Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

• A one-step procedure to control strain path changes in sheets, without unloading.
• Strain path controlled by displacements in main directions of a cruciform shape.
• Experimental and predictive forming limits are greatly influenced by prestrains.
• Premature necking is observed for high prestrains in uniaxial tension.

Optimization of sheet metal forming processes requires a very good knowledge of material forming ability, more especially for aluminum alloys which generally exhibit a poor formability at ambient temperature. During the forming of industrial parts, very complex strain paths are usually observed and can affect the formability of the sheet. In this work, in order to investigate strain path effects on formability, an innovative one-step procedure is proposed to control the strain path changes with a single test, without unloading. The test is based on the use of a cruciform shape loaded with a planar biaxial tensile device. Strain path is controlled by the displacements in the two main directions of the cruciform specimen. For a given non-linear strain path type, experimental forming limit points are greatly influenced by the level of prestrain which can either improve or reduce formability. The same tendency is observed when using a predictive tool based on a finite element model of the same cruciform shape and a rather good correlation is observed between experimental and numerical results.