A Fracture Criterion for Cold Forming

Abstract

Fracture under combined stresses has been studied with the objective of evaluating existing fracture criteria and providing data for the formulation of new criteria. Upsetting, bending, rolling, and plane strain tests were used for the fracture study. By utilizing various combinations of test specimen height-to-diameter ratio and die contact surface friction conditions, a wide range of stress and strain ratios can be developed in the test specimens. The procedure involves measurement of the tensile and compressive strains at the fracture site. Two grades of carbon steel and one grade of stainless steel were tested in upsetting, bending, rolling, and plane strain deformation. The results are expressed as a forming limit in a total compressive strain versus total tensile strain diagram. The data are also consistent with the Cockcroft fracture criterion.