Strength Scaling in Fiber Composites

A research program was initiated to study and isolate the factors responsible for scale effects in the tensile strength of graphite/epoxy composite laminates. Four layups 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 = to 4, 3, 2, and 1, respectively. The full scale specimen sizes was 32 piles thick as compared to 24, 16, and 8 piles for the 3/4, 2/4, and 1/4 specimen sizes respectively. Results were obtained in the form of tensile strength, stress-strain curves and damage development. Problems associated with strength degradation with increasing specimen sizes are isolated and discussed. Inconsistencies associated with strain measurements were also identified. Enhanced x ray radiography was employed for damage evaluation, following step loading. It was shown that fiber dominated layups were less sensitive to scaling effects compared to the matrix dominated layups.