Kacem Saï

Physical basis for model with various inelastic mechanisms for nickel base superalloy

Abstract Two mechanical behaviour models for N – 18 alloy are proposed. The material is a powder metallurgy nickel base superalloy hardened by 60% volume of the ordered γ′ phase. The behaviour of alloy N – 18 is modelled by classical constitutive equations involving plasticity and creep. The experimental data used include stress relaxation and creep tests. An updated version of the first model is proposed and compared to the experimental data set. A new model is also presented with equations based on physical concepts. Material parameter identification is performed for each model, and experimental results are in good agreement with theoretical simulations.

Anisotropic (Continuum Damage Mechanics)-based multi-mechanism model for semi-crystalline polymer

In this work, the anisotropic damage of semi-crystalline polymers is investigated. The model, developed within a thermodynamic framework, includes the following features: (i) the degree of crystallinity; (ii) the hydrostatic pressure effect; and (iii) the damage anisotropy. The adopted tensorial damage variable is based on the Continuum Damage Mechanics approach under the energy equivalence assumption. For the quantification of the anisotropy, a parameter called “shape factor” is defined as the ratio between the void mean diameter and the void mean height. This parameter is linked to the main axial and the main radial damage components. Experimental data taken from the recent literature using the tomography technique were selected to assess the model capability. Finite element simulations of notched round bar specimens subjected to tensile test stopped at three key loading stages are systematically compared with experimental data. The proposed model was able not only to accurately simulate the macroscopic response of the material, but more interestingly, to reproduce the spatial distribution of the shape factor. This demonstrates the anisotropy effects of the material under study induced at different stages of the deformation.

Micromechanical Modeling of the Ratcheting Behavior of 304 Stainless Steel

Numerical simulations of 304 austenitic stainless steel (SS304) cyclic and ratcheting responses are performed using polycrystalline plasticity models. On the basis of the polycrystalline model of Cailletaud and Pilvin (1994, “Utilisation de modeles polycristallins pour le calcul par elements finis,” Rev. Eur. Elem. Finis, 3, pp. 515–541), a modification of the β rule that operates the transition between the macroscopic level and the grain level is proposed. The improvement of the transition rule is obtained by introducing a “memory variable” at the grain level, so that a better description of the local stress–strain behavior is provided. This new feature is calibrated by means of previous simulations using finite element (FE) aggregate models. The results of the updated polycrystalline plasticity model are in good agreement with the macroscopic responses.

Capabilities of the Multi-mechanism Model in the Prediction of the Cyclic Behavior of Various Classes of Metals

The paper deals with an evaluation of the multi-mechanism (MM) approach capabilities in the prediction of the cyclic behavior of different classes of metallic materials. For this objective, the tests detailed in (Taleb, Int J Plast 43:1–19, 2013a) have been simulated here by the MM model. In these tests, six alloys were considered: two ferritic steels (35NCD16 and XC18), two austenitic stainless steels (304L and 316L), one “extruded” aluminum alloy (2017A) and one copper-zinc alloy (CuZn27). The specimens have been subjected to proportional and non-proportional stress as well as the combination of stress and strain control at room temperature. The identification of the material parameters has been carried out using exclusively strain controlled experiments under proportional and non-proportional loading paths performed in the present study for each material. The model may describe a large number of phenomena with twenty five parameters in total but, it appears that for a given material under the adopted conditions, the activation of all parameters may be not necessary. Our attention was focused mainly on the capabilities to predict correctly the cyclic accumulation of the inelastic strain including the shape of the hysteresis loops. The comparison between test responses and their predictions by the MM model are generally satisfactory with relatively small number of material parameters (between eight and thirteen according to the material). One can also highlight the capability of the MM model to describe a transient ratcheting without activation of the dynamic recovery term in the kinematic variables. Finally, the MM model deserves improvement for a better description of the cyclic behavior of anisotropic materials.

Finite element modelling of burnishing process

Abstract Burnishing is a process of finishing plane or cylindrical surfaces by plastic deformation. It employs a ball or rolling tool pressed against the workpiece. It does not only improve surface finish but also generates compressive residual stresses throughout the surface layers. In this study, a finite element (FE) modelling was performed to provide a fundamental understanding of the burnishing process on a carbon steel workpiece. The simulation results were focused on surface roughness, residual stresses and the influence of burnishing parameters (feedrates and penetration depth) on surface roughness and residual stress. The authors have noted that burnishing improves surface quality and introduces compressive residual stresses. These results were successfully compared with experimental data which were obtained some years ago by the authors.

Multi-mechanism modeling of inelastic material behavior

This book focuses on a particular class of models (namely Multi-Mechanism models) and their applications to extensive experimental data base related to different kind of materials. These models (i) are able to describe the main mechanical effects in plasticity, creep, creep/plasticity interaction, ratcheting extra-hardening under non-proportional loading (ii) provide local information (such us local stress/strain fields, damage, ….). A particular attention is paid to the identification process of material parameters. Moreover, finite element implementation of the Multi-Mechanism models is detailed.

Applications of Multi-Scale Models to Numerical Simulation and Experimental Analysis of Anisotropic Elastoplastic Behavior of Metallic Sheets

In this paper anisotropic mechanical behavior of AA2024 aluminum and Ti6Al4V titanium alloys were studied using three different approaches: unified, multi-mechanism and polycrystalline. The theoretical formulations of studied elastoplastic models are first described. Thereafter, some numerical results concerning the simulation of a uniaxial tension test applied to thin metallic sheets are presented. Comparison between experimental results (taken from the literature) and numerical simulations shows that the multi-mechanism and polycrystalline models describe slightly better the anisotropy when considering all the directions. Finally, numerical simulations of a deep drawing test of AA2024 aluminum thin sheets will be analyzed.

Finite Element Analysis of PMMA Stretch Blow Molding

The electric bubbles are a useful product made of PMMA material. They are produced by the stretch blow molding process. Thickness, which reflects the quality of the electric bubble, is a crucial parameter that deserves special attention for the molding process. In this work, finite element simulations of the stretch blow molding process are performed aiming at the determination of the preform geometry to ensure homogeneous thickness of the finished product. The geometrical parameters of the preform are optimized allowing a better homogeneity thickness compared to existing data. The predicted parameters allow the improvement of the thickness distribution. The standard deviation of the thickness is reduced to about 95% compared to the existing bubble.