Effects of strain hardening behavior on the strain path dependence of forming limit stresses in a steel tube

The strain path dependence of forming limit strains and stresses of a steel tube subjected to combined axial load and internal pressure are investigated for linear and combined stress paths using a tension-internal pressure testing machine. The combined stress paths consist of two linear stress paths and include unloading between the first and second loadings. The strain hardening behavior of the steel tube for both linear and combined stress paths is observed in terms of the equivalent stress-equivalent plastic strain ((sigma) over bar-(epsilon) over bar) curves. Forming limit stresses in stress space drop on a single curve irrespective of the strain paths when the (sigma) over bar-(epsilon) over bar, curves for the given linear and combined stress paths are on a single curve (isotropic hardening), while the forming limit stresses for other combined stress paths generally become less than those for the linear stress paths when the material exhibits the cross effect in the second loadings. It is therefore concluded that the strain path dependence of forming limit stresses is much affected by the strain hardening behavior of the material for the given loading path. A new method for evaluating forming limits for combined loadings is proposed utilizing a limit curve in an equivalent plastic strain-stress ratio space.