Interlaminar Strains and Compressive Strength Reductions Due to Nested Layer Waviness in Composite Laminates

Abstract

Layer waviness, or out-of-plane layer undulations in a composite laminate, has been shown to produce significant reductions in compressive strength. The object of this research was to directly correlate experimentally determined strain fields and compressive strengths with analytical predictions. Two layer wave formations were intentionally fabricated into otherwise wave-free carbon/epoxy cross-ply laminates: a single wavy layer and three nested wavy layers. Moire interferometry, mechanical testing, and finite element analysis were used to determine interlaminar strains and compressive strengths. Results indicate that under compression loading, large interlaminar shear and normal strains, comparable to the magnitude of the applied strain, occur as a result of layer waviness. Experimentally, the three nested layer wave formation produced a significantly larger strength reduction than the single layer wave. Using the maximum stress as well as the Hashin failure criteria, however, only a slight additional reduction in compressive strength was predicted for the three nested layer wave case as compared to the single layer wave. These results suggest that although the finite element model properly predicts deformations and interlaminate strains associated with layer waviness, simple stress-based failure criteria are not adequate for predicting compressive strength reductions associated with naturally occurring formations of layer waviness.