Residual Shank Torque of Bolted Joints: A Numerical Investigation

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.

SPR Characteristics Curve and Distribution of Residual Stress in Self-Piercing Riveted Joints of Steel Sheets

Neutron diffraction was used to describe the residual stress distributions in self-piercing riveted (SPR) joints. The sheet material displayed a compressive residual stress near the joint, and the stress gradually became tensile in the sheet material far away from the joint. The stress in the rivet leg was lower in the thick joint of the softer steel sheet than in the thin joint of the harder steel sheet. This lower magnitude was attributed to the lower force gradient during the rivet flaring stage of the SPR process curve. This study shows how the residual stress results may be related to the physical occurrences that happened during joining, using the characteristics curve. The study also shows that neutron diffraction technique enabled a crack in the rivet tip to be detected which was not apparent from a cross-section.

Effects of Material Property and Structural Design on the Stress Reduction of the Joints in Electronics Devices

Electronics devices consist of silicon chips, copper leads, substrates and other parts which are jointed to each other with solder, conductive adhesive or other materials. Each coefficient of thermal expansion is different and it causes strain concentrations and cracks. We analytically investigated the stress reduction structure at the edge of the joints such as Sn-Ag-Cu solder or Cu/Sn alloy between the silicon chip and copper lead. At first, we examined the influence of the joint thickness and fillet at the joint edge on the stress. In the joint without fillet, the stress at the end of the joint increased depending on the thickness of the joint. The fillet of the joint increased the stress of the Cu/Sn alloy joint and the stress was increased depending on the thickness, though the fillet decreased the stress of the solder joint. We suggested the copper lead with slits to reduce the force of constraint. We compared the effects of the structure parameters of the slits on the stress reduction. The height was a more effective parameter than the width and the pitch. In the case of solder joint, the slits of the copper lead reduce the stress more effective in the thick joint than the thin joint. However, in the case of Cu3Sn joints, the slits reduced the stress more effectively in the thin joint than thick joint.