An investigation of fracture criteria for predicting surface fracture in paste extrusion

In the extrusion of pastes, fractures may be found on the surface of product. Such fractures compromise strength and are often unacceptable aesthetically. Here some theoretical criteria for predicting the onset of surface fracture, using the elastic–plastic finite element method are evaluated and the success of these criteria in predicting recent observations is assessed. Two criteria based on stress fields successfully predicted an increase in the depth of fracture cracks with extrusion ratio. However these criteria, which are dependent on the deforming zone stresses and extrudate residual stresses, respectively, do not successfully predict the increase in fracture with increasing die entry angle observed experimentally. Three criteria based on ductile fracture are also investigated and difficulties associated with their accurate evaluation in extrusion problems highlighted. However, all three successfully predict the increase in fracture with increasing die entry angle. In considering the effect of extrusion ratio on surface fracture, two of these criteria should be at least qualitatively correct while for the third this is unlikely.