Effect of tow gaps on impact strength of thin composite laminates made by Automated Fiber Placement: Experimental and semi-analytical approaches

Automated Fiber Placement is currently being used to manufacture large and complex composite structures. However, the final composite products may include manufacturing defects such as gaps and overlaps, which may reduce the mechanical performance of the structure. The effect of these defects on the compression strength and also medium velocity impact loading with the impact energies of 15 J 50 J have been experimentally investigated earlier. However, there is still a lack of knowledge in the study of impact response of and damage propagation in composite plates at low-velocity impact loading in the presence of the manufacturing defects. In this paper, the effect of periodically induced gaps on the low-velocity impact response of the thin composite plates has been experimentally investigated. For this purpose, quasi-isotropic Carbon/Epoxy polymer composite plates have been manufactured with AFP process, including periodical patterns of gaps, and the obtained impact responses of the plates have been compared with the results of the baseline samples. The baseline sample is a similar sample that has been manufactured by hand layup technique. Furthermore, a two degree of freedom mass-spring model is also proposed to account for the effect of the manufacturing defect on the impact response of the laminates with induced defects. The model includes a non-linear damage model to account the delamination propagation during the impact process. Ultrasonic C-Scan analysis has also been performed to capture the projected delamination pattern. Results indicate that the AFP manufacturing defects can reduce the impact resistance of the composite plates by about 17% and also has an effect on the delamination area of the samples for low levels of impact energy. Microscopic observation is further performed to investigate the interaction of manufacturing defects and damage caused by impact. It is shown that delamination initiation likely occurs in the gap area.