Modesar Shakoor

A new finite element approach for modelling ductile damage void nucleation and growth—analysis of loading path effect on damage mechanisms

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.

Advances in Level-Set Modeling of Recrystallization at the Polycrystal Scale - Development of the Digi-μ Software

The mechanical and thermal properties of metallic materials are strongly related to theirmicrostructure. An accurate and quantitative prediction of microstructural evolutions is then crucialwhen it comes to optimize the forming process. Recently a new full field approach, based on a Level-Set (LS) description of interfaces in a finite element (FE) context has been introduced to model 2D and3D primary recrystallization (ReX), including the nucleation stage [1, 2], and has been extended to takeinto account the grain growth (GG) stage [3, 4]. The ability of this approach to model also the Zenerpinning (ZP) phenomenon without any assumption concerning the shape of second phase particleswas also demonstrated [5]. Moreover, recent developments have also illustrated the capability of thisapproach to take into account the characteristics of twin interfaces during grain boundary motion [6,7]. Current work concerns also the improvement of the numerical cost of this new approach [8]. Allthese developments are necessary to account for the microstructural complexity of ReX phenomenon.

Experimental-Numerical Validation Framework for Micromechanical Simulations

A combined experimental-numerical framework is presented in order to validate computations at the microscale. It is illustrated for a flat specimen with two holes, which is made of cast iron and imaged via in situ synchrotron laminography at micrometer resolution during a tensile test. The region in the reconstructed volume between the two holes is analyzed via Digital Volume Correlation (DVC) to measure displacement fields. Finite Element (FE) simulations, whose mesh is made consistent with the studied material microstructure, are driven by measured Dirichlet boundary conditions. Damage levels and gray level residuals for DVC measurements and FE simulations are assessed for validation purposes.

Numerical modeling of ductile fracture at the microscale combined with x-ray laminography and digital volume correlation.

Predicting ductile fracture for complex loading paths is essential within the framework of metal forming processes. Most models are developed and used at the macroscopic scale and do not account explicitly for material microstructures. This paper describes a methodology aiming at understanding and modeling ductile damage mechanisms at the microscale. This methodology relies on the acquisition of X-Ray laminography pictures during in-situ tensile tests, digital volume correlation (DVC) to measure 3D displacement and strain fields in the bulk and 3D finite element (FE) modeling of the heterogeneous microstructure including ductile damage mechanisms. The methodology is illustrated on nodular graphite cast iron. FE simulations of the heterogeneous microstructure are conducted and compared with DVC results and the influence of boundary conditions is discussed.