Porosity effect on residual flexural strength following low energy impact of carbon fibre composites

Studies of the combined effects of the presence of porosity (as it may result from partially effective cure cycles) and of low-energy impact damage on the residual properties of CFRP laminates have led so far to controversial results. In particular, it is not clear from the literature whether the presence of voids would blunt crack propagation following impact or rather would promote damage development. These effects would respectively either increase or reduce post-impact residual strength, relative to that of the laminate with virtually no voids, as the result of an optimal manufacturing procedure. With this in mind, different cure cycles have been applied to produce carbon fibre-reinforced polymer (CFRP) composites with various levels of void content, which were subjected to low energy impact damage (3, 4.5 and 6 J) and then to post-impact flexural strength measurement. Damage assessment using micro-focus computed tomography (μCT) was used to complement traditional ultrasonic C-scans, which proved ineffective on the high-porosity samples. Three cure-cycles were investigated: one which led to high porosity (average void content 4 vol%) and two conventional low-porosity cure cycles, only one of which included a post-cure cycle. This study has found that, despite a lower initial flexural strength, higher residual flexural strength was retained after impact in the high-porosity material than in the low-porosity one. This is explained by the lower extent of impact damage observed in the high porosity material, where voids had the effect of suppressing delamination propagation.