FRACTURE-TOUGHNESS AND THE EXTENTS OF PRIMARY VOID GROWTH

It is generally believed that, for an otherwise constant microstructure, the crack tip opening displacement at fracture (delta(IC)) will scale with the primary particle spacing, X0. 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 delta(IC) would scale as X0(R(V)/R(I)), where R(V) is the radius of the void as viewed normal to the fracture surface and R(I) 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 ln (h/2R(I)), where h is the depth of the void, and that delta(IC) would scale as L ln (h/2R(I)), where L is some microstructural distance, taken here to be X0. These two approaches have been tested for materials for which delta(IC) ranges from about 7 to over 150-mu-m. 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.