Véronique Aubin

Cyclic plasticity of a duplex stainless steel under non-proportional loading

The low-cycle fatigue behavior of a duplex stainless steel, 60 % ferrite - 40 % austenite, is studied under tension-compression/torsion loading at room temperature. The influences of loading direction and loading path are analyzed. It is shown that the duplex stainless steel has an isotropic behavior under tension-torsion loading in monotonic as well as in cyclic conditions. The loading path induces an over-hardening on cyclic hardening of duplex stainless steel, but lower than the one on austenitic stainless steels. The effect of loading history is studied in terms of strain amplitude, mean strain and loading path. It is shown that only histories in strain amplitude and loading path have a small effect on the stabilized stress.

Cyclic behaviour of a duplex stainless steel under multiaxial loading: Experiments and modelling

Abstract The low-cycle fatigue behaviour of a duplex stainless steel, 60 % ferrite — 40 % austenite, is studied under tension-compression/torsion loading at room temperature. It is shown that the duplex stainless steel has an isotropic behaviour under cyclic proportional loading. The non-proportional loading paths induce an extra-hardening, but lower on duplex stainless steel than on austenitic stainless steels. Three models able to account for the extra-hardening are identified and tested on the experimental data base. Two of them give accurate predictions.

Prediction of the Scatter of Crack Initiation under High Cycle Fatigue

Under fatigue loading, the number of cycles to failure and its associated scatter increase when the loading level decreases. The High-Cycle Fatigue (HCF) regime is thus characterized by a large scatter in the number of cycles to failure [1]. Cracks initiation represents an important part of the lifetime of the structures. A stochastic method is used to study the fatigue crack initiation prediction in the 316L austenitic stainless steel. The present work proposes to show that this scatter can be attributed to the random orientation of individual grains, which influences the crack initiation localization. The stresses in grains are determined by finite element computations (FEM [2]), using a configuration representative of a polycrystalline aggregate. This approach takes into account the crystallographic orientations of the grains in the aggregate as well as the deformation incompatibilities between neighbouring grains due to crystalline anisotropic elasticity and elasticplasticity [3]. Then, the scatter of the number of cycles to crack initiation is derived from the FEM stress fields using two fatigue crack initiation criteria: an usual one, Mura’s criterion [4] and a more recent one [5], based on Discrete Dislocation Dynamics (DDD) simulations and taking into account plastic slips, cross slip and stress tensor components.

Cyclic Softening Behaviour of a Duplex Stainless Steel: Microstructural Origin and Modelling: Part I: Correlation between Cyclic Behavior and Dislocation Microstructure*

Abstract This first part aims to study the correlation between the cyclic softening observed during low-cycle fatigue in a 2507 duplex stainless steel (DSS) alloyed with 0.17% N and the dislocation structure development. Tension-compression and tension-compression- torsion fatigue tests were carried out under total strain control at Δεteq/2 = 0.5%. The dislocation structures were characterized by transmission electron microscopy during cycling with interrupted tests. The austenite showed planar arrangements of dislocations during all the fatigue life, the ferrite showed evolution from a homogeneous distribution of dislocations to a structure of dislocation bundles in tension-compression, and to a refined wall structure in tension-compression-torsion. It is proposed that the evolution of the dislocation structure in the ferrite plays an important role during the cyclic softening in the DSS.

Dislocation Structures of Duplex Stainless Steel in Uniaxial and Biaxial Cyclic Loading

Austenitic-ferritic duplex stainless steel has been subjected to uniaxial and biaxial nonproportional cyclic loading with the same equivalent strain amplitude. The dislocation structures in specimens fatigued to fracture using both types of loadings were studied and compared. Uniaxial cyclic loading, both in austenitic and in ferritic grains, produces simple structures due to activation of predominantly one slip system. Non-proportional cyclic loading results in formation of cell and wall structures and thus in higher stress response of the material.

Yield Surface and Complex Loading Path Simulation of a Duplex Stainless Steel Using a Bi-Phase Polycrystalline Model

In order to model the elasto-viscoplastic behaviour of an austenitic-ferritic stainless steel, the model initially developed by Cailletaud-Pilvin [1] [2] and used for modeling single-phase polycrystalline steel is extended in order to take into account the bi-phased character of a duplex steel. Two concentration laws and two local constitutive laws, based on the crystallographic slips and the dislocation densities, are thus simultaneously considered. The model parameters are identified by an inverse method. Simple tests among which tension test at constant strain rate and at different strain rates and uniaxial tension-compression test are used during the identification step. The predictive capabilities of the polycrystalline model are tested for non-proportional loading paths. It is shown that the model reproduces the over-hardening experimentally observed for this kind of loading paths. Then, yield surfaces are simulated during a uniaxial tension-compression test: it is shown that the distortion (i.e. plastic anisotropy induced by loading path) is correctly described.

Load History in Fatigue: Effect of Strain Amplitude and Loading Path

The low-cycle fatigue behavior of a duplex stainless steel, 60 % a - 40 % y, is studied under tension-compression/torsion loading at room temperature and under strain control. It is shown that the duplex stainless steel has an isotropic behavior under cyclic proportional loading. The loading path induces an extra-hardening on cyclic hardening of duplex stainless steel but lower than that on austenitic stainless steels. The effect of loading history is studied in terms of strain amplitude, mean strain, and loading path. It is shown that only histories in strain amplitude and loading path have an effect on the stabilized stress.

Cyclic Softening Behaviour of a Duplex Stainless Steel

Abstract This second part aims to propose an extension of the bi-phased polycrystalline model shown in a previous contribution [1], in order to simulate the cyclic softening observed in an austenitic-ferritic stainless steel under uniaxial and non-proportional biaxial cyclic loading [2]. Assuming that the cyclic softening for uniaxial and biaxial non-proportional cyclic loading is due to a rearrangement of dislocations in hard (rich in dislocations) and soft (poor in dislocations) zones in ferritic grains [3], the ferritic single crystal law is modified. The new model parameters are identified from TEM observations [3] and by inverse procedure. Finally, the new model is validated for other cyclic loading paths. A good agreement is observed between simulation and experimental data.

Micromechanisms of Damage in Multiaxial Fatigue of an Austenitic-Ferritic Stainless Steel

Austenitic-ferritic stainless steels, also called duplex stainless steels, are designed to combine the properties of both ferritic and austenitic phases in order to optimize mechanical strength, ductility and corrosion resistance. They are therefore used for components submitted to corrosive environments as for instance in the chemical, petrochemical or nuclear industry or for the transport of chemical products.

Introduction au numéro thématique

Les structures industrielles sont le plus souvent des assemblages de composants de propriétés mécaniques différentes. Le choix du mode d’assemblage est guidé par des considérations techniques comme le poids, l’encombrement, la compatibilité des matériaux, la résistance de la structure... associées à des considérations économiques comme le coût. La prise en compte de l’assemblage dans un calcul de structure pose encore de nombreux problèmes aux industriels. En effet, bien que le réel ignore les assemblages, ceux-ci constituent des zones d’hétérogénéités et/ou de discontinuités avec de forts gradients de microstructures et de sollicitations, où se localiseront des problèmes susceptibles d’avoir de lourdes conséquences sur la fiabilité mécanique des structures. L’étude de la tenue des assemblages nécessite une approche pluridisciplinaire, multi-échelle, et la prise en compte des assemblages dans un calcul de structure nécessite des calculs fortement couplés entre la thermique, la métallurgie, la chimie, la mécanique... Un certain nombre de travaux de recherche sont développés depuis quelques années autour de la mécanique des assemblages. Le colloque MECAMAT qui s’est tenu à Aussois en janvier 2003, intitulé Assemblages : des matériaux à la structure , se proposait de faire le point sur l’état de l’art de la prise en compte, dans un calcul de structure, des différents problèmes mécaniques soulevés par les assemblages, depuis leur élaboration jusqu’à leur utilisation. Le colloque traitait des joints soudés, des joints collés, des assemblages par contact (rivetage, boulonnage, vissage), mais aussi d’exemples d’assemblages multi-matériaux plus spécifiques. Des développements de techniques de modélisation spécifiques, nécessaires aux calculs des structures assemblées, étaient également présentés. Le présent numéro thématique de la revue Mécanique & Industries se propose d’offrir un panel aussi large et représentatif que possible des travaux présentés à ce colloque.