Estimating microscopic defect size from Acoustic Emission Monitoring of Mode I, Mode II and Mixed-Mode I/II delamination propagation in GFRP laminates under quasi-static loads

In-situ acoustic emission (AE) monitoring of standard quasi-static fracture tests on fiber-reinforced polymer (FRP) composite specimens proves that delamination propagation results from the stochastic occurrence of micro- or meso-scale matrix cracks. The AE signal activity correlates with the observed speed of delamination propagation. Faster delamination propagation hence is rather due to a higher number of matrix cracks per unit time than a significantly increasing average crack size. Approaches for roughly estimating the average size and the size distributions of these matrix cracks with AE monitoring are presented and discussed. This has implications for micromechanical modelling of delamination propagation. The contribution focusses on glass fiber-reinforced epoxy, but the method is also applicable to carbon fiber reinforced thermosets and thermoplastics as well.