| The forming-limit diagram for bulk-forming processes is studied on the basis of micro damage development from metallurgical inclusions. Ductile fracture by growth and coalescence of voids at inclusions is investigated with the aid of plasticine models. Commonly observed fracture phenomena in the workability testing of metals, e.g. slant cracking, the higher fracture limit incurred by homogeneous deformation, and matrix hardening characteristics, have been duplicated and explained by the model. A phenomenological criterion of the linkage of voids, which incorporates inclusion parameters and matrix hardening characteristics, is postulated.; Using a facility for superimposing hydrostatic pressure during mechanical testing, the micro damage process in steels containing inclusions was studied. In addition to changes in the void formation mechanism, the forming limit characteristics are also altered. Both the workability index and tensile fracture strain are found to increase linearly with increasing superimposed pressure. Concomitant transition of the fracture mechanism under pressure is examined through fractographic analyses. The nucleation and growth of voids at inclusions is severely retarded by the superposition of pressure. Furthermore, the sensitivity of the fracture strain to pressure is influenced by the interfacial bond strength between the inclusion particle and the steel matrix. A workability model, based on void growth under pressure, is proposed to explain the observed variation in forming-limit curves.; The damage resistance of inclusion particles was further studied using hot-rolled 1045 steel bars containing aligned sulfide stringers. Variation in workability due to the orientation between inclusion stringers and the tensile stress is analysed and correlated with the transition in damage development and fracture mechanism. Both the workability index and tensile fracture strain are found to decrease linearly with the total projected inclusion length on the fracture plane, in accordance with previous results on other iron-based materials. |
