We experimentally investigated the effect of the mass of a drop hammer in impact punching with polyvinyl alcohol (PVA) gel as the working medium. There is an optimal mass of the drop hammer. For constant initial energy, if the mass of the drop hammer is small, the impulse (change in momentum) becomes small, and punching cannot be accomplished. On the other hand, if the mass of the drop hammer is large, the speed of the pressurizing ram will be small for the same kinetic energy, the flow pressure of the PVA gel will not be sufficiently raised, and again, punching cannot be accomplished.
In this study, a new peen-forming technique by means of vibration peening using a rectangular solid pin is developed. It is possible to produce a smaller x-direction (short side of the pin) curvature radius Rx than the z-direction (long side of the pin) curvature radius Rz . Vibration peening, which projects the pin into the specimen using the reciprocating motion of the electric hammer, is applied. In the experiment, Rz / Rx increases as the pin-tip radius r decreases. The motion of the pin is analyzed by the dynamic explicit finite-element method (FEM). The pin reciprocates between the hammer and the specimen and collides with the specimen 17 times in 0.2 s. The number of collisions agrees with the experimental result of 10 to 18 times in 0.2 s. The plastic strain distributions are determined using the maximum velocity of the pin. Peen forming is analyzed by the following method. In Step 1, the initial velocity is set for multiple pins, and plastic strain distributions generated by collisions are analyzed by the dynamic explicit FEM. In Step 2, plastic strain distributions analyzed in Step 1 are input to the specimen to analyze the deformation by the static implicit FEM. The analysis results indicating the increase in Rz / Rx with decrease in r agree with the experimental results.
The aim of this study was to examine the effect of accumulated equivalent plastic strain on the ductile fracture criterion. Spheroidized medium-carbon steel JIS-S55C was adopted as the test material. An investigation into the ductile crack initiation behavior of round-bar tensile specimens with/without circumferential notches was carried out. Tensile prestrain was applied by cold drawing. The stress triaxiality and accumulated equivalent plastic strain were calculated by the finite element method (FEM). The ductile fracture limit of tensile prestrained steel was expressed as a function of the stress triaxiality and accumulated equivalent plastic strain. The validity of the ductile fracture limit was confirmed by the prediction of chevron crack generation in multipass drawing.
Automobile body parts produced by the hot stamping process show excellent shape fixability with an ultra high tensile strength of 1.5 GPa. We investigated the effect of flow stress during forming and transformation after forming on shape fixability in the hot stamping process. Referring to the experimental and FEM coupled simulation results, we discussed the mechanism behind the excellent shape fixability in the hot stamping process. Steel of 0.2% C was used for hot stamping in this study. IF steel and SUS 304, which have different transformation behaviors from that of 0.2% C steel, were used for comparison. The temperature of forming start was varied from 400 to 800 °C. After hot stamping, the shape fixability of the parts was evaluated. The result showed that the excellent fixability in hot stamping was caused by not only low flow stress during stamping, but also martensitic transformation. When martensitic transformation occurs after stamping, excellent shape fixability is obtained without any relation to the flow stress during forming. Accordingly, it was concluded that the stress introduced by hot stamping is relaxed and becomes uniform throughout the thickness during martensitic transformation. In addition, the application of tensile stress due to thermal contraction also contributes to the decrease in spring back.