Test And Crystal Plasticity Numerical Study On The Evolutionof Metals Subsequent Yield Surface

Metal structure under complex loading inevitably cause plastic deformation during service.To ensure the security,it is necessary to thoroughly understand the plastic behavior of metal materials under complex loading and to establish a reasonable constitutive model.And to calculate the plastic deformation accurately,it depends on whether the yield surface can be reasonably described.But a large number of studies have shown that classical macroscopic constitutive model of plastic deformation in the process of evolution of the yield surface and a description of the associated plastic flow rule,there is a big difference with the results of experimental observation.Therefore,in order to better describe the plastic failure behavior of metal structures in practical engineering,it is necessary to carry out a more in-depth study.In order to test the subsequent yield surface of material under different predeformation and to study its evolution rule,this paper uses the pure copper respectively by using single sample method and multi-sample method from a different number of the measuring point,the testing sequence,the effects of different translational strain and the measured yield surface appears concave phenomenon and so on are studied.Besides,the rationality and effectiveness of the two kinds of test methods have been analyzed.On the basis of this,Chaboche macroplastic constitutive model is used to simulate multi-sample method to test the subsequent yield surface and explore its applicability.And then,based on the crystal plasticity constitutive model with the Bauschinger effect considered,a representative unit model with periodic boundary conditions satisfied and a real test consistent whole finite element model with the characteristics reflected of deformation of polycrystalline materials are used to simulate the single sample method and multi-sample method test the yield surface.Finally,the phenomenon of concave in the subsequent yield surface testing by the thin wall tube specimen and its reasons are discussed,and the numerical analysis of the plastic behavior is carried out on two different scales of the material,the infinitesimal and the structural whole.The research has mainly made the following progress:1、By means of single sample method and multi sample method test,it is found that the yield surface of the measured is deep different to the yield surface of the classical plastic theory,no matter which method is adopted.2、Found that the two methods of subsequent yield surface shape and size also have obvious difference.The test results of the single sample method is very sensitive to the test path,test sequence and the number of test points,but the multi-sample method test results are not be affected by the above factors.In the case of small sample material differences between,with multi-sample method test than the single sample method is more reasonable and reliable.3、Test results found Chaboche model can describe the movement and swell-shrink of the subsequent yield surface in stress space,but can’t reflect the change of the yield surface shape(curvature)when the small translational strain is used to defined.For the evolution of the subsequent yield surface defined by large translational strain,it is acceptable to to describe it with the classical Chaboche macroscopic plastic constitutive model.4、The representative unit model combine with crystal plasticity calculation are used to simulate the subsequent yield surface test of T2 copper under different monotonic preload and cyclic loading,reproduce the evolution law of the measured subsequent yield surfaced,but the simulation test result will not be concave phenomenon.The conclusion is consistent with the classical plastic theory which derived from the basis of Drucker’s uniform material.5、When the finite element model of hollow thin wall circular tube based on crystal plastic theory is used,the simulation of the subsequent yield surface tested by multi-sample method and single sample method can reproduce the real test process.The simulation results are consistent with the measured results,both with an inner concave phenomenon,which are contrary to the corollary of the yield surface described by the classical plastic theory.6、The whole specimen model of crystal plasticity simulation and the subsequent yield surface of real test does not satisfy the classical theory of plasticity yielding surface convex conclusions,but the polycrystalline unit model simulation test results in the "yield surface convex" this point is consist with the classical plasticity theory,which show that the subsequent yield surface tested by thin-walled cylindrical specimen appears concave is mainly caused by the structural response of the specimen.In this paper,the crystal plasticity constitutive model is used to simulate the yield surface test process,and it can reasonably reflect the true micro deformation mechanism of metal materials and describe the evolution of yield surface.By comparing the test results of different test methods and combining the simulation results of two finite element models,the reasonable explanation of concave point of the subsequent yield surface of thin-walled tube specimen is given.