Nondestructive evaluation of thermal stress-induced damage in thin composite laminates

Nondestructive evaluation of thermal stress-induced failure in the free edg e region of thin composite laminates (1 mm thick) has been carried out. The damage zone which arose under a cryogenic cooling was examined using the u ltrasonic C-scan and optical microscopy. Characteristic behavior of thermo- acoustic emission (AE) detected from the composite specimen during the heat ing and cooling cycle indicated the degree of damage which had occurred dif ferently for as-moulded and cryogenically-treated specimens with various la y-up angles. Analysis of the vibration spectrum data obtained with short be am specimens exhibited that the failure occurrence caused an obvious decrea se in resonant frequency and some considerable increase in flexural damping ratio. Total AE energy showed a feature similar to the flexural damping ra tio which increased with increasing crack density. Consequently, it is thou ght that thermo-acoustic emission in association with flexural damping migh t be employed as a tool for nondestructive evaluation of thin composite lam inates. (C) 2001 Kluwer Academic Publishers.