Investigation of compression after impact (CAI) strength of sandwich panels

Sandwich panels are used in industrial fields where lightness and energy absorption capabilities are required. In order to increase their exploitation, a wide knowledge of their mechanical behaviour also in severe loading conditions is crucial. Light structures such as the one studied in the present work, sandwich panels with aluminium skins and Nomex honeycomb core, are exposed to a possible decrease of their structural integrity, resulting from a low velocity impact. In order to quantitatively describe the decrease of the sandwich mechanical performance after an impact, an experimental campaign of Compression After Impact tests (CAI) has been performed. Sandwich panel specimens have been damaged during a low velocity impact test phase, using an experimental apparatus based on a free fall mass tower. Different experimental impact energies have been tested. Damaged and undamaged specimens have been consequently tested adopting a Compression After Impact procedure. A deep analysis on the experimental data has been carried out in order to find relation between the residual strength of the panel and the possible relevant parameters. The results show a clear reduction of the residual strength of the damaged panels compared with undamaged ones. Nevertheless, a reduced dependency between the impact energy and the residual strength is found above a certain impact energy threshold. Afterwards micromechanical finite element models have been adopted to reproduce sandwich panels subjected to CAI tests. The results of the numerical model, regarding both the estimation of the buckling load and specimen’s shape, closely match the experimental data. At the end, two analytical models for intact sandwich panels have been evaluated and the thin face approximation value, a parameter above which the sandwich panels are considered as thin face sheet composites, has been modified for sandwich panels with Aluminium skins and Nomex honeycomb core. Moreover an empirical factor (K) has been defined and multiplied with analytical formulae in order to allow the analytical model to predict the critical load of impacted sandwich panels. The results of the analytical models for intact sandwich panels and for the impact ones (corrected by means of K) well fit the experimental data.