Strain instability and fracture at the surface of upset cylinders

Abstract

A refined method has been developed for measurement of the localized surface strains during axial compression of cylinders. The method has been applied to 1045 steel cylinders in various conditions to study fracture under combined stress states. Because the refined measurement method is used, perturbations are observed in the strain path which would otherwise not be apparent. These perturbations precede surface fracture in every case. It is shown that geometric similarities exist between the strain perturbations in upsetting and those in trough formation preceding fracture in stretch forming of sheet material. In addition, metallographic examination of transverse sections of the upset cylinders reveals the presence of subsurface void formations prior to fracture. It is concluded that the surface fractures in upset cylinders involve localized necking of material between inhomogeneities, and it is suggested that an accurate model of this behavior may give some understanding of the influence of material parameters on fracture.