Yield Phenomena of Post Biaxially Strained Metals

In sequential of multistep forming operations, the material which has already undergone permanent deformation is expected to behave differently in each stage of deformation than the as-received material. The effect of such changes on (i) the materials ability to distribute strain uniformly, (ii) the strain limits, and (iii) the residual ductility have been studied for a mild steel and for a dual-phase steel. Steels of both materials were balanced biaxially prestrained for different levels of strains and then uniaxial properties and forming limits were determined. Both materials were found to suffer a rapid loss of uniform strain and residual ductility. Forming limits of both steels were found to decrease with prestrain except for stretch-type deformation in the sheet plane. The prestrain produces strain softening1 in mild steel; whereas, this is observed only in 90 deg to the rolling direction in dual-phase steel. In balanced biaxially prestrained (5 percent major and minor strain) dual-phase steel, during hemispherical punch stretching, necking mostly occurs in the rolling direction as opposed to the transverse direction in the as-received material. Such effect can be explained in terms of loss of stability which is observed in uniaxial property of this material.