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Simulating 3D crack propagation in polycrystalline microstructures of steel using finite deformation crystal plasticity, gradient enhanced damage and the XFEM -- modelling aspects and numerical issues

le 16 mars 2017

Stefan Loehnert, Senior Engineer à l'Institut de Mécanique des Milieux Continus de l'Université Gottfried Wilhelm Leibniz de Hanovre en Allemagne, interviendra lors du séminaire.

During forming processes of steels within the polycrystalline microstructure cracks might nucleate and propagate and eventually lead to the failure of the material. In addition, during the time the formed engineering part is in service, the damage development emanating from the production of the part might have an influence on its lifetime.

Hence, it is important to know about the initiation and growth of microcracks and their effect on the macroscopic behaviour of the material and the engineering part.

Here, a method to calculate crack propagation within the polycrystalline microstructure of steels under finite deformations is presented. The method incorporates the XFEM in combination with level set techniques and a gradient enhanced damage model in combination with finite deformation crystal plasticity. The gradient enhanced damage can be interpreted as a void volume fraction, and its value along the crack front is used to determine whether the discrete part of the crack propagates. The crack propagation direction is defined as the direction of fastest void volume growth. A combination of different level set techniques is used to calculate the geometry update of the crack in 3D in an accurate and robust way. Projection techniques are required for internal variables as well as for the primary field variables to ensure good convergence behaviour.

Due to complicated crack geometries and crack propagation the XFEM enrichments of nodes might lead to badly conditioned global system matrices due to possible near linear dependencies between standard and enriched degrees of freedom or between enriched degrees of freedom.  One way to overcome this problem is to stabilise the element stiffness matrices by adding some small artificial stiffness to those eigenmodes that should not have a nearly zero eigenvalue. This technique has been presented for the case of linear elastic fracture mechanics problems before. Here it is extended to finite deformation problems with inelastic and possibly softening material behaviour.
Type :
Séminaires - conférences
Lieu(x) :
Campus de Cachan
Amphi Emedia
Bâtiment Léonard De Vinci - ENS Cachan
61, avenue du Président Wilson 94230 Cachan
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