Won-destructive detection of fatigue damage in thick composites by pulse-echo ultrasonics

The non-destructive evaluation (NDE) of fatigue damage in thick polymer composites used in large ships is examined by the use of low-frequency pulse-echo ultrasonics. The initiation and growth of fatigue-induced damage was monitored with ultrasonics during cyclic flexural loading of glass-fibre-reinforced polymer (GRP) composites between 7 mm and 35 mm thick. Fatigue loading the GRP to high stresses (above 50% of the static failure stress) induced extensive damage that consisted of debonding of the surface glass fibre tows together with a complex array of cracks and delaminations within the composite, and this damage was easily detected by means of ultrasonics. Cyclic loading the composite to relatively low fatigue stresses (below 50% of the failure stress) caused much less damage. Short cracks developed within the resin-rich regions between the glass fibre plies and within the fibres tows, and this damage was just detectable by using ultrasonics. The cracks induced by low-stress fatigue were difficult to detect because they mostly grew parallel to the transmission path of the ultrasound waves. The ability to detect damage induced by either low or high fatigue stresses improved with increasing thickness of the GRP. The implications from this study for the NDE of fatigue-induced damage in thick composite ships by the use of pulse-echo ultrasonics is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.