| Research Interests |
General: Mechanics of materials. Micromechanical and multiscale modelling of the mechanical behavior of solids with experimental integration. Prediction of mechanical properties of heterogeneous media using homogenization and full-field approaches. Coupling between Finite Element analysis and homogenization techniques. Constitutive laws. Plastic deformation, texture development and microstructure evolution of polycrystalline metals and minerals and nanostructured materials.
Second-order homogenization of viscoplastic polycrystals: The computation of large-strain mechanical behavior and texture evolution of viscoplastic (VP) polycrystals using self-consistent (SC) models is nowadays a standard approach in the materials science community. For this, several “classical” SC approximations for non-linear materials are available (e.g. the "secant", "tangent" and "affine" formulations), all of them based on linearization schemes at the local level that use information on mechanical field averages only, disregarding higher-order statistical information (i.e. average field fluctuations) in the grains. However, the above assumption may be questionable for single-phase aggregates of highly anisotropic grains or for two-phase polycrystals, where a strong directionality and large variations in local properties are to be expected. The non-dependence with higher-order statistical moments is particularly critical for the treatment of those highly-contrasted materials, since such information is essential to capture—in an average sense—the effect of the strong deformation gradients that are likely to develop inside grains which are highly anisotropic, or adjacent to another phase. Therefore, in order to overcome the above limitations we have implemented inside the code VPSC [1] a rigorous non-linear homogenization scheme that takes into account information on the average field fluctuations at grain level.
[1] The code VPSC—developed and maintained by R.A. Lebensohn and C.N Tomé—is a multipurpose polycrystal plasticity research code, based on the knowledge of the mechanisms of slip and twinning that are active in single crystals of arbitrary symmetry. VPSC can be used to predict the effective stress-strain response, texture evolution, anisotropy, etc., and it is presently used as a predictive tool for metallic and geological material systems, for parameter identification, interpretation of experimental results and multiscale calculations, in academic and industrial applications, etc., by numerous research groups worldwide (at present, the VPSC code distribution list has more than 100 registered users).
Homogenization of the dilatational viscoplastic behavior of anisotropic porous polycrystals: The originally incompressible VPSC formulation has been recently extended to deal with porous polycrystals taking into account: a) the dilatational deformation associated with void growth , and b) second-order statistical information in terms of average field fluctuations inside the constituent grains. Such extended model allows us to account for porosity evolution in voided polycrystals, while preserving the anisotropy and rate sensitivity capabilities of the VPSC formulation. This extended VPSC model has been applied to address the problem of coupling between texture, plastic anisotropy, void shape, triaxiality, rate sensitivity and porosity evolution.
Simulation of forming of anisotropic materials by direct coupling between SC polycrystal models and Finite Element (FE) codes: Simulating the forming of anisotropic materials, such as zirconium, requires a description of the anisotropy of the aggregate and the single crystal, and also of its evolution with deformation (due to texture development and hardening by slip and twinning). As a consequence, using polycrystal constitutive laws inside FE codes represents a considerable improvement over using empirical constitutive laws, since the former provides a physically-based description of anisotropy and its evolution. We have developed a polycrystal constitutive description for pure clock-rolled Zr deforming under quasi-static conditions at room and liquid nitrogen temperatures, using tensile and compressive experimental data to adjust the constitutive parameters of a SC polycrystal model. This model was in turn implemented in an explicit FE code, assuming a full polycrystal at the position of each integration point. This methodology was successfully applied to simulate the inhomogeneous three-dimensional deformation of zirconium bars deforming under four-point bend conditions.
Macroscopic anisotropic yield functions and FE analysis to account for the effect of twinning on anisotropy and work-hardening: We have recently combined physically-based and experimentally-adjusted models for description of the twinning process and the interaction between twinning and slip systems in zirconium and a macroscopic yield criterion that accounts for the effect of twinning on yielding under quasiestatic (elastoplastic) and dynamic (elasto-viscoplastic) loading conditions, using a multiscale methodology based on: texture measurements, uniaxial mechanical tests, SC polycrystal models, interpolation techniques at macroscopic level, and FE analysis. We are also in the process of applying the above methodology to titanium.
Full-field formulation based on Fast Fourier Transforms for the calculation of the micromechanical behavior of plastically deformed 3-D polycrystals: This formulation—originally developed as a fast algorithm to compute the linear and nonlinear response of composites using as input a digital image of their microstructures—has been adapted by us to treat 3-D polycrystals deforming by dislocation glide. The FFT-based model provides an exact solution of the governing equations, has better performance than a Finite Element calculation for the same purpose and resolution, and can use voxelized microstructure data as direct input. Among the applications of this formulation we can mention: a) validation of predictions of statistical models on effective properties, field fluctuations, global and local texture evolution and hardening in plastically deformed cubic and hexagonal materials; b) direct comparison with intragranular orientation maps and micromechanical fields measured by imaging techniques (OIM, DIC, etc) in plastically deformed Cu polycrystals; c) study of the effect of microstructure on strain localization; d) prediction of subgrain structure formation, GND densities and implementation of length-scale-sensitive hardening laws.
|
| Research Highlights |
Ricardo Lebensohn is a Researcher of the Materials Science and Technology (MST) Division at Los Alamos National Laboratory (LANL, USA). Before joining LANL in 2003, he was a Professor at the Physics Department of the National University of Rosario (UNR, Argentina), and a Researcher of Argentina's National Research Council. He received his PhD degree in Physics from UNR in 1993, and did his postdoctoral training in the GPM2 Laboratory of Grenoble (France). In 1997 he was awarded a Fulbright Scholarship to work at University of California, Berkeley (USA), and has also worked as Guest Scientist at Technical University of Hamburg-Harburg (Germany), RISOE National Laboratory (Denmark), University of Paris-Nord and Ecole Polytechnique (France). He has been involved in research on Mechanics of Materials, in the area of modelling the mechanical behavior of heterogeneous materials, for over 15 years. He is the coauthor of the widely-used ViscoPlastic SelfConsistent (VPSC) code for the prediction of the mechanical behavior and the texture development of polycrystalline materials. He has published over 40 peer-reviewed articles in international archival journals, and given over 20 invited lectures in international conferences, Universities, laboratories and industrial research centers in South America, United States and Europe. |
Structure/Property Relations, MST-8 
