Mechanisms of plastic deformation prior to ductile fracture in a low alloy steel

The fracture toughness of metallic materials is related to their ductility. Thus the fracture process is characterized by the mechanisms of plastic deformation which take place prior to fracture. In this study, such a characterization was obtained on tensile specimens of a low alloy steel containing niobium. The dislocation cell structures formed in the necked sections of tensile specimens were examined as a function of local strain, expressed as reduction in area. It was found that these cell structures decrease in size as the strain is increased up to a certain diameter where the diameter stays constant with further increase in strain. This behavior is related to observations on the variation in the fracture criterion JIc with grain size to provide qualitative understanding of the dependence of fracture toughness on microstructural and dislocation arrangements in metallic materials.