Prying Action in a Bolted Cantilever Analyzed as a Receding Contact Problem

Abstract

Bolts are often used to hold flat components together while transmitting a tensile (separating) load. When the components are relatively rigid and the transmitted load exceeds the combined bolt preloads, complete separation of the components can occur, resulting in the load simply being shared among the bolts. However when the components are flexible, the tensile load may cause elastic partial separation, in which case there is a concentrated contact force at the separation boundary. This prying force adds significantly to the load carried by the bolts. It therefore creates an unexpected risk of bolt yielding accompanied by a deleterious tension loss upon unloading. In the case of repeated loading it can cause bolt fatigue. In this paper, we perform a novel receding contact beam analysis of prying action in the bolted joint of a cantilever-loaded beam/plate. The flexural deformation of the plate induces prying forces that result in a substantial increase in the bolt force well above that which would be predicted by the simple beam reaction force. The design parameters are the joint dimensions, material properties, the bolt preload, and the applied force. It is modeled using two beam theories (Euler–Bernoulli and Timoshenko) to provide insight into the roles of the main parameters. A comparison with a finite-element analysis of the three-dimensional elasticity problem shows a reduction in the bolt force of 10%–26% compared with the beam theories, which are more conservative. This difference is consistent with the idea that a one-dimensional beam model is artificially stiffened compared to a full three-dimensional analysis.