An experimental study of the damage growth in composite laminates under tension-fatigue after impact

Abstract

The behavior of damaged laminates under cyclic load is crucial to the damage tolerance of composite structures. In this experimental study, we investigate impact damaged laminates under tension–fatigue loading. For that purpose, a tension after impact (TAI) specimen is designed, which exhibits a similar impact behavior like a specimen of the established compression after impact (CAI) standard. A set of quasi-static TAI tests is conducted as a reference for the investigation under cyclic load. A 30 J impact was chosen as the baseline, causing a delamination-dominated barely visible impact damage. For the cyclic test, fifteen TAI specimens were distributed over three load levels. The tests reveal a widespread fatigue effect, edge delamination, and a significant growth of the impact-induced delamination. The damage propagates mainly in the load direction and in the interfaces close to the impact side. The observed delamination growth can be explained through to the fiber cracks caused by the impact. Moreover, the growth rate exhibits severe outliers, whose damage propagates exceptionally fast. These outliers can be traced back to the morphology of the impact damage. The qualitative pattern of fiber fracture massively influences the delamination propagation. Based on this dependency we outline the need for a qualitative evaluation of impact damage and its scatter.