A Fracture Criterion for Porous Materials and Its Application to the Shape of Sintered Preforms in Forging

Ductile fracture of porous metals during forging was studied. The effect of the shape of sintered copper preforms on the forming limit is examined in upsetting with flat dies at room temperature. Cylindrical preforms with concave ends were found to show less barrelling during deformation and had a larger upsetting limit compared to those with flat ends. A fracture criterion for porous materials was proposed as a function of the history of the hydrostatic component of the stress. The criterion was applied to upsetting of sintered copper preforms. For calculation of the forming limit, the histories of the stress components at the equatorial free surface were obtained applying the plasticity equations for porous materials. The change in the density distribution during upsetting was measured.