Mechanisms of ductile fracture in crack-tip fracture process zones

The strength and toughness of engineering alloys, under conditions where general yielding and sub-critical crack growth precede catastrophic fracture, are critically dependent on the nucleation, growth and coalescence of microvoids in fracture process zones that are usually subjected to mean - normal stress approaching sigma(m)/2k = 2.0. It is shown that at these high mean-normal stress levels, microvoid nucleation and spontaneous void coalescence can become the controlling process of ductile fracture, with negligible dilational-plastic void growth prior to microvoid coalescence and the formation of a fracture surface, This effect is the direct result of high mean-normal stresses promoting the microvoid coalescence process when the newly-nucleated voids:Ye still of the same size and spacing as the void-nucleating particles, Under these conditions virtually all the dilational void-growth is confined to the final void-coalescence :Fracture surface and is the result of a highly - localised limit-load failure (or internal microscopic necking) of the intervoid matrix. The results suggest that in plane-strain fracture process zones, where sigma(m)/2k approximate to 2.071 ductile fracture at the microvoid-nucleation strain is likely to occur whenever the volume fraction of micron-sized particles exceeds a value of V-f approximate to (0.02. On the other hand, in plane-stress fracture process zones, where sigma(m)/2k approximate to 0.5, ductile fracture is unlikely to be observed at the microvoid-nucleation strain and the normal three-stage process of nucleation, growth and coalescence should always operate, Certain metalworking processes of the extrusion and bar-drawing types may also exhibit nucleation-controlled ductile fracture, due to the very high mean-normal stresses that can be generated on the axis of symmetry of the plastic-working zones.