A fracture criterion of aluminum alloys in hot metal forming

Aluminum alloys have been tested under uniaxial tensile conditions at various temperatures and strain rates to determine the strain to fracture. This strain depends on both temperature and strain rate. It has been observed that the strain to fracture attains a maximum for some tests performed at a constant strain rate. The regime where fracture occurs before this maximum is reached is referred to as regime a and the other regime as regime b. In the case of regime a, it is demonstrated that the strain to fracture may be assumed to be a function of a single argument, the Zener-Holloman parameter. This hypothesis is confirmed by torsion tests carried out by other researchers. In the case of regime b, however, the strain to fracture does not follow this rule. Based on the experimental results, an extension of a fracture criterion applicable to cold metal forming processes is proposed to account for the effects of temperature and strain rate. A special form of this criterion for free surface fracture and its application to fracture of a circular hollow cylinder are discussed.