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International audienceA combined computational-experimental framework is introduced herein to validate numerical simulations at the microscopic scale. It is exemplified for a flat specimen with central hole made of cast iron and imaged via in situ synchrotron laminography at micrometer resolution during a tensile test. The region of interest in the reconstructed volume, which is close to the central hole, is analyzed by Digital Volume Correlation (DVC) to measure kinematic fields. Finite Element (FE) simulations, which account for the studied material microstructure, are driven by Dirichlet boundary conditions extracted from DVC measurements. Gray level residuals for DVC measurements and FE simulations are assessed for validation purposes

International audienceIn this work, a new finite element framework is developed and applied to the study and modeling of ductile fracture mechanisms at the microscale. More particularly, a body-fitted meshing and remeshing methodology is introduced and applications to void coalescence are investigated. Though most studies focus on periodic arrangements of voids, it was proven in experiments as in simulations that random void clusters have a major influence on void growth and coalescence. With the method proposed in this paper, various void arrangements can be addressed and their effect on void growth and coalescence can be studied at large plastic strain and various stress states

International audienceThe present paper addresses the challenge of conducting Finite Element (FE) micromechanical simulations based on 3D X-ray data, and quantifying errors between simulations and experiments. This is of great interest, for example, in the study of ductile fracture as local comparisons and error indicators would help understanding the limitations of current plasticity and damage models. Standard methods used in the literature to conduct FE simulations at the microscale are often based on multiscale schemes. Relevant mechanical fields computed in an FE simulation at the specimen scale are used as boundary conditions for the micromechanical simulation, where the real microstructure is meshed from 3D X-ray images. These methods hence rely on an identification of material behavior at the macroscale, say, using force measurements and 2D surface images. In an earlier work by the authors, a method for conducting micromechanical simulations using measured boundary conditions thanks to Digital Volume Correlation (DVC) was proposed. The interest of this DVC-FE approach is that it uses solely 3D X-ray images acquired in-situ during the experiments. Thus, FE simulations are directly conducted at the microscale, with no dependence on specimen scale simulations or multiscale schemes. This method also includes a methodology to perform local error measurements with respect to experimental observations. In this paper, both multiscale schemes and this DVC-FE approach are applied to new experimental results on a nodular cast iron specimen with machined holes. Ductile fracture due to the nucleation, growth and coalescence of microscopic voids between the machined holes is observed in-situ thanks to synchrotron 3D imaging. The objective of this paper is to assess the accuracy of boundary conditions for each approach and conclude on the optimal choice. Based on both average and local error measurements, it is shown that void growth is underestimated with multiscale schemes, while predictions are significantly improved with the DVC-FE approach

SUMMARYMechanical computations in multiphase domains raise numerous difficulties from the generation of the initial mesh to its adaptation throughout the simulation. All alternatives to mesh adaptation, such as level-set methods, have the well-known drawback of inducing volume conservation issues. In this paper, a moving mesh method is coupled to a topological mesh adaptation technique in order to track moving and deforming interfaces in multiphase simulations, with a robust control of mesh quality. Level-set functions are used as intermediaries to enhance the mesh adaptation technique with a volume conservation constraint, which is compatible both with implicit and with body-fitted interfaces. Results show that this method has the same advantage of permitting important displacements, deformations, and topological changes (coalescence of interfaces, for example) as a standard level-set method, while volume diffusion is drastically reduced.

A two-dimensional finite element (FE) model is presented to model the nucleation and void growth stages in ductile damage phenomena on the microstructure scale. This model is based on a level-set (LS) method coupled with an advanced re-meshing strategy. Both nucleation modes (interface debonding and inclusion fracture) are modelled through the introduction of micro-voids according to stress-based criteria. The LS method and mesh adaptation are used to accommodate the topology modification of the microstructure and to model multiple void nucleation and growth for different loading paths. The enhanced FE model is adopted to analyse the key features of the damage mechanisms on the micro-scale. The effects of inclusion orientation and of a complex loading path on nucleation and void growth are addressed. Good agreement is found with available experimental and numerical data found in the literature. The results exhibit that the loading path is a key point in damage growth. The proposed FE framework is an efficient technique to study damage phenomena on both simple and realistic microstructures. In the future, such an approach can be used to calibrate macroscopic ductile damage models for a complex loading path.

An identification framework is introduced herein to calibrate material parameters at the microscale in order to analyze ductile damage. It is applied to study a dog-bone sample, which is made of spheroidal graphite cast iron, loaded in tension and imaged via in situ microtomography. The region of interest is analyzed via Digital Volume Correlation (DVC) to measure kinematic fields. Finite Element (FE) simulations, which account for the studied microstructure that is explicitly meshed thanks its 3D image, are driven by Dirichlet boundary conditions extracted from DVC measurements. The plastic behavior of the ferritic matrix is calibrated via integrated DVC. The three mechanisms of ductile damage are then analyzed in view of the predictions of numerical simulations at the microscopic scale.