Remarks on unstable ductile crack growth

From a review of various theories of fracture, it is concluded that to describe initiation of fracture in plane strain, the T contour integral is the most embracing, but probably more relevant to circumstances of high constraint, than to low constraint cases. An R-curve picture of stable ductile crack growth is a measure of total plastic dissipation of work rather than of toughness and expression though J is a characterising description valid whilst a J field exists for ω >> 1, or as an attempt to define a normalized work rate independent of size and shape, for the flat fracture component, through use of the factor eta/Bb. Either argument gives results identical for small amounts of growth by defining a material tearing resistance, T/sub mat/, describable through (dJ/da)/sub mat/. The inequality that leads to instability is expressed from a characterizing term (dJ/da)/sub appl/, or through arguments of compatibility of displacement, or as a balance of energy rates. All reduce to the same dominant terms for small amounts of growth, ω >> 1, though subject to numerical differences according to various approximations. For larger amounts of crack growth the methods diverge. The physical argument of a balance of energy rates is valid in concept is the existence of T/sub mat/ unique for all configurations and sizes is not established and assessment of which terms for T/sub appl/, are truly of second order is not yet clear. Suggestions are made for assessing the magnitude of the term ω >> 1 to ensure the relevance of J theory to plasticity. The energy balance concept is expressed as an effective toughness by combining the size and shape factors eta and Bb with (dJ/da)/sub mat/