Impact of thick CFRP laminates: the effect of impact velocity

Carbon fibre reinforced polymer laminates of the order of 24 mm thick are being considered for the wing skins of large civil aircraft. These skins are susceptible to impact damage, which has implications on their long-term structural integrity. This can lead to conservative design. Impacts are often classified by the incident kinetic energy of the impactor alone, but there is some evidence that the impact velocity can have an important effect on the response and damage of a panel. Two panel geometries have been studied, each with 13 different impacts of constant impact energy using explicit finite element (FE) simulations. There is a significant effect observed due to different impact velocities on the panel response. Most importantly, the maximum stress in the panel was found to vary with impact velocity. Two experiments were carried out as a preliminary investigation into this effect. A simulated impact on a quasi-static test machine and a dynamic drop weight impact were carried out, and the residual tensile strength found in each case. The dynamically impacted specimen was found to have a 20% lower strength than the specimen with a simulated impact using a quasi-static test machine. Two further FE models, modelling the experiments more closely, found that the dynamically impacted panel experienced a higher stress state. This indicates that the reduction in tensile strength is due indirectly to inertial stiffening and a stress localisation effect.