Effect of strain hardening behavior on forming limit stresses of steel tube subjected to nonproportional loading paths

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 (first and second) linear stress paths and include unloading between the first and second loadings. The strain hardening behavior of the steel tube for many linear and combined stress paths is observed in terms of equivalent stress–equivalent plastic strain image curves. The forming limit stresses for the linear and some combined stress paths fall on a single curve in stress space irrespective of the stress paths when the corresponding image curves observed for the given stress paths are on a single curve. On the other hand, the forming limit stresses for some combined stress paths are lower than those for the linear stress paths when the specimen exhibits a low strain hardening rate immediately after the change in stress paths. It is, therefore, concluded that the forming limit stress of the steel tube is not fully path-independent, and that the path dependence of forming limit stress is strongly affected by the strain hardening behavior of the material for given loading paths.