Predicting short beam shear strength reduction in carbon/epoxy laminates containing voids

The presence of voids in composite materials reduces the material’s strength, particularly matrix-dominated properties such as short beam shear (SBS) strength. Until now there has been no satisfactory way to reliably predict the effect of voids on SBS strength. Instead, there has been a reliance on using average void content combined with extensive testing to assess the linear reduction of strength with increasing void content to determine a 'knock-down' factor. Here, the reduced cross-sectional area is used to predict SBS strength of laminates containing voids. A detailed characterisation of the internal void location and morphology for numerous samples was obtained through X-ray CT scanning, prior to mechanical testing. The ‘critical plane’ in each sample, i.e. the length–width plane with the maximum cross-sectional area of voids was located and used to predict SBS strength. To additionally account for non-uniformity in the shear stress distribution, a stress scaling factor (SSF) is introduced.