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This paper attempts to predict the occurrence of central bursting defects in the plane strain extrusion process using upper bound method. For this purpose, the material under deformation is divided into three deformation zones. These deformation zones are separated from each other by the shear boundaries as the exponential functions. Then, an admissible velocity field, including the radial and the angular velocity components are developed for each deformation zone. Strain rates components are determined and mathematical relationships for internal, shear and friction powers are obtained. For a given process conditions, the total power toward geometrical shape of shear boundaries entrance and existence is optimized. Intersection position of the entrance and existence shear boundaries of deformation zone on centerline represents the occurrence of central bursting defects. The effect of process parameters, including semi die angle, reduction in area and friction factor on the defects and the extrusion force are investigated. The analysis results with the FEM simulation (Deform software) and the results of the published papers are compared. The results showed that increase of friction factor and increase the reduction in area decreases the probability of central bursting defects.

Keywords: Central bursting defects, Plane strain extrusion, Upper bound analysis

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