Theoretical and Experimental Investigations of the Mechanism of Ductile Fracture of Metals.

For ductile fracture in a free smooth surface, validity of the shear stress hypothesis is assumed. In the microscopic range, it is based on void initiation, void growth, and final internal reduction. The aim was to estimate the ductility of those metals in which voids are initiated at nonmetallic inclusions. Based on observations of the development and appearance of ductile fracture in free surfaces of bend test specimens of two steels in the hot and cold rolled conditions, a model of plastic flow limitation by plastic shear band instability is proposed. The forming limits to be expected according to this model even outside the shear bands, and hence the ductility, are graduated reasonably by inclusion content and strain hardening ability. Taking as basis the inclusion distribution in the surroundings of small tensile surface cracks of the specimens, a comparison of expected forming limits with measured ductility showed applicable agreement for hot rolled steel. The ductility of cold rolled material is partly underestimated by this method. Possible reasons are suggested.