Fatigue Crack Growth Rate at Material and Geometry Transitions in Glass-Epoxy Composites

An element test specimen with ply drops, intended to be representative of a composite structure of varying thickness such as the main laminate in a wind turbine rotor blade structure, is used to investigate the fatigue damage initiating from a ply drop under cyclic tension-tension loading. The focus is to measure the growth rate of delamination cracks propagating from a thin towards a thicker section. Several delaminations initiate from tunneling cracks - cracks between the ply drops and resin reach areas - after very few load cycles. All except one delamination crack propagate for a number of cycles but eventually stop growing. The only delamination crack that continued to grow has the characteristic that its growth rate increases as it propagates to thicker sections of the element specimen. The experimental findings are supported by finite element results.