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

The damage process in composite laminates subjected to cryogenic cooling was monitored employing a thermo-acoustic emission (AE) technique. The thermo-AE signals processed with a short-time Fourier transform could be classified into three different types which were correlated with individual microfracture processes. In the initial stage of cryogenic cooling, very strong AE signals with low and high frequency bands were dominantly detected showing that large cracks accompanying fiber breakages were developed mainly. With an increase in the cooling time, weak emissions with low frequency bands became prevalent indicating the propagation of microfractures in the matrix and/or fiber-matrix interface. Similar types of AE signals, however, having weak amplitudes, were also observed for the cryogenically-treated specimens during thermal heating and cooling load cycles. Thus, analysis of thermo-AE behavior through the thermal load cycle led to the nondestructive evaluation for the cryogenic damage of composites.