Monitoring of microfracture processes in composite laminates by thermo-acoustic emission

Abstract

Composite laminates subjected to cryogenic impacts were monitored using a thermo-acoustic emission (AE) technique. The thermo-AE signals being processed through a short-time Fourier transform could be classified into three different types in relation with individual microfracture processes. In the initial stage of cryogenic impact, large cracks accompanying fiber breakages were developed mainly and very strong AE signals with low and high frequency bands were dominantly detected. During keeping the composite in a cryogenic cooling temperature, strong emissions became less and weak emissions with low frequency bands prevailed indicating the propagation of microfractures in the matrix and/or fiber-matrix interface. For the cryogenically-treated specimens similar types of AE signals having weak amplitudes were also observed during thermal heating and cooling load cycles. It is concluded that analysis of thermo-AE behavior detected through the thermal loads was useful for the nondestructive monitoring of the cryogenic microfracture processes of composites.