Effect of friction on a crashworthiness test of flat composite plates

The diffusion of fiber reinforced plastics in crashworthiness applications is continuously growing thanks to the excellent balance between high mechanical performances and low weight, resulting in most cases in a Specific Energy Absorption (SEA) of composite structures higher than that of the corresponding metallic structures. In this paper, a new fixture to test composite plates applying an in-plane load has been used to investigate the effect of the impact velocity and of the friction caused by the fixture on the SEA of carbon fiber reinforced epoxy plates. The tests have been carried out using a drop tower testing machine and the effect of the friction has been studied varying the clamping force given by the fixture. Splaying is the main failure mechanism found in the specimens during the tests; SEA values (43.6 kJ/kg in average) increase with the clamping force due to the higher friction level induced by higher clamping force; impact velocity does not significantly influence the results. To avoid an overestimation of the SEA due to the excessive friction force (+5.6% when the clamping force increases from 0.8 kN to 8 kN), a Polytetrafluoroethylene (PTFE) coating has been applied to the anti-buckling supports to reduce the friction. The effect of this modification has been studied by carrying out a new test in which the specimen slides between the anti-buckling supports with a given clamping force. A significant reduction (-48% with same clamping force) of the friction force is obtained when the lubricant is applied.