Experimental Study On Subsequent Yield Surface Evolution Of Nodular Cast Iron

The thin-walled tube of QT600-7 nodular cast iron are used in this paper,the multi-sample method was adopted,the initial yield surface test and subsequent yield surface test under pro-load of proportional path and non-proportional path was proceed,through different ratio of tension and torsion.Proportional path include pre-tension,pre-torsion and pre-tension&torsion,non proportional path along diamond path.Through the analysis of initial yield surface and subsequent yield surface,the influence of pre-load path,pre-deformation degree and offset strain value on the evolution of subsequent yield surface was studied.At the same time,the influence of plastic strain increment on the plastic flow direction is studied,and the relationship between plastic flow direction and normal direction of load plane of initial yield surface and subsequent yield surface was discussed.Experimental results show that:1.the subsequent yield surface moves in the direction of pre-loading,the smaller of the offset strain,the larger of the moving.The effect of pre deformation on the yield surface with small offset strain is larger,and the larger the pre deformation is,the larger the deformation of yield surface is.The smaller the offset strain value is,the larger the moving of subsequent yield surface in the direction of pre-loading is,and so with the deformation;the " sharp angle" phenomenon will appear at the pre-loading direction and the "concave" or flat phenomenon will appear at the pre-loading reverse direction.The shape of the subsequent yield surface approach to initial yield surface gradually will the increase of offset strain.2.the larger the plastic strain increment is,the more stable the plastic flow direction is.When the offset is large,the different offset strain has little effect on the plastic flow direction.The relationship between the plastic flow direction and the normal direction of the load surface,is related to the deformation degree of the yield surface,and the smaller the yield surface deformation is,the better the correlation is.