Correlation between the acoustic emission and damage modes during splitting of carbon fibre reinforced poly-ether-ether-ketone composite

Abstract

Failure of composite materials often involves more than one damage mode, such as matrix cracking, fibre breaking, fracture of the fibre-matrix interface, delamination and fibre pull-out. Splitting of a thin ply unidirectional reinforced material along its fibre direction [1] is possibly the most simple damage mode to be found in a real composite. The composite will fail by a single damage mode, i.e. matrix cracking, as long as the reinforcing fibres are perfectly aligned and the crack propagation is confined in a direction at zero degrees to the fibre axis. Slight deviation of the crack path or of the fibre alignment will cause the crack front to intercept the fibre-matrix interface. If the interface is weak, fracture of the fibre-matrix interface will occur as a second damage mode in the failure process. The crack front will then continue to propagate within the fibre-matrix interface or kink out back into the matrix. This phenomenon will cause matrix cracking and fracture of the fibre-matrix interface to alternately dominate the failure process. The crack front may also kink out from the fibre-matrix interface into the adjacent fibre to cause fibre breaking. In this case the failure process will be governed by a combination of three damage modes, i.e. matrix cracking, fibre-matrix fracture and fibre breaking. If the fibre-matrix interface is strong enough, the crack front which intercepted the interface will continue to propagate into the fibre to cause fibre breaking. Thus failure of the composite will be contributed to by combination modes of matrix cracking and fibre breaking. Understanding the failure process of the composites is necessary for designing and working with these materials. More important than that, monitoring the integrity of a composite under service will only be possible when the above-mentioned damage modes are fully understood. Acoustic emission is a promising technique which is receiving increasing attention for monitoring the integrity of components as well as investigating the failure mechanism of the materials, The ability of the technique to provide information on damage progression in real time gives it advantages [2] over other non-destructive techniques for continuous monitoring. It is a common aim among researchers and engineers to establish a correlation between damage modes and the characteristics of their acoustic emission signals. Peak amplitude is the most frequent acoustic emission parameter which has been manipulated [3-5] in order to distinguish the