Springback prediction during press forming has become more important in the recent background that high-strength steels have been applied to white body parts. The springback prediction accuracy depends on the estimation of recovery strain after die release. It is well known that unloading behavior after pre-strain shows a nonlinear stress-strain relationship, so a material model considering the nonlinear elastoplastic behavior is necessary for accurate springback predictions. In this study, the nonlinear behaviors of high-strength steel sheets under uniaxial and biaxial stress states were measured. A new material model that takes the nonlinear behavior into consideration was proposed, and implemented into finite element method (FEM) software. The press-forming tests of curved hat-shaped parts and the corresponding springback analyses were performed and the springback angles were evaluated. The springback predictions made using the proposed model showed good agreement with experimental results.
The ratio of tooling feature size to minimum thickness becomes small in microscale products and significantly affects the deformation behavior in micro-sheet forming. In this study, the effect of this relative tooling feature size on the drawing characteristics, and effects that can improve the drawability, such as the friction holding effect, hydrodynamic lubrication effect and compression effect of blank edge by radial pressure, in micro-hydromechanical deep drawing (MHDD) are investigated by finite element method (FEM) simulation. The results show that the microdrawing characteristics in MHDD can be improved by applying counterpressure. However, the required fluid pressures for friction holding and hydrodynamic lubrication effects increase when the ratios of punch diameter and/or die shoulder radius to thickness decrease, although the compression effect of blank edge by radial pressure is independent of the relative tooling feature size. It is found that a high fluid pressure is helpful in improving the microdrawing characteristics at the microscale with a small relative tooling feature size.
One of the causes of quenching cracks is regarded to be the stress that occurs owing to quenching. Quenching stress is affected by the material shape. In order to prevent the quenching crack in a polymer quenching process, it is important to understand the relationship between the material shape and the quenching stress. Therefore, the relationship between the flange thickness of a flange shaft and the quenching stress was investigated. As a result of quenching stress analysis, it was confirmed that the radial stress increased with decreasing flange thickness. Then, a quenching test utilizing test material corresponding to an actual product size was conducted and the residual stress was measured after quenching. The radial residual stress increased with decreasing flange thickness, and this tendency corresponds to the result of the quenching stress analysis. In addition, the generation mechanism of the quenching stress was considered using the results of quenching stress analysis. As a result, the causes of quenching cracks have been found to be not only time lags of the martensitic transformation between the surface and the inside along the axial direction, but also the difference in the transformation behavior between the flange part and the shaft part.
Press forming for the mass production of carbon-fiber-reinforced thermoplastic (CFRTP) sheets has recently been studied intensively in order to reduce the weight of transport machinery and to improve collision safety. The authorsexamined the possibility and practicality of the fine punching of CFRTP sheets using a press machine employed for press forming. The following results were obtained. (1) In the shave punching of CFRTP sheets, materials were separated by cutting them regardless of the relatively large shaving allowance, resulting in a smooth cut surface. Moreover, shave punching was also successfully applied to very thick sheets, with the increase in shaving load being extremely small even when the sheet thickness was increased. (2) In vibro-punching, a conventional punching die was attached to a numerical control (NC) servo press, and a vibrating punch was induced to cut into the materials during the process. With this vibration motion, fine punching became easier to perform than conventional vibro-punching．
In this study, the effects of pre-straining and surface friction on fatigue properties have been investigated. Three grades of 590-MPa-class steel sheets that have different roughnesses and yield ratios were used. At first, the steel sheets were subjected to 10% tension and 10% compression as a pre-straining process. Then, the surfaces of the steel sheets were burnished with a contact force of 19.1 kN. After that, plane-bending fatigue tests were carried out using these specimens. The evaluation by this fatigue test has revealed the following. A low roughness of the steel surface diminishes the effects of pre-straining and surface friction on the fatigue properties. Fatigue strength increases after applying surface friction in the case of complete smoothing of the initial roughness. Otherwise, fatigue strength decreases in the case of residual concavities after applying surface friction. These residual concavities are caused by the work-hardened surface after the tension-compression process. For the fatigue deterioration due to the presence of the residual concavities, finite element simulations have implied that tensile residual stress is generated at the residual-concavity tips after applying surface friction. This residual stress probably deteriorates the fatigue strength.