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Considering a material at the ductile plateau, fracture tests on small specimens in generalized yielding conditions are common. We need to extract from them adequate information to characterize the fracture resistance properties of the material. We also need to predict initiation, crack growth, and maximum load for a crack found in a ductile structure. But the real problems are three-dimensional; for instance, semi-elliptical surface cracks or through-cracks in “small” thicknesses (tunneling and mixed-mode fracture). Moreover, they are not only in the symmetrical Mode I case (angled crack extension).
The common denominator of all these phenomena is the ductile fracture processes in the material at the crack border; these micromechanisms extend over some characteristic length which needs to be introduced at a crack tip because of the very intense strain gradient. We thus need a ductile fracture damage function belonging to the continuum mechanics frame and related to the history of stresses and strains averaged over such a characteristic volume. In this numerical feasibility study, using elastic-plastic finite-element computations and guided by a ductile fracture model in three stages—void nucleation, void growth, and coalescence—we tried such a differential damage history, in a most simplified form. We integrated this during the whole stress and strain history in each finite element along the crack path, and we studied the influence of the mechanical and numerical parameters playing a role in this methodology. We describe herein the evolution, which results from this criterion, of some parameters used in the literature as initiation and crack growth criteria.
ductile fracture, void growth, finite elements, elastic-plastic deformation, generalized yielding, crack initiation, stable crack growth, instability, crack propagation
Maître de Confénces à l'Ecole Polytechnique et à l'Ecole des Mines, Laboratoire de Mécanique des Solides, Ecole Polytechnique, 91128 Palaiseau
Adjoint au Chef, Département Calcul de la Division des Fabrications, Framatome, Saint Marcel