Universidad del País Vasco / Euskal Herriko Unibertsitatea (UPV/EHU) in Spain.
The global aim of ESR5 is the formulation of a viscoplastic strain gradient continuum theory applicable to multi-coupled modelling.
Classical continuum solid mechanics theories, such as linear or nonlinear elasticity and plasticity, have been used in a wide range of fundamental problems and applications in mechanical and materials engineering. Although initially designed to describe deformation phenomena and processes at scales ranging from millimetres to meters, and therefore observable by the naked eye, these theories were applied in the last century to model phenomena at the atomic scale (elastic theory of dislocations).
However, recent experimental observations at the micrometre or nanometre scale with newly probes such as nano-indenters and atomic force microscopes have suggested that classical continuum theories are no longer sufficient for a precise and detailed description of the corresponding deformation phenomena. These drawbacks have led to the recent development of theories that attempt to capture such phenomena via dependencies on plastic strain gradients.
The purpose of strain gradient modelling is twofold. First, it provides a continuum model that can account for size effects observed during deformation, machining processes or material processing. Second, it is well-known that severe deformation leads to strain localization, e.g. adiabatic shear banding. Finite element simulations of localization phenomena usually exhibit spurious mesh-dependence (dependence of the results on finite element size and orientation) that can be regularized by means of strain gradient models. Coupling with thermal diffusion can lead to regularization but associated length scales are generally too small. Material length scales, related to microstructural features like grain size or dislocation structures, then play an essential role for the understanding and simulation of localisation processes.
The different steps of the PhD work in close cooperation with other ERS contributions will be as follows:
- formulate the macroscopic strain gradient model
- identify material parameters from experimental results obtained in ENABLE (ESR1, 2, 3)
- identify characteristic length scales using in particular from the results of the multiscale polycrystalline approach of ESR4
- simulate regularized strain localization phenomena including thermal coupling
- extend the work to more severe loading conditions (including remeshing techniques) and relate to the experiments and simulations of ESR7 (Machining high strain rates), ESR 8 (FSW, high strain), ESR (Additive Manufacturing, high temperatures).
- transfer of the result to ESR6 (High Performance Computing).
Model and element development will be performed following an implicit scheme. Reaching severe deformations and simulation of realistic processes may require the extension to an explicit solver.