High rate testing of composite fastener joints with and without clamp-up

Abstract

The NASA Advanced Composites Consortium (ACC) High Energy Dynamic Impact (HEDI) group has been investigating experimentally the effects of testing speed and fastener clamp-up on the behavior of single lap shear composite fastener joints. Two fastener types, a titanium pin and a Hi-Lok® fastener, were used to simulate the two conditions of clamp-up force (zero and non-zero). The tests were performed at the rates of 21×10-6 m/s, 2.54 m/s, 7.62 m/s, and 12.7 m/s. Strain gages and digital image correlation (DIC) were used to measure deformations, displacements, and strain fields. This paper elaborates results from tests conducted at 12.7 m/s for pin and Hi-Lok® fastener types. While the strains at bearing failure initiation appeared to be slightly rate sensitive for test articles employing pin joints, no trend could be associated with that for the fastener joints. The strain levels corresponding to bearing failure initiation were much higher for test articles using Hi-Lok® joints (with washers), because of the clamp-up. The strain rates measured at a remote location for the pin and the Hi-Lok® joint approached 200 s-1 and 130 s-1 respectively, at 12.7 m/s test speed. For the pin joint test articles, the peak radial strain rate experienced by the CFRP laminates underneath the hole, as close as possible to the bottom edge (bearing dominated region), was significantly higher and reached around 2500 s-1 on average, also at 12.7 m/s. Test articles employing Hi-Lok® exhibited higher load bearing capacity at all loading rates.