Lakhdar Taleb

A micromechanical modeling of the Greenwood¿Johnson mechanism in transformation induced plasticity

Abstract An experimental analysis of transformation induced plasticity (TRIP) considering bainitic transformation in a 16MND5 steel (SA508Cl3 in ASTM norm) has been reported recently in the International Journal of Plasticity. Its main results have shown that among the existing models used in practical applications, the one proposed by Leblond 10 years ago seems to be one of the models which describes the more accurately the phenomenon. It was also noticed that this model leads to a singularity in the description of the TRIP at the beginning of the transformation under low applied stress. This singularity is usually avoided by the introduction of an arbitrary threshold. The TRIP norm at the end of the transformation seems also slightly overestimated by this model. The work which is presented here is devoted to an investigation of these discrepancies from a reevaluation of the micromechanical model as originally used by Leblond et al. A more complete formulation taking into account the elasticity in both phases will be developed and solved resulting in an improved model enabling a better description of the experimental results and removing the singularity mentioned above. A comparison between experimental results and the predictions of the proposed model is given.

Experimental analysis of transformation plasticity

Abstract Transformation plasticity is an irreversible strain observed when metallurgical transformation occurs under small external stress lower than the yield stress of the weaker phase. This paper is devoted to an experimental analysis of that phenomenon in ferrous alloys. The particular case of the steel which composes vessels in French nuclear reactors is considered. This steel is called 16 MND 5 and A 533 respectively in AFNOR and ASTM norms. The main results proposed in literature concerning transformation plasticity on this material were analyzed and perfectible aspects are pointed out. The results of several performed tests are analyzed. The transformation plasticity coefficient, the kinetics as well as the dependence on the norm and the direction of the applied stress are particularly studied. The experimental results are compared to the predictions of the main existing models.

On the validation of the methods related to cyclic behavior of metallic structures

Abstract This work includes two ‘classical’ experimental and numerical parts. In the first part, an experimental device has been carried out and two tests have been performed on a structure subjected to a biaxial prescribed force (tension and torsion) and cyclic thermal loadings. The effect of short term mechanical overloads in tension is also analyzed with the objective to check the validity of an approach proposed previously in order to take into account that effect. The numerical part has two goals : evaluate the ability of Chaboches viscoplastic model to describe the overloads effect and compare the assessment of some simplified methods to our experimental results. We have considered two categories of methods. The first concerns the methods based on the elastic analysis (3 Sm and efficiency rules), the second category is related to some more recent methods based on an elastic analysis combined with an elastoplastic analysis limited to the first cycle (Gatts and Talebs methods). The following results have been obtained: • The forecasts using the approach proposed previously in order to describe the primary overloads effect is confirmed by our tests. • The Chaboches model has a tendency to overestimate the evolution of the strain increments induced by the successive overloads. • The predictions given by some methods based on elastic analysis (the efficiency rule in particular) do not always seem conservative. On the other hand, Gatts and particularly Talebs methods give some conservative forecasts for the studied cases.

On the Cyclc Behavior of 304L and 316L Stainless Steels

This work is devoted to a survey of the ratcheting phenomenon and follows the study presented in [1]. Two current austenitic stainless steels 304L and 316L are considered and tested under cyclic stress and strain control at 350°C. Under stress control ratcheting seems very small under proportional as well as non-proportional stress control. The elastic shakedown steady state exhibited by both materials may be explained by their capabilities to develop very significant cyclic hardening (especially isotropic) at 350°C.

Effect of the random spatial distribution of nuclei on the transformation plasticity in diffusively transforming steel

The γ → α diffusive phase transformations of steels can lead to transformation plasticity (TRIP) if accompanied of an external loading stress or a pre-hardening of the parent phase. The most current approaches of its modelling are based on the Greenwood-Johnson mechanism; they call upon hypotheses (about microstructure morphology, constitutive laws...) which are at the origin of discrepancies between predictions and experimental observations in particular loading cases. Some of these restricting hypotheses can be eliminated with a micromechanical finite elements approach, where the elastoplastic interactions between phases are determined at the micro-scale of a volume element containing hundreds of growing particles (Barbe et al., 2005, 2008). This paper deals with the effect of the spatial distribution of product phase nuclei on the global kinetics and transformation-induced plasticity (TRIP) in the volume element. Two distributions are considered: uniform random and at preferential nucleation sites of a Voronoi tesselation mimicking austenite microstructure.

Effects of the austenite grain size on transformation plasticity in a 35 NCD 16 steel

In this work, the influence of the austenitic grain size (AGS) on the transformation induced plasticity (TRIP) was experimentally studied. The study was undertaken in the case of the martensitic transformation of a 35NCD16 steel. Transformation induced plasticity tests (TRIP tests) were carried out under various conditions of austenitisation leading to different grain sizes of austenite. Two studies have been performed leading to different observations. In the first study, where the heating rate during austenitisation was relatively fast (50°C/s), the results seem indicate that the transformation induced plasticity is higher when the austenitic grain size is smaller (Boudiaf et al., 2008). In the second study we have considered a slower heating rate during austenitisation (0.5°C/s) on specimens manufactured from another batch of the same steel. Different results were then obtained where TRIP increases slightly with the grain size. However, the uncertainties pointed out in assessing TRIP incite to be more careful. The work is still in progress in order to clarify the observed difference evaluating in particular the role of the heating rate during the austenitic transformation.

Effect of Pre-Deformation on the Cyclic Behavior and Fatigue of 304L SS

This study deals with the effect of the loading history on the cyclic behavior and the fatigue life of a 304L stainless steel at room temperature. The experiments have been performed using two specimens’ categories. The first one (virgin) has been submitted to only classical fatigue tests while in the second category, prior to the fatigue test; the specimen was subjected to a pre-hardening process under either monotonic or cyclic strain control. Cyclic softening followed by cyclic hardening are observed for the virgin specimens while only cyclic softening is exhibited by the pre-hardened specimens. The obtained results show that fatigue life is strongly influenced by the pre-hardening: the latter seems beneficial under stress control but detrimental under strain control, even in the presence of a compressive mean stress. The results are discussed regarding the cyclic evolution of the elastic modulus as well as the isotropic and kinematic parts of the strain hardening in different configurations: with or without pre-hardening, stress or strain control.

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.

Experimental Analyses about Ratcheting of Extruded 2017A Aluminum Alloy Subjected to Proportional and Non-Proportional Loadings

This work deals with the study of ratcheting for extruded 2017A aluminum alloy subjected to different loading conditions: unsymmetrical one-dimensional stress control as well as multi-dimensional combinations of stress-stress or stress-strain control. All tests have been conducted within the small strain assumption conditions at room temperature. The results point out the absence of ratcheting under unsymmetrical torsional stress control which enlarge the result given in (Taleb, L., 2013, Int. J. Plast., 43 (2013), 1-19) in which the absence of ratcheting under unsymmetrical tension-compression is demonstrated for the same material. However, under constant axial stress and cyclic shear strain, ratcheting is clearly observed. The results may be explained mainly by the evolution of the isotropic cyclic hardening exhibited by the material.

Analyses about the Anisotropic Behavior of a 2017A Aluminum Alloy

The study presented in this paper uses a multi-mechanism model (MM) in order to simulate the cyclic behavior of an anisotropic aluminum alloy 2017A subjected to complex loading. Two sets of parameters were used. The first set is proposed in [1]; it is identified considering cyclic tests performed under strain control following proportional and non-proportional paths. The second set of parameters has been identified recently on a larger database [2]. In this work, we propose to evaluate the capability of the according to the set of parameters under consideration. Note that the identification process in both cases was performed using strain controlled experiments while the evaluation of the model uses stress controlled experiments.