Cure cycle effect on impact resistance under elevated temperatures in carbon prepreg laminates investigated using acoustic emission

Voids in composites have been a perennial problem, since the amount of porosities is deemed to bear a strong relation with the degradation of service performance of laminates. On the other side, the effect of porosity on impact resistance of laminates is often dependant on their distribution in the material, especially with respect to the location and severity of impact damage in it. In this study, the influence of void content percentage on the residual flexural strength of CFRP laminates impacted at very low energy, in the region of 1 J, at different temperatures was investigated. Laminates were fabricated using 16 layers of Cycom 977-2 prepreg material in a [0/90/90/0]2S layup with different void contents in the range from 1 to 7% by varying cure conditions. Low velocity impact tests were conducted on three categories of laminates, defined as high pressure cured, low pressure debulked and low pressure non debulked respectively, each of these at ambient temperature (30 °C) and elevated temperatures of 50, 70 and 90 °C. Post-impact residual flexural strength of the laminates was measured by three-point bending tests followed in real time by acoustic emission (AE) monitoring. From the separation of frequency bands and of their amplitude levels, identification of the different failure modes, such as matrix cracking, delamination and fibre failure, was performed. The results indicated that in the case of very low impact energy high porosity laminates, such as non debulked ones, may possess slightly higher residual flexural strength and an enhanced delamination resistance with respect to debulked ones.