Influence of stress relaxation on clamp-up force in hybrid composite-to-metal bolted joints

This paper presents an experimental investigation of the influence of stress relaxation on clamp-up load in hybrid composite-to-metal bolted connections. Loss of bolt clamp-up load may influence the strength and fatigue life of the connection. This study focuses on the effects of bolt retightening, use of tapered-head bolts versus protruding-head bolts, and briefly addresses environmental effects including temperature. All tests were conducted for a time period of at least three months in order to estimate primary and secondary stress relaxation effects. Bolt clamp-up load curves were fit to three different but similar equations for stress relaxation using a least squares method. The bolt retightening tests show that some of the preload in the connections can be maintained with periodic retightening of the bolts. Little difference in relaxation was observed when comparing tapered-head bolts with protruding-head bolts, given that roughly equal stress relaxation rates were observed. In a limited amount of testing, it was observed that temperature shifts caused more rapid stress relaxation rates and that post cure of the composite is essential to achieve a stable thermal response.