Failure mode discrimination in mode II fracture of glass/epoxy laminates using acoustic emission technique

The intention of this work is to discriminate the failure modes in Mode II fracture of glass/epoxy laminates on the basis of the frequency of the elastic waves emitted when the material is strained. Failure investigation was performed on end notched flexure (ENF) samples prepared with a Teflon insert in the midsection to act as delamination initiator, by performing acoustic emission (AE) monitoring on four-layer glass/epoxy laminates with different fibre stacking sequences, namely [0/0/0/0], [0/90/90/0], [+45/-45/+45/-45], [0/+45/-45/90] and [90/+45/45/0]. Unidirectional [0/0/0/0] and [0/0] laminates were also prepared as lap jointed samples and tested in shear. Moreover, pure epoxy resin specimens were tested in tensile and flexural loading to compare with composite laminates. AE monitoring by the analysis of mode II fracture was able to discriminate all the failure modes, including matrix cracks (longitudinal and transverse), interface failures, fibre micro-buckling and fibre fracture by the measurement of the different primary frequency ranges of AE signals. Considerations on the influence of fibre stacking sequence on the severity of particular failures and on the sequence of events over time leading to failure were also reported