Acoustic emission analysis of the interlaminar resistance increase during Mode I delamination with fibre bridging in composite laminates

Abstract

This study investigates the damage mechanisms and associated interlaminar toughness (G_IC) increase in Mode I delamination with large-scale fibre bridging for a carbon fibre/epoxy composite. Using Acoustic Emission (AE), Wavelet Packet Transform (WPT), and scanning electron microscopy, four damage modes were identified: matrix cracking, interface debonding, fibre pullout and fibre breakage. These modes are combined in the fibre bridging process. Cluster analysis of AE signals correlated each mode to a specific AE signature. The AE energy rate (AEER), defined as the cumulative AE energy per unit of crack propagation length, revealed that fibre pullout, with an AEER at least an order of magnitude higher than other modes, is the dominant toughening mechanism for G_IC increase. Matrix cracking and interface debonding have a moderate effect, whereas fibre breakage has little effect on the G_IC increase. The magnitude of G_IC during delamination propagation also correlates with the instantaneous cumulative absolute energy per AE counts (d(AEE)/d(Counts)), defined as the ratio of the differential of cumulative AE absolute energy to the differential of cumulative counts. This ratio increases with delamination growth and finally stabilizes. These correlations provide a basis for evaluating damage mechanisms and designing composite toughening strategies.