Forming limit analysis for two-stage forming of 5182-O aluminum sheet with intermediate annealing

► Effective plastic strain was proposed to account for the pre-strain and annealing effects. ► A combined theoretical and experimental investigation was conducted. ► σ-FLDs for a two-stage forming with an intermediate annealing step were generated. ► σ-FLDs show independence of pre-strain and annealing. ► Post-annealing stress-based forming limit curves converge to a single curve. Stress-based forming limit diagrams for a two-stage forming technique with an intermediate annealing step were generated for aluminum alloy 5182-O with a new experimental/theoretical methodology. It was demonstrated that unlike strain-based forming limits, stress-based forming limits are independent of pre-strain levels, pre-strain paths and annealing conditions, and converge to a single forming limit curve that is close to the stress forming limit of the as-received material. For this purpose, AA5182-O specimens were pre-strained to various levels in uniaxial, plane strain, and equibiaxial tension; smaller shapes were extracted, annealed, and tested in limiting dome height (LDH) tests. Strain fields for the forming limit diagrams were measured with stereo digital image correlation during each LDH test, where the onset of localized necking was identified to construct the forming limits. In the calculation of stress-based forming limit curves, only the as-received material properties were required, while the effects of pre-strain and annealing were accounted for with a calculated constant “effective plastic strain.” An analytical method is described to develop a new experimental/theoretical methodology effectively predicting the pre-strain and annealing effects. This enables reliable prediction of formability in multi-stage forming processes interrupted by annealing treatment employed to recover additional stretchability in critical areas of a formed part.