Investigation of forming limit diagram for tube hydroforming considering effect of changing strain path

The forming limit diagram (FLD) has been widely used as a measure of the maximum formability of a material in tube hydroforming (THF). The geometric shape of the FLD varies owing to the influences of many factors, especially the strain path. Therefore, discussing the change rule of FLDs under various strain paths has practical significance. In the present study, strain paths generated from THF are classified as simple or complex ones. The FLDs for THF are established based on Swift’s diffused necking criterion and Hill’s localised necking criterion along both simple and complex strain paths. Through a comparison of the FLDs obtained from various strain paths, the influences of changing strain path are revealed. Some THF experiments under various strain paths are performed to verify the theoretical analysis. The theoretical analysis and experimental results prove that the position of the FLD changes with different strain paths. Compared with the FLD position established along a simple strain path, the FLD position under a two-stage linear strain path moves in the upper left direction with an initial uniaxial tensile strain path and in the lower right direction with an initial equibiaxial tensile strain path.