Predicting residual strength of pre-fatigued glass fibre-reinforced plastic laminates through acoustic emission monitoring

Glass fibre-reinforced plastic beams were pre-fatigued in four-point bending up to selected portions of their fatigue life. The samples were then monotonically brought to failure while recording their acoustic emission response. The trend of the cumulative event counts was markedly affected by the specimen load history: for a given stress level, a higher activity was obtained from samples subjected to a higher number of cycles. A good empirical correlation was found between the material residual strength and the total event counts detected at maximum stress applied during pre-fatiguing cycles. Moreover, the correlation was improved when a previous model, relying on fracture mechanics concepts, was utilised. However, in both cases, a difficulty was experienced in evaluating the residual strength of the composite laminate after a small number of fatigue cycles. Under these conditions, the modulus decrease was seemingly more sensitive to the effects of fatigue than the acoustic emission response.