Tensile Fracture of Drawn Copper and Mild Steel

Annealed OFHC copper and SAE 1018 steel were reduced by multipass drawing from diameters of 25.4 mm (and smaller) to 11.8 mm. A comparison was made of the experimental draw stresses and those calculated by Sachs’ and Avitzur’s equations and fair agreement exists. The drawn bars were subsequently reduced in diameter by 10 percent in order to provide gage sections and then were pulled in tension to fracture. It was found that in multipass draws some work softening occurs. The oxygen-free copper showed indications that fracture was initiated at the center of the specimen. The internal fracture grew to the near shape of a sphere and separation did not occur until the load had almost decreased to zero. The mild steel apparently also fractured in the center, but complete separation took place immediately after the tensile stress reached its maximum. The fracture theories of Latham and Cockcroft, as well as that of Chen and Kobayashi, were examined and it was found that fair agreement existed. It was also found that for these particular tests, the maximum tensile stress criterion gave more convincing results.