Research On The Identification Of Plastic Anisotropic Properties By Spherical Indentation

As the effective approach on the characterization and identification of material properties,indentation techniques are still key and hot studies.As a new technique,there are many advantages on indentation techniques.For example,simple specimen preparation,no destruction,no limitation for specimen structure and shape,and conveniently testing in local volumes such as polycrystalline materials or weld regions and so on.This paper focuses on the approach on the identification of plastic anisotropic properties by spherical indentation.The main contents and innovative achievements are presented as follows:1.This paper optimizes the calculation solution of anisotropic coefficients for Yld91 and Yld2004 yield criterions: In some special stress conditions,the calculation equations of anisotropic coefficients are simplified and system of nonlinear equations was established.Trust-region techniques based on the nonlinear least-square method are used to solve these equations.The method is unaffected by Jacobian matrix,and has steady robustness and high efficiency.2.Yld91 yield criterion and Yld2004 yield criterion are incorporated with the stress integration algorithm,Hollomon hardening law and elastoplastic rate constitutive equations to create the corresponding constitutive models,respectively.The constitutive models corresponding to Yld91 and Yld2004 yield criterions are implemented into ABAQUS software using UMAT subroutines,and their effectiveness and reliability are verified by finite element simulations.Meanwhile,a flexible and universal structure form on the secondary development of constitutive models is proposed.3.FE model of spherical indentation is constructed.UMAT subroutines corresponding to Yld91 and Yld2004 constitutive models are used into FE simulations of spherical indentation.Instrumented indentation experiments,and uniaxial compression experiments along different directions are conducted.4.For sheet metal materials,this paper first applies more advanced anisotropic constitutive models as Yld91 and Yld2004 yield criterions to FE analyses of spherical indentation.Based on FE analyses,the method to determine the optimal location(2r/d and l/D)of the data acquisition points is improved and applied to anisotropic case: at some directions from the rolling direction,the distribution gradients of equivalent stress and equivalent plastic strain,frictional effect on them are studied respectively,to determine the trust-region of optimal location.In the indenting process,plastic anisotropy is identified from the trust-region of optimal location and the results are analyzed.Some results are demonstrated as follows: indentation on Yld2004 is abler to fully identify difference in equivalent plastic strain than that on Hill48 and Yld91 when indentation depth ht is relatively smaller.Plastic anisotropy identified from indentation on Yld2004 or Yld91 is more applicable at larger strains than that on Hill48,and that on Yld2004 is more applicable than that on Yld91,overall.The method on the determination of the optimal location is demonstrated to be also applicable and valid for anisotropic materials.5.For out-of-plane anisotropic materials,new modified ECM is proposed: it can describe stress field,strain field and displacement field for out-of-plane plastic anisotropy,and also characterize piling-up and sinking-in effect.Based on new modified ECM,the representative flow stress-strain method is modified,to determine flow stress-strain curves along different directions.The flow stress-strain curves are compared with uniaxial compression experiments.Based on new modified ECM and the representative flow stress-strain method,a new reverse approach on the identification of plastic anisotropic properties is proposed,which is combined with the trust-region techniques based on the nonlinear least-square method.The new reverse approach is verified by FE simulations of spherical indentation and indentation experiments.Finally,it is demonstrated that the proposed reverse approach is reliable and useful for identification of plastic anisotropic properties of various anisotropic materials,and new modified ECM is right.