Quantitative non-destructive characterization of damage in graphite-epoxy composites

We investigated the effect of controlled damage on the residual strength, failure mechanism and acoustic emission (AE) activity of graphite epoxy composites. The controlled damage was introduced by indenting unidirectional tensile specimens using a hardness machine with a diamond indentor. The specimens were loaded parallel to the fiber direction. The damage progression mechanisms operating upon subsequent loading and the failure modes were found to be different for sound and indented specimens. The difference in mechanical behavior affected also the AE, as displayed by the statistical moments of the peak amplitude distribution. Specifically, the variation of moment values with stress level were found to be different for the two groups. This difference can be used to sort sound from defective material during loading. In addition, this approach can in future be used as a means to ratify the soundness of structures, if a reliable database can be generated.