Pre-Strain Influence on Forming Limit Diagrams of Aluminium Alloy Sheet using Biaxial Cruciform Testing: A comparison between Continuous and Discontinuous Approach

The present work investigates the influence of pre-strain on the forming limit curves (FLCs) of 2.5 mm thick cold rolled aluminium alloy 5052 sheets through finite element (FE) simulations using biaxial testing of cruciform samples. Three distinct strain path conditions, such as uniaxial, plane strain, and biaxial, are considered in this work. For each strain path condition, two levels of pre-strain are evaluated. Limited literature suggests that continuous and discontinuous strain path change may significantly influence the forming limits. This work delves deeper into this aspect by systematically incorporating both continuous and discontinuous strain path change in simulations. In the continuous strain path change, the strain path transitions directly from one condition to another without unloading during the simulations. In contrast, the discontinuous strain path change incorporates unloading and reloading of sample when transitioning from one strain path to another. The FE simulation results are presented as forming limit curves for different strain paths. The findings indicate that pre-straining under uniaxial conditions followed by biaxial loading enhances the limit strain in the positive region of the forming limit diagram (FLD). Conversely, biaxial pre-straining followed by uniaxial loading increases the limit strain in the negative region of the FLD. This trend is observed under both continuous and discontinuous strain path changes. However, the continuous strain path change yields higher forming limits as compared to the discontinuous strain path change.