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

采用伺服控制拉伸内压试验机，研究了外径76mm、壁厚3.9mm的5000系铝合金A5154-H112挤压管的各向异性塑性变形行为及成形极限。详细测量了初始屈服轨迹、不同加工硬化水平下的塑性工作轮廓，以及线性和组合应力路径下增量塑性应变向量的方向。研究发现，在主应力空间中构造的实测工作轮廓形状相似，且在恒定应力比下，增量塑性应变向量的方向基本保持不变。在各向同性硬化假设下，采用霍斯福德屈服函数或Yld2000-2d屈服函数预测的加工硬化行为与线性和复合应力路径下的观察结果非常吻合。因此发现该材料几乎是各向同性的加工硬化。这两种屈服函数都是预测材料各向异性塑性变形行为的有效现象学塑性模型。在应变空间和应力空间中绘制了成形极限曲线。前者给出了常规成形极限曲线（FLC），后者给出了成形极限应力曲线（FLSC）。结果表明：在不同的应力路径下，成形极限处的应力状态在主应力空间上趋近于一条曲线；当在应力空间中观察时，FLC的路径依赖性消失。我们已经证实，这种效应不是当前材料在高应变水平下应力-应变曲线的饱和趋势的结果。因此，FLSC概念对于预测金属管材和板材在平面应力状态下的失效是有价值的，特别是在多阶段成形中。

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.