The effect of alternating the sequence of variable-energy repeated impact on the residual strength and damage evolution of composite laminates

This article investigates the effect of alternating the sequence of variable-energy repeated low-velocity impact on the residual strength of carbon fiber reinforced polymer (CFRP) laminates. Quasi-isotropic CFRP specimens were impacted using three impact energy levels, that is, Low (L), Medium (M), and High (H). Every group, out of three groups in total, was impacted consecutively using one of three sequences: LMH, HML, and MLH. Delamination and its growth after each impact was quantified using a phased-array ultrasonic probe. The impacted specimens were then subjected to compression after impact loading with in situ acoustic emission (AE) and digital image correlation (DIC) measurement. Mechanical results showed that the residual strength of the specimens was not significantly affected by alternating the impacts sequence. However, the AE, sentry function and DIC results revealed that the impact-induced damage in HML configuration was activated much earlier than MLH, and the damage induced in LMH configuration was activated later than both.