Deux exemples de manifestation de la localisation : mécanisme de déformation et critère de ruptureTwo examples of localisation occurence: deformation mechanism and fracture criterion

Numerical simulation, essential tool for vehicles dimensioning, is poorly adapted for simulating areas which present high stress and strain gradients. Solutions are proposed to model two localisation examples issued from very different fields. Aluminium foams are cellular materials which deform by fracture of the cell walls. Damaged zones stay localised between undeformed blocks.The selected model (Thesis in progress by J.S. Blazy, ENS Mines Paris) is a compressible potential (Abouaf 1985) in which a constant has been replaced by a function depending on the porosity and which governs the initiation of localisation. This model makes possible to predict the occurence of the strain localisation. Ductile fracture of metallic alloys appears suddenly in a zone of only about ten–fifty micrometers. Analysing the damage mechanisms leads to consider fracture as an instability acting on the damage growth. Such a mechanical instability criterion being established, fracture prediction by numerical simulation is then possible because this instability occurs in a part which does not yet contain any significantly high gradients.