Acoustic emission and fatigue crack growth in rigid polyurethane foam

Abstract

Compact tension specimens of a rigid polyurethane foam have been tested in fatigue and crack growth has been monitored visually and by means of acoustic emission (AE). During the load cycle it has been found possible to resolve the AE activity into four regions: the crack faces “un-sticking”, fracture events at or close to peak load, a period of zero AE just after peak load, and AE associated with crack closure lower down the unloading part of the cycle. The fracture AE has been found to increase rapidly with crack length — consistent with a seventh power dependence on ΔK — and to occur during every cycle at high ΔK values, but to be absent in an increasingly greater proportion of cycles as ΔK is decreased below about 40 kPa m1/2. AE data obtained on samples in which crack growth occurred across the layers of foam, through the high density inter-layer skins, show that the technique is very sensitive to the crack retardation effect associated with these skins well before this retardation is detectable visually.