Plastic Behavior Of Metals Under Tension-Torsion Loading

Metal materials have been widely used in engineering structures, components and parts subjected to complex loading. In order to insure the safety of them, the plastic behavior of metal materials should be comprehensively understood and reasonably described by constitutive model. Metal plastic behavior under complex loading is still difficult to be accurately described because of the intricacies of deformation mechanisms. This is leading to that the yield surface evolution and the associated plastic flow law cannot be expressed reasonably by the conventional constitutive models. Therefore, in order to improve the constitutive description, large quantities of experimental, theoretical and numerical studies are demanded.In this thesis, the experimental analyses of the yield surface evolution of a45steel, polycrystalline copper and pure aluminium are carried out by using single-specimen and multiple-specimen methods. The limitations of single-specimen method were investigated, and the reliability of multiple specimens’test was proven by experimental numerical comparative analysis. The description ability on yield surfaces of conventional plastic theory, taking Chaboche visco-plastic model, for example, was investigated. A sub-model method was proposed for the cross-scale analyses considering the global specimen and the local polycrystalline aggregate. The macro mesoscopic plastic behavior was researched with crystal plasticity theory and classical plasticity theory.The main conclusions and achievements obtained as follows:1. The single-specimen method and multiple-specimen method to determine a yield surface were compared. The results show that the shape of the yield surface with the single-specimen method is related to the loading sequences and the number of testing points. The yield surface appears "concave" feature when the first yield point is tested in the opposite to preloading direction, otherwise this phenomenon will not occur. The shape of yield surface with the single-specimen method was affected by loading path, and the result is obviously unreasonable and deviates from the plastic basic theory. The yield surface tested by multiple-specimen methods will not lead to "concave" feature. It was proven that multiple-specimen method is more reasonable to test yield surface. This research provides test cases to question the single-specimen method that adopted by many foreign scholars.2. Using the multiple-specimen method, the evolution of the subsequent yield surface and the plastic flow rule of a45steel were presented under tension-torsion loading. The effects of preloading paths, unloading points, yield definitions and prescribed pre-strain on the subsequent yield surface were explored. The experimental results of yield surface for a45steel are as follows:the shape of the subsequent yield surfaces strongly depends on the preloading direction; the curvature of the subsequent yield surfaces in the loading direction is larger than that in the opposite direction. The plastic flow directions are not always normal to the subsequent yield surfaces, and are related to the yield definitions. By the yield definitions with smaller offset strain, the distortion of the subsequent yield surfaces becomes more prominent, and the plastic flow directions have larger deviation from the normal directions of yield surface. Based on the larger plastic strain definition, the subsequent yield surfaces are close to a column surface. The results reveal that the conventional plastic theory cannot reasonably predict yield surface determined by a fairly small offset strain, and not fit to fatigue analysis.3. The rationality of Chaboche model, including back-stress and yield radius, was discussed. The integral algorithm of the model which considering visco-plasticity and the nonlinear kinematic hardening was deduced, and the user subroutine UMAT for ABAQUS was compiled. The simulation confirmed that the present model can be applied to describe the hysteresis of a45steel under the cyclic loading with different strain amplitude. The experiments of subsequent yield surfaces of a45steel were simulated with Chaboche model by single-specimen and multiple-specimen methods. The results demonstrated it can reasonably predict the translation and expansion of yield-surface, but can not describle the distorsion, such as "sharp corner" and "blunt rear", when yield was defined by a fairly small offset strain.4. A sub-model method was developed to investigate the plastic behavior considering a global test specimen and polycrystalline aggregate model. The global specimen was analyzed by the macroscopic elasto-plastic constitutive model, the displacement boundary conditions were applied to the representative volume element (RVE) with polycrystalline aggregate. The cross-scale analysis was implemented from macro-specimen scale to micro-grain scale based on the crystal plasticity theory and classical plastic plasticity theory. This calcalation method can obviously reduce the total computing time and simulate precisely the micro-inhomogeneity of deformation.5. Combined tension-torsion test under pre-tension and pre-torsion deformation, the yield characteristic of polycrystalline copper in grain scale was investigated by applying crystal plasticity theory associated with polycrystalline aggregate model. Through the research on the shape and the evolution of the subsequent yield surface, the effects of different loading paths and different yield definitions on the subsequent yield surface were explored. The heterogeneous statistical analysis of the polycrystalline copper under different loading paths is also performed. And further more, the effect of loading history on subsequent yield surface, and on micro heterogeneous distribution were estimated. The results by the analysis based on crystal plasticity calculation combined with the sub-model method were compared with experimental results, and they were in reasonable agreement.6. Comparing to the experimental processes of subsequent yield surfaces of a pure aluminium under complex cyclic loading, the the evolution of subsequent yield surface were simulated at grain scale by applying crystal plasticity theory associated with polycrystalline aggregate model, which considering the microstructural characteristics and grain deformation of polycrystalline materials. It was confirmed that numerical simulation of polycrystalline aggregate RVE model can reasonably estimate the distortion of the yield surface evolution along different directions—"sharp corner and blunt rear", and the results coincide reasonably with experimental observations. It indicated that the method proposed by author can be applied to present the plastic deformation under cyclic loading at grain scale. In addition, this research provides a reference for improving the description of the present plastic behavior.