A fracture criterion for sheet metals subjected to draw-bending is investigated using the concept of the forming limit stress criterion. An experimental apparatus that is capable of draw-bending sheet specimens with forming speeds （approximately 100 mm・s-1 ） comparable to those of real press forming machines is designed and built. The test materials are an ultralow-carbon steel sheet and a 590MPa high-strength steel sheet. Specimens undergo bending-unbending under tension when passing over a die profile. The magnitudes of true stress σDB at which the stretch-drawn specimens fractured are precisely determined from the measured data of the drawing force and the cross-sectional area of the specimen after fracture. Moreover, multiaxial tube expansion tests are performed to measure the forming limit stresses σPT of the test materials under plane-strain tension. It is found that σDB is larger than σPT by 6-18%. Therefore, it is concluded that the forming limit stress criterion is effective for the fracture prediction in draw-bending.
In stretch flange forming, the sheet-edge quality is important since it affects the appearance of fractures at the edge. In this study, the effect of the edge trimming conditions of the blank sheet on both the sheet-edge quality and flange-up formability is investigated. It is found that in the case of edge trimming of a sheet with high tensile strength and poor ultimate ductility, a large burr generates at the edge, which easily causes fractures, when the shearing clearance between the upper and lower dies is large. This tendency in trimming for flange-up forming is not observed in hole piercing for hole expansion. It is clarified that the burr is caused by excessive bending behavior under large shearing clearance conditions, and that the forming limit in flange-up forming is independent of the clearance unless a large burr generates. Then, a new shearing die structure, which suppresses excessive bending, is developed in order to avoid the creation of large burrs, and consequently, high formability is attained.
The isotropic hardening model for an elastic-plastic material have often been used for simulation of cold forging processes, by which, however, it is difficult to represent the Bauschinger effect properly. On the other hand, considering the Bauschinger effect is essential for more precise simulation of the forming process as well as the strength of the forged part without heat treatment. One example is a cold-forged bolt of stainless steel. In the present paper, for two multistage cold forming processes of non-heat-treated stainless steel bolts, the simulations of the forming process and the strength were performed by FE analysis using the YU model that is one of the kinematic hardening models being able to express the Bauschinger effect. As a result, the Bauschinger effect during the forming process can be expressed properly. Moreover, by carrying out the analysis on the tensile test of the forged bolt, the difference in strength by between the isotropic and the kinematic hardening models can be estimated clearly. Furthermore, the effect of the forming process on the strength of the forged bolt can also be evaluated appropriately.
The fold-forging method, by which a stepped plate is folded into a crank throw shape, is one of the forming process of the built-up crank throw. In the bending process by the fold-forging method, surface defects occur occasionally. When these defects occur, they must be removed. Therefore, extra machining allowance is needed for clearing the surface defects, resulting in low material utilization. In order to prevent surface defects in the bending process, it is important to understand the generation behavior of surface defects, and thus, it was investigated. As a result of the bending test, it was confirmed that surface defects are generated by material overlap. In addition, it was clarified that the overlap originated from a minor surface defect near the bending point. In order to prevent the overlap, the actual bending process was improved. As a result, the surface defect dose not occur and material utilization is improved.