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Forming limit diagram (FLD) is a measure of the formability of a sheet material. The major-minor strain pairs that are closest to the neck on multiple specimens of various strain paths are utilized to construct a boundary between safe and unsafe zones. The challenge to obtain the FLD is the determination of incipient necking. Three approaches to determine the limit strains have been investigated and compared in this research in order to establish the optimal one for implementation: (1) commonly used Bragard criterion ( 1)e Br with periodic grids; (2) tracking the region of large local strains from strain history to locate the instance when critical major strain ( 1)e cr happens; (3) post-processing of strain history to locate the inflection in the major strain rate curve 1 max (e&&) at the onset of localization. The last criterion of inflection in strain rate 1 max (e&&) carries both a numerical and a physical meaning towards developing an understanding of flow localization, formability and fracture.

The constitutive behavior of both direct-chill cast (DC) and continuous cast (CC) AA5754 sheet materials has been investigated up to large strains using a modified shear test. The modified shear sample prevents rotation of the shear zone compared to the ASTM standard B831 test (2005, “Standard Test Method for Shear Testing of Thin Aluminum Alloy Products,” 2005 Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 02.02, pp. 601–603). The results show that the effective stress and effective strain curves from shear tests match those obtained from uniaxial tension, but only by incorporating the material anisotropy using the Barlat–Lian yield function. The flow stresses of both materials saturate at large strains; however, the fracture strain of the CC material is significantly lower than that of the DC material.

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In this article, we study the Portevin-Le Chatelier (PLC) bands and their influences on strain localization and fracture in continuous cast (CC) AA5754 aluminum sheets. Three types of tensile tests are conducted:(1) tensile samples are pulled directly to fracture at 223 K, (2) tensile samples are pulled at 223 K to initiate diffuse necking followed by unloading and reloading to fracture at room temperature, and (3) tensile samples are pulled at 223 K to localized necking and unloaded followed by reloading to fracture at room temperature. Furthermore, in situ V-bending test coupled with deformation mapping using digital image correlation is used to study damage at large strains. The results show that PLC bands detect favorable geometrical sites for shear band initiation. The formation of shear bands precedes damage and damage is a consequence of shear band formation.