Sheet metal forming-limits for complex strain paths

Abstract The three non-quadratic yield criteria for a material with in-plane isotropy proposed by Hill (1979) were used to determine the effect of strain path on the forming-limit diagram (FLD). By this means, the experimentally determined FLD for different strain paths for Armco, copper, aluminium and steel were transformed to forming-limit stress diagrams (FLSD) in stress coordinates. Calculations were made for values of exponent m of 1.6, 1.8 and 2.0 in the yield criteria. The best yield criteria and values of m were found on the basis of good linear correlation between the calculated values of the principal stresses σ 1 and σ 2 at the point of instability. The FLSD obtained at the best yield criteria and values of m were then transformed to FLD, by means of which the formability of materials for different strain paths can be determined directly.