Numerical investigation on the influence of tightening in bolted joints

In a bolted joint, the preload level resulting from the tightening torque represents a very important parameter governing the stresses distributions involving the joint under the real loading conditions. This paper deals with the development of a Finite Element (FE) model for the investigation of the effects of some selected preload levels on the stress-strain states affecting both bolt and plate in a single lap joint. The aim of this FE model is to support the design phase of strain gauges instrumented bolt to evaluate experimentally the rate of tensile load applied to the joint that the bolt absorbs with different preloads. The test article consists of two steel plates, a steel bolt and an aluminum nut. The results herein presented showed firstly that, without bolt preload, the tensile load applied to the joint is completely transferred to the bolt and that the load transferred to the bolt almost linearly decreases as the preload increases. Moreover, at a selected preload level, the transversal and longitudinal stresses (with respect to the load direction) increase as the tensile load increases, while the stress along the plate thickness direction decreases, reaching negative values. On the other hand, at a selected tensile load level, the transversal and longitudinal stresses as well as the stress along the thickness direction decrease as the preload level increases. Predicting the mechanical behaviour of the only bolted joint, if the same bolt model will be used to simulate the mechanical behaviour in a hybrid single-lap joint, possible imperfections of the model will have to certainly be linked to the modelling of the adhesive.