Nondestructive Evaluation of Thermal Stress-Induced Damage in Thin Composite Laminates

Abstract

Nondestructive evaluation of thermal stress-induced failure in the free edge 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 ultrasonic C-scan and optical microscopy. Characteristic behavior of thermo-acoustic emission (AE) detected from the composite specimen during the heating and cooling cycle indicated the degree of damage which had occurred differently for as-moulded and cryogenically-treated specimens with various lay-up angles. Analysis of the vibration spectrum data obtained with short beam specimens exhibited that the failure occurrence caused an obvious decrease in resonant frequency and some considerable increase in flexural damping ratio. Total AE energy showed a feature similar to the flexural damping ratio which increased with increasing crack density. Consequently, it is thought that thermo-acoustic emission in association with flexural damping might be employed as a tool for nondestructive evaluation of thin composite laminates.