Assessment of Work-Hardening Characteristics and Limit Strains of Anisotropic Aluminum Sheets in Biaxial Stretching

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Abstract
In this work, commercially pure aluminum sheets in both the as-received and annealed conditions are tested in uniaxial and biaxial tension. Biaxial stretching is performed in dies giving different degrees of stress biaxiality. Resulting effective stresses and plastic strains are estimated according to the original Hill’s theory in the two situations where planar anisotropy is either neglected or taken into consideration. In both situations discrepancies between biaxial and uniaxial flow curves are observed. By analyzing the above uniaxial and biaxial test results according to the flow rule associated with a yield function recently proposed by Hill, a new material index describing the anisotropic behavior has been evaluated. This new material behavior description realized a satisfactory agreement between work-hardening characteristics of the tested aluminum sheets under various biaxial stress systems. The same tested aluminum sheet materials have been then tested in order to determine their forming limit curves. Correlation between these curves and the theoretical predictions of limit strains according to various instability analyses, is sought through the use of the above description of material work-hardening.

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