Impact Damage and Crashworthiness of Composites

MERL has instrumented drop weight impact test facilities that are used for a variety of non-standardised impact tests such as tool drop simulations on representative panels, damage tolerance studies and shock loading of components.

MERL has used these impact testing facilities for internal research studies. One such study has been to investigate the technology required to implement protective surface layers for application to critical parts of composite structures. The primary function of the surface protection layers in this study was to reduce the damage arising from a low velocity impact event such as an accidental tool drop, which often can lead to hidden or barely visible impact damage. The surface layer protects the structure below, through energy absorption and through redistribution of stress around the impact site. Other goals are to reduce the difficulties associated with damage site identification and to reduce potential problems from moisture uptake through damaged sections after an impact event has occurred.

To enable multiple impacts to be withstood, the primary effort has been to evaluate the feasibility of using an elastomeric surface layer and the feasibility of having multi-layered protection with an embedded elastomeric layer. Natural rubber, rubber blends, fabric-reinforced rubber and thermoplastic elastomers have all been examined as potential components in protective surface layers. In this study, special elastomer compounds (rubber or related compounds) and attachment methods have been developed.

The following figures illustrate the effect of a polyurethane surface layer in reducing impact damage in a GFRP panel from a 50J impact. The first sectioned panel had no surface protection and shows extensive impact induced delaminations throughout the GFRP panel. These delaminations have been highlighted using a dye penetrant method. The second panel had a polyurethane surface layer at the impact point. In this case the GFRP panel can be seen to have suffered stress whitening, but no delaminations were introduced by the 50J impact event.