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Most automobile body members are fabricated from formed steel sheets joined by spot welding, and these welded joints typically tend to cause fatigue failure owing to local stress concentrations. In this study, fatigue tests were conducted under cyclic torsion loads using spot-welded rectangular section members made of 590 MPa grade dual-phase high-strength steel to investigate the springback effect on fatigue properties. These members were assembled using the rectangular sections that were formed under different blank holder forces, as this would change the amount of springback. Furthermore, the prediction of the fatigue life of these members was attempted through springback analysis, an elastic analysis together with a method of fatigue life prediction for spot welds using nominal structural stress. In this prediction, residual stress caused by springback was regarded as the mean stress. Concerning the experimental results, every fatigue failure was initiated close to the spot weld edge located at the center of the tested members, and fatigue life decreased with springback amount increasing. In addition, the predicted results were generally in agreement with the experimental results in terms of the effect of springback.

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The study deals with the development of a diamond burnishing tool with a hydraulic control system for discontinuous workpiece surface finishing. Burnishing can be regarded as an alternative surface finishing technology. However, applying a commercial burnishing tool to a discontinuous surface is almost impossible owing to its configuration. We developed a hydrostatic burnishing tooling system and demonstrated the capability of the system developed. To apply burnishing to an interrupted surface, we tried to develop a hydrostatic burnishing tooling system. The system developed permits a minimum displacement of the pushed depth as well as in-process burnishing force monitoring. Some experimental results demonstrated the capability of the hydraulic burnishing tool to be applied to an interrupted surface. Also we discussed in this report is the relationship between compression depth and surface texture, determined by comparing the hydraulic burnishing tool developed with a commercial one.

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A layered workpiece consisting of a surface layer (sleeve) of the high-carbon steel S45C and an inner layer (core) of the low-carbon steel S15C was used to study the conditions for gear extrusion. The specifications of the gear examined are as follows: module m=1.25 - 3.0 mm, number of teeth Z=18 - 11, and whole depth h=2.25 module. The conditions for obtaining a layered gear with a carbon content that allows the hardening of only the surface layer by high-frequency hardening were investigated in terms of sleeve thickness, reduction in area, and other factors. The result showed that such a layered gear could be produced, and that the punch pressure required was far below the strength of the punch material. Because high-frequency hardening allows processing within a short time period, it appears possible to produce such gears in a production line including heat treatment. However, because the sleeve and core were not jointed, the extrusion of the core and a low deformation resistance in ahead of the sleeve were observed.

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