The effect of anisotropy on the localization and failure of aluminum alloys under biaxial loads

Date

2019-08

Authors

Chen, Kelin

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Abstract

Development of a robust finite element model capable of simulating ductile failure of thin-walled Al-alloy structures under complex loading conditions requires: (a) A suitably calibrated constitutive model of the material that includes the prevailing plastic anisotropy, and (b) an appropriately extracted material hardening response to large enough strains. This work addresses these two issues through the analysis of the response up to failure of thin sheets in hydraulic bulge tests, and the large deformation response of tubes under combined shear and tension. Hydraulic bulge tests have been used to extract the material hardening response of sheet metals to large strains. The extraction requires proper knowledge of the plastic anisotropy in the sheets. To this end, the non-quadratic anisotropic yield function of Barlat et al. [2005] was calibrated using a series of uniaxial and biaxial tests and two data from the bulge test. The calibration uses an iterative scheme for evaluating the stress state at the apex. The calibrated yield function is then used to extract the material hardening. The veracity of the scheme is demonstrated by using the calibrated yield function and hardening response in a 3-D finite element model to successfully simulate the bulge test up to failure. The second part of the project simulates the response of Al-alloy tubes under combined tension and torsion. Experiments conducted in parallel with this study have shown that following initial plastic deformation, strain localizes in a narrow zone and grows significantly before rupture. Here, the non-quadratic anisotropy yield function is calibrated using the set of tension-torsion experiments conducted, supplemented by a set of pressure-tension experiments. The constitutive model is then used to extract the material hardening response from a simple shear test accounting for the rotation of the material frame. The two constitutive components are then implemented in a 3-D finite element model to simulate a set of tension-torsion experiments. It is demonstrated that the constitutive model and hardening material response can reproduce the experimental structural responses, the onset of localization and its evolution to strains that correspond to the measured failure strains. This is achieved without the artificial introduction of material softening.

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