Finite Element Simulation of Bolted Joints and Magnesium Bolt-Load Retention Behaviour

Engineering stress analysis of bolt joints requires knowledge of the stiffness of the joints. One convenient way to estimate stiffness of joints is to use effective stress area equations because only part of the joint surface carries load. In this work, the existing effective stress area equations (developed about 50 years ago) were assessed using a washer/joint contact finite-element (FE) model. It was found that these equations may underestimate the effective area and the joint stiffness by as much as 50%.

Typical bolt-load retention (BLR) test fixtures were modeled using a commercial FE code. In the simulation, the creep stress-strain behaviour of a magnesium alloy was described by a three-parameter power-law relation that was fitted from the compressive creep test results of a high-pressure die-casting (HPDC) Mg alloy (AM50). The bolt-load vs. time relationships from FE simulation were compared with those obtained experimentally, and they showed good agreement. The experimentally verified FE model was then used to test the sensitivity of the geometry of the cylindrical BLR sample and test fixture on BLR behaviour. In this way, the applicability of the use of existing effective stress area equations to calculate the initial nominal stress in a bolt joint was studied.