Shearing is a commonly used method of cutting materials for former from bars or wires. However, the cut face shape of a material after shearing is worse than that obtained with machine, saw, or laser cutting because droop and burr are formed at the edge of the cut face. It is necessary to alter the cut face shape after shearing for use in former. The cut face of a ductile material fractured by torsion is perpendicular to the longitudinal direction of bars or wires without droop or burr. In this research, we developed a new shearing method in which shearing is combined with twisting to achieve a short cutting time with a length high accuracy and a cut surface without droop or burr. A prototype machine for shearing a material with torsion was developed. Low-carbon steel bars (diameter: 1.96 mm) were used for the experiment. The specimens were annealed at 1123 K for 2 h. The specimens were sheared after being twisted at several torsion angles. As a result, the height of droop and burr decreased in the case of shearing with torsion, as compared with that in the case of only shearing.
Shearing is a commonly used method of cutting materials for former from bars or wires. However, the cut face shape of a material after shearing is poor because droop is formed at the edges of the cut face. It is necessary to alter the cut face shape after shearing for use in former. In this research, we developed a new shearing method in which shearing is combined with twisting to achieve a short cutting time with a length high accuracy and a cut surface without droop. Annealed low-carbon steel bars (diameter: 1.96 mm) were used for the experiment. The specimens were twisted after a static shear force was applied to the bar. As a result, the droop height decreased markedly. In addition, to restrict the work hardening area by applying torsion, the bar was twisted after shear stress, which was slightly less than the yield stress, was generated at the clearance with the loading of a large static shear force. The equivalent stress at clearance exceeded the yield stress, when the torsion was small because the shear stress was large. Therefore, the twisted area became narrow, and work hardening was restricted.
The localization of plastic deformation has been discussed as “stationary discontinuity” characterized by a vanishing velocity of an acceleration wave. Therefore, on the basis of the coincidence of the onset strains between localized deformation and acceleration waves of vanishing velocity, diagrams of diffuse necking, localized necking and forming limit are analyzed by applying the proposed acoustic tensor, which is based on the generalized Christoffel tensor derived by one of the authors, and by solving the cutoff conditions of a quasi-longitudinal wave to determine the onset strains of deformation localization. The purpose of the present study is to analyze the effect of superimposed hydrostatic pressure on localized deformation using the proposed theory of ultrasonic wave velocities propagating in plastically deformed solids. From the analytical results, the catastrophic ductility increase is caused by the mode change of localized deformation beyond the critical hydrostatic pressure.
In this study, the stretch flanging fracture of a C-S test piece, which has a curved part (C-part) connected to a straight part (S-part) fabricated using high-tensile-strength steel sheet, is investigated. Both experimental and FEM analyses show several strain concentrations on the edge of the flange, and deformations due to such concentrations can be explained by the consecutive occurrence of the braking effect of the stress gradient. The first concentration of strain, which triggers several strain concentrations, is induced by a nonuniform stress. The cause of the nonuniform stress is considered to be the variation in flanging-up angle during the stamping process. To verify this, a C-S test piece with an additional part (obtained by adjusting the edge contours of the curved part) is also investigated. We concluded that pieces having an additional linear and a serrated edge located outside the additional part are optimal for high formability.
Bent sheet metals are now used widely because their welded part that can cause the debasement of product strength can be reduced by a bending process. Bent plates are contained in many products, and their use is rapidly increasing. However, such plates tend to undergo stronger defective deformation than thin sheet metals, and thin and thick sheet metals differ in accuracy and shape of their products in the bending process. As for the accuracy of bending process and defective deformation, we need to rely on the interpretation by technicians to determine the operational conditions. Therefore, it is necessary to clarify defective deformation phenomena in the bending process of a thick plate. In this study, the effects of the sizes of the punch and die on spring-back angle were examined. By reducing die open width it is possible to control spring-back angle however it is also necessary to consider the crack of the plate. Meanwhile, punch radius hardly affects spring-back angle. Note that it is necessary to consider that damage remains on the inside surface of the plate as caused by the punch and the shape of the plate on central portion in the longitudinal direction. As a result, an optimum die open width is proposed for the suppression of defective deformation in the thick plate bending process of A5083.
Welding is widely used for constructing of many structures. Residual stress is generated near the bead because metal near the bead is heated. Tensile residual stress degrades fatigue strength. Several methods of reducing of residual stress are developed. Heat treatment and shot peening are practically used. These methods are time-consuming and require special devices. In this study, a new method of reducing residual stress is proposed. In this method, vibrations are used during welding, namely, ultrasonic vibration and low-frequency vibration. The effectiveness of the method is examined experimentally. It is concluded that tensile residual stress is reduced when ultrasonic vibration or low-frequency vibration is used. When ultrasonic vibration and low-frequency vibration are used simultaneously, the reduction rate of residual stress is large. Results of the experiment are examined by simulation qualitatively. The yield stress immediately after welding is very low because the temperature of the metal near the bead is very high. Then, an analytical model considering the yield of metal is proposed. From the simulation method, results of the experiment are demonstrated qualitatively.