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Strain gradient modelling of plasticity confinement effects in metals at small scales

par Kazemi Hatami, Mahdi
Président du jury Delplancke, Marie-Paule
Promoteur Massart, Thierry,Jacques
Publication Non publié, 2016-04-29

Thèse de doctorat

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Résumé :

The present manuscript addresses the computational modeling of size effects in the plastic behavior of metals at small scales. It is experimentally observed that the mechanical properties of metals are strongly affected when at least one microstructural length scale is scaled down to the micro/nanometer range or when the size of the object stands in the micron or sub-micron range. In such cases from a continuum point of view, the role of plastic strain gradients is considerable in controlling hardening properties. Classical theories of plasticity cannot predict such behavior since they don’t take any intrinsic material length scale parameter into account. For such cases, strain gradient plasticity has been developed to represent size effects. This project focuses on using a phenomenological strain gradient plasticity model to represent some aspects of size effects in metals.To this end, the strain gradient viscoplastic formulation with isotropic material response, based on the formulation developed by Borg et al. (2006), was implemented. Moreover, the strain gradient crystal viscoplastic formulation, according to the development of Borg (2007b), was implemented in a finite strain 2D setting. An extension of the finite strain rate-independent isotropic formulation (Niordson and Redanz (2004)), initially implemented by Mazzoni-Leduc (2010), to plane stress was performed and exploited. As a first application of the research, the rate-independent strain gradient formulation was first used to model the material behavior of Transformation Induced Plasticity (TRIP) assisted multiphase steels. This is done by an extension of the model, developed in Mazzoni-Leduc (2010) for local phase transformation features, by applying a special averaging scheme incorporating experimentally observed transformation kinetics of the phase transformation. Results show that the model stands in a good qualitative agreement with the experiments. The model is shown to have potential for material properties optimization as a perspective. As a second application, the strain gradient viscoplasticity formulations, for both the isotropic implementation and for the crystal plasticity effects, are used to model the compression of Copper micro-pillars. Computationally, the confinement effect is modeled, and experimental data are used to validate the approach. Results show the necessity of considering orientation-dependency of the material. The experimental plastic confinement effect is captured in a qualitative manner. Extension of the model to 3D and studying the grain size effect on bi-modal polycrystals are among the future plan of the work.