Instability criterion of materials in combined stress states and its application to orthogonal cutting process

The loadings of variable combined stresses may cause different instabilities of materials such as the shear localization instability or non-localized thermal softening instability. The purpose of this paper is to present a theoretical analysis of instability behaviors of material under the loading conditions of combined stresses and apply the analytical results to interpret the removal mechanisms of chip materials in cutting process. First, the perturbation analysis of material instability under the loading conditions of combined stresses and relatively critical conditions are presented, followed by discussions on relations between the instability behaviors of materials and the loading conditions. Then, the removal mechanisms of chip materials in cutting process are studied by numerical simulation. The formation of continuous and discontinuous chips is shown closely related to the plastic flow and instability behaviors of the chip material, and the transformation between the two types of chips is found dependent on the ratio of plastic work dissipated by dilatation deformation and that dissipated by shear deformation.

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