Abstract

During operation, the overall stress state of a screw is a function of the direct stress generated by the axial preload, and the external loads, plus the shear stress due to the residual shank torque. All in all, the higher the residual torque, the lower the direct stress the screw can withstand prior to yielding. The residual shank torque stems from the tightening torque, part of which flows through the shank and it is, only partially, released after wrench removal, thanks to springback phenomena involving both the screw and the joined elements. This phenomenon has been tackled in a previous experimental and analytical work by the authors, which investigated the effect of the stiffness and frictional parameters of the joint on the amount of residual shank torque. Such research was based on a sleevelike specimen, and, in fact, the results were strictly applicable to the case of slender cylindrical joint. The present contribution aims at assessing the effect of the same parameters on the residual shank torque, namely: the ratio between the torsional stiffness of the screw and of the plates, the friction coefficients (underhead and thread). Nonetheless, thanks to a novel three-dimensional finite element model, the parameters have been varied in a much wider range, in order to analyze all the likely operating conditions. The model is capable of predicting the residual shank torque for both the cases of slender and thick joint (plate-like joint). The model has been developed in the Ansys R17 software, but the methodology can be extended to other codes with minimal changes.

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