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

The non-destructive evaluation (NDE) of fatigue damage in thick polymer com posites used in large ships is examined by the use of low-frequency pulse-e cho ultrasonics. The initiation and growth of fatigue-induced damage was mo nitored with ultrasonics during cyclic flexural loading of glass-fibre-rein forced polymer (GRP) composites between 7 mm and 35 mm thick. Fatigue loadi ng the GRP to high stresses (above 50% of the static failure stress) induce d extensive damage that consisted of debonding of the surface glass fibre t ows together with a complex array of cracks and delaminations within the co mposite, and this damage was easily detected by means of ultrasonics. Cycli c loading the composite to relatively low fatigue stresses (below 50% of th e failure stress) caused much less damage. Short cracks developed within th e resin-rich regions between the glass fibre plies and within the fibres to ws, and this damage was just detectable by using ultrasonics. The cracks in duced by low-stress fatigue were difficult to detect because they mostly gr ew parallel to the transmission path of the ultrasound waves. The ability t o detect damage induced by either low or high fatigue stresses improved wit h 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 r eserved.