MONITORING THE TENSILE DEFORMATION IN REAL UNIDIRECTIONAL KEVLAR EPOXY COMPOSITES

Two acoustic emission (AE) parameters, event count rate ( ER) and the skewness of the peak amplitude distribution ( PAD), were found to correlate with mechanical and damage mechanisms in unidirectional Kevlar-epoxy composites loaded in tension. The ER of the AE reached a local maximum at about 0.3-sigma-cu (sigma-cu = composite tensile strength), a minimum at about 0.6-0.7-sigma-cu, and increasing ER rates were noted as failure stress approached, with a peak at final failure. Along with our proposed approach to evaluate the PAD as a tool for characterizing processes, damage mechanisms and failure modes, the third statistical moment of the PAD, the skewness, was found to be able to distinguish between various mechanisms. From the beginning of loading until completion of the fibre straightening process, 0.6-0.7-sigma-cu, the value of the skewness steadily decreased, and then gradually increased to final failure. This behaviour is explained by a model based on the non-elastic mechanisms which contribute to the monitored AE. At final failure, the PAD's skewness value was affected by macroscopic failure modes. When this mode comprises fibre fracture with little matrix and interface splitting, the skewness decreased; when fibre failure was accompanied by matrix and interface splitting, its value changed slightly or even increased.