Failure modes in fiberglass after multiple impacts: an acoustic emission and digital image correlation study

The nature and extent of damage in multiple times impacted E-glass/epoxy laminates subjected to compression loading has been investigated, using a correlation of passive image processing and online acoustic emission techniques. Sixteen-ply unidirectional glass fiber reinforced plastic laminates were fabricated, on which low velocity repeated impact tests were conducted at an energy of 8.67 Joules, followed by post-impact compression tests accompanied with acoustic emission monitoring and online recording of the complete loading using a digital camera. The combination of different methods suggested that damage modes during impact change were depending on whether the laminate had been impacted already or not. This compression after impact loading the accrued importance of fiber microbuckling, as assessed from acoustic emission frequency analysis, for growing number of impacts and for higher compression stress. Moreover, different growing levels of critical stress for the laminates during loading were established, linked to damage initiation (first acoustic emission events detected), to damage propagation, (digital images start detecting an increase of damaged area), and to unstable damage growth (maximum compression after impact stress is reached).