对称拉压循环下金属屈服硬化的各向异性与细观变形转动的不均匀性

ANISOTROPY OF YIELDING/HARDENING AND MICROINHOMOGENEITY OF DEFORMING/ROTATING FOR A POLYCRYSTALLINE METAL UNDER CYCLIC TENSION-COMPRESSION

考虑金属材料晶体细观塑性变形流动的各向异性性质以及晶体滑移的非线性运动硬化,以Voronoi多晶集合体作为材料的代表性单元(RVE),用晶体塑性模型描述金属材料的细观本构关系,对多晶纯Cu进行了晶粒尺度的对称应变循环细观分析.证实了本文模型和方法可以用于描述多晶金属材料不同应变幅的循环滞回特性,并可以用于估计金属材料在循环加载过程中后继屈服面形状和曲率与预加载方向相关的变化.分析发现:在对称应变循环中,随循环数增加,多晶材料宏观拉伸方向的内部细观应变分量的统计变异系数(COV)不断增加,材料内部晶格的不均匀细观转动也越来越显著.这些结果表明,循环过程中应变分布差异和晶格转动差异逐渐增大,从而导致材料内部微结构越来越不均匀.

The mesoscopic analyses at a grain level are carried out for a pure copper under symmetrical strain cyclic loading. The Voronoi crystal aggregation which consists of a number of randomly arranged crystal grains is adopted in the analysis as a RVE （representative volume element） of a metal material, and a crystalline plastic model is applied to describe the grain＇s mesoscopic constitutive relationship, by which the mesoscopic plastic slipping in anisotropic grain follows the nonlinear kinematic hardening law. Through numerical simulation it is confirmed that the present method can be applied to simulate the hysteresis for a polycrystalline material under cycle loading of different strain amplitudes, and it can be used to estimate the subsequent yield surface shape and its curvature change of the material in the cycle loading process, which are relative with the preloading direction. According to statistical analysis, it is found that the coefficient of variation （COV） for internal axial strain in the macro tension direction of the polycrystalline RVE considerably increases with the cyclic number in the symmetrical strain cycle loading, and the statistical lattice rotating of the material increases with loading cycle number. These results indicate that the internal material＇s microstructure will tend to heterogeneity, since the coefficients of variation for the inhomogeneous strain and rotation angle increase with the cyclic number.