis gave the best results. All mic rocracking theories based on a strength-failure criteria gave poor res ults. All microcracking theories using one-dimensional stress analyses , regardless of the failure criterion, also gave poor results.