Effect of a Strain-Hardening Rate at Uniform Elongation on Sheared Edge Stretching

Edge failure during stretching of sheared edges limits the use of sheet steels in a number of product applications. The shearing process causes a highly strained region adjacent to the shear face, called the shear-affected zone. In the present study, the strain-hardening rate at uniform elongation, Z, is used as an empirical measure of cohesive strength at the interface of the various phases in steel microstructures. The higher the value of Z, the lower the macro strain when voids begin to form that lead to decohesion of the interface and subsequent failure. The data from four different studies are used to show that the true circumferential strain at failure in a hole expansion is a direct function of Z for most microstructural conditions. Sheet steels that exhibit better performance than that which would be expected for their Z values have one or more of the following characteristics—an increase in ferrite strength, lower carbon martensite in DP steels, or TRIP steels. A hot-rolled ferrite/pearlite microstructure is the only case of decreased true circumferential strain at failure for a given value of Z.