Material Modeling and Forming Limit Analysis of Tubular Materials for Automotive Use Subjected to Biaxial Stresses

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The anisotropic plastic deformation behavior and forming limits of extruded 5000 series aluminum alloy tubes, A5154-H112, of 76mm outer diameter and 3.9mm wall thickness is investigated, using a servo-controlled tension-internal pressure testing machine. Detailed measurements were made of the initial yield locus, contours of plastic work for different levels of work-hardening, and the directions of the incremental plastic strain vectors for both linear and combined stress paths. It is found that the measured work contours constructed in the principal stress space are similar in shape, and that the directions of the incremental plastic strain vectors remain almost constant at constant stress ratios. The work-hardening behavior predicted using Hosford's or the Yld2000-2d yield functions under the assumption of isotropic hardening agrees closely with the observations for both linear and combined stress paths. The material is thus found to work-harden almost isotropically. Both yield functions are effective phenomenological plasticity models for predicting the anisotropic plastic deformation behavior of the material. Forming limit curves are plotted in strain space, as well as in stress space. The former gives a conventional Forming Limit Curve(FLC) whereas the latter gives a Forming Limit Stress Curve(FLSC). It is found that FLCs depend significantly on the strain/stress paths, but the stress states at the forming limit lie close to a single curve in the principal stress space for a variety of stress paths ; the path-dependence of the FLC vanishes when viewed in stress space. We have confirmed that this effect is not a result of the saturation tendency of the stress-strain curves of the present material at high strain levels. The FLSC concept is therefore valuable for predicting the failure of metal tubes and sheets in plane stress states, particularly in multistage forming.