Experiment Research On Subsequent Yield Surface And Plastic Flow Under Complex Loading Of Polycrystalline Copper

In the present thesis, experimental results of thin-walled tubes of annealed polycrystalline copper with the combined tension-torsion loading are introduced to study the rules of yield and subsequent yield and plastic flow which can provide reasonable bases for constitutive behaviour. It is also discussed that the different descriptions of subsequent yield surface due to the different yield definitions and the evolution of subsequent yield surface on the (σ - 3^1/2Ï„)coordinates and Ï€ plane. According to theses analyses, it is found that the shape of subsequent yield surfaces mainly depend on the preloading direction. The phenomena of vertex at curve is observed to be along the preloading direction of corresponding subsequent yield surface after torsion pre-strain, and the shape of yield curve near the vertex is related to different yield conditions. The result shows that: the larger the offset strain is defined, the more the subsequent yield surface will expand in the direction without preloading, and the less the vertex phenomena will be observed.An experiment investigation is carried out on the plastic flowing characteristics of annealed polycrystalline copper subjected to proportional and non-proportional loading. The changes of the corresponding angels, θ₁ between plastic strain rate and deviatoric stress and θ₂ between plastic strain rate and deviatoric stress rate under the cyclic and cruciform straining test are discussed. The results show that the additional hardening is stronger on circular strain path than on the cruciform strain path. Under the cyclic strain path, θ₁ and θ₂ fluctuate periodically in a cycle and the evolutions of plastic strain rate direction will strongly depend on the shape of the strain path.The experimental analysis also shows that the influence of the yield definition is not only on the shape of subsequent yield surfaces but also on the plastic strain rate direction. Under the combined loading path after pre-torsion, the larger the offset strain is chosen the closer the direction of plastic strain rate vectors and deviatoric stress vectors will be. Otherwise the direction between the two vectors might be dispersed.In this thesis, a non-linear kinematic hardening constitutive model is introduced. The parameter determination of this model was investigated and a novel method is proposed. By using this method a set of parameters for the annealed polycrystalline copper was determined. Furthermore the finite element analyses associated with the parameters for uniaxial and biaxial cyclic loading have been performed.The yield , subsequent yield and the rules of plastic flow are the foundations of plastic theory. In the present thesis, the results show: the yield , subsequent yield and the rules of plastic flow for the descriptions of the classical plastic theory are not completely correct and reasonable. For theses shorts of plastic theory, it will bring serious errors in the prediction of metal strength and especially in the case of cyclic loading.