Characterization of transverse failure in composites using acoustic emission

Abstract

Composites materials provide a strong, lightweight material suitable for engineering applications. Degradation of these materials is by propagation of a range of defects which ultimately control failure. Acoustic emission (AE), an ultrasonic method for materials characterization, provides one of the most sensitive techniques for monitoring this material degradation and has advantages over both conventional ultrasonic and radiographic methods. Since composite materials used for engineering applications are manufactured by cross-ply methods, transverse failure is an important damage mechanism in controlling the initiation of damage in a complete composite. These defects cause stiffness degradation which may be lifetime limiting in, for instance, stiffness dependent aerospace applications, ultimately leading to initiation of delamination. Transverse samples of glass-polyester resin have been monitored using acoustic emission as they are taken to mechanical failure. Results presented consider the AE produced by transverse composite material in cured and post-cured conditions. The pattern of AE indicates the fact that the resin toughness has increased due to post cure. Weibull statistics have been applied to acoustic emission event parameters in the case of a transverse composite, although this particular application of the statistics does not bear a fundamental relationship to the microscopic damage mechanisms. Weibull statistical parameters associated with AE signals have proved useful in signalling the changing condition of these materials and monitoring the onset of damage. Transverse cracking in fibre bundles is considered, with acoustic emission providing a sensitive method for real time detection of cracking. AE provides a unique view into the micromechanics of defect initiation and growth.