Ductile crack growth—II. Void nucleation and geometry effects on macroscopic fracture behavior

Abstract

Many metals that fail by void growth and coalescence display a macroscopically planar fracture process zone of one or two void spacings in thickness; outside this region, the voids exhibit little or no growth. A finite element model of this mode of failure was described in Part I of this paper [J. Mech. Phys. Solids, 43, 233–259 (1995)]. A row of void-containing cell elements is placed on the symmetry plane ahead of the initial crack. The cells incorporate the softening characteristics of hole growth and dependence on stress triaxiality. These cells are embedded within a conventional elastic-plastic continuum. Under increasing strain, the voids grow and coalesce to form new crack surfaces, thereby advancing the crack. Parametric studies reveal that the important microstructural parameters in the model are D and f₀, characterizing the spacing and the initial volume fraction of voids on the fracture plane. Using this model, Xia et al. [J. Mech. Phys. Solids, 43, 389–413 (1995)] have successfully predicted details of the load, displacement and crack growth histories—collectively referred to as the macroscopic fracture behavior—of four specimen geometries, which give rise to significantly different crack tip constraints under fully plastic conditions. Here we study the quantitative effects of void nucleation by a stress and strain criterion on the macroscopic fracture behavior. This behavior is compared with predictions using a similar volume of voids present from the very beginning. Geometry effects on macroscopic fracture behavior under contained and fully yielded conditions are discussed for the three-point-bend specimen and the center-crack-panel loaded in tension. Here the objective is to show the connection between the crack growth resistance and the fracture environment, namely, the constraint ahead of the crack tip and the tensile stress on the fracture plane.