Impact of thick CFRP laminates: the effect of impact velocity

Carbon fibre reinforced plastic laminates of the order of 24mm 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 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 quasi-static impact 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 quasi-statically impacted specimen. Two further finite element models, modelling the experiments more closely, found that the dynamically impacted panel experienced a higher stress state. This indicates that the reduction in strength comes mainly from inertial stiffening and a stress localisation effect.