Fracture toughness and the extents of primary void growth

Abstract

It is generally believed that, for an otherwise constant microstructure, the crack tip opening displacement at fracture(δ, _IC) will scale with the primary particle spacing,X _o. However, fracture toughness is also influenced by the microstructure as well as by particle spacing and volume fraction. It has been proposed that the extents of primary void growth could be used as a measure of the influence of microstructure on toughness. Two approaches to using the extents of primary void growth have been suggested. In one, it was proposed that δ,_IC would scale asX _o(R_v/R_I), whereR _v is the radius of the void as viewed normal to the fracture surface andR, is the radius of the particle nucleating the void. In the second, it has been proposed that a measure of the true local strain would be Inh/2R ₁), whereh is the depth of the void, and that δ_IC would scale asL Inh/2R ₁), whereL is some microstructural distance, taken here to beX _o. These two approaches have been tested for materials for which δ_IC ranges from about 7 to over 150 μm. The results suggest that the two proposed scalings hold only for high-strength microstructures of low constrained ductility containing primary particles of small spacings and which blunt smoothly and not to vertices.