Formability of steel sheet in high velocity impact

Abstract

High velocity deformation can be quite effective in increasing the forming limits of metal sheets as well as effectively treating some other common metal forming problems, such as wrinkling and distortion. This study examines the high velocity formability of cold rolled sheet steel as developed in impact with a curved punch. To study forming over large areas, a steel sheet sample was electromagnetically launched, at a shaped punch at velocities of 50–220 m/s, using a flat spiral electromagnetic coil and an aluminum driver sheet. Estimates of impact velocity were obtained with the help of a high-speed camera. It was observed that the impact velocity of the samples increased nearly linearly with the energy of launch. The failure strains of all the steels were dramatically increased beyond those obtained in tensile tests as expected from the forming limit diagram. The forming limits of steels with both very low and high quasi-static ductility were similar in high velocity forming. Formability seemed to depend largely on local boundary conditions as dictated by the punch/tool geometry used. The strain distribution obtained depended on the shape of the punch used for impact. Thus it appears that in high velocity impact, the quasi-static ductility of the material is not of primary importance to the material's formability. At high velocities, high strains to failure can be obtained with steels having very low quasi-static ductilities. Based on these observations, one may be able to develop forming operations that take advantage of this dramatic improvement in formability. This could enable the use of high-strength low-cost materials in a number of complex components.