STRENGTH SCALING IN FIBER COMPOSITES

Factors responsible for scale effects in the tensile strength of graphite/epoxy composite laminates have been studied. Four layups, (+30n/-30n/90(2n))s, (+45n/-45n/0n/90n)s, (90n/0n/90n/0n)s, and (+45n/-45n/+45n/-45n)s, were chosen with appropriate stacking sequences so as to highlight individual and interacting failure modes. Four scale sizes were selected for investigation, including full-scale size, 3/4, 2/4, and 1/4, with n equal to 4, 3, 2, and 1, respectively. The full-scale specimen size was 32 plies thick as compared to 24, 16, and 8 plies for the 3/4, 2/4, and 1/4 specimen sizes, respectively. In general, large-size specimens exhibited a lower tensile strength. Fiber dominated layups were found to be less sensitive to scale effects compared to the matrix dominated layups. It has been observed that the presence of transverse ply cracks, which were associated with the ply thickness, were largely responsible for both strength and failure mode variations in different size specimens. In some cases, such cracks were found in virgin laminates, which meant that subscale models and full-scale laminates of the same layup were effectively dissimilar.