![]() The energy criterion is a necessary criterion for crack growth, but not a sufficient one. However, no crack growth occurs, because the stresses and plastic strains at the crack tip are still small. This also means that at any stress different from zero the energy criterion is fulfilled (apart from the work for void formation). ![]() Therefore the R-curve must start from zero (at zero stress the size of the plastic zone is zero), as it requires no energy to form a plastic zone of zero size. The latter is presumably small in comparison with the energy dissipated in the plastic zone. ![]() In a ductile material, this is the work for the formation of a new plastic zone at the tip of the advancing crack, plus the work required for the initiation, growth, and coalescence of microvoids. The R-curve represents the energy required for crack growth.
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