The significance of indentation in the inspection of carbon fibre-reinforced plastic panels damaged by low-velocity impact

Low velocity impact tests were carried out at different energy levels on three types of T400/934 angle-ply laminates, using an instrumented drop weight apparatus. After impact, the indentation depth and the residual tensile strength were measured as a function of impact energy. Using the results obtained and experimental data available in the literature, an indentation law, allowing for the prediction of the impact energy from the depth of indentation, was assessed. The indentation law found seems to have a quite general applicability, being scarcely affected by the fibre type and orientations, and matrix type. A previous formula, modelling the residual tensile strength decay as a function of impact energy, was proven capable to correctly predict the residual strength results for all the laminates tested. Combining the indentation model and the residual strength model, a closed form model, explicitly correlating the residual strength and the indentation depth, was obtained. The theoretical predictions were in very good agreement with the experimental results. It is shown that the new model assumes a simple analytical form for a given laminate, permitting the material characterisation by a minimum of test data.