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This paper evaluates the brittle fracture risk for a C–Mn weld in the upper shelf of the brittle-to-ductile transition: the criterion considered is based on a critical stress σth, with the failure probability related to the volume around the crack where the maximum principal stress exceeds σth. The weld shows a complex microstructure with two types of melted zone. SEM observations showed that the main cleavage sites are located in the coarse grain zone, near an inclusion. In test analyses, the material’s heterogeneity in the weld metal is considered to estimate local fields at the weld pass scale.

International audience; In this paper, polycrystalline bi-phased microstructure calculations were performed using finite element (FE) method and compared to mechanical fields measured experimentally using a digital image correlation (DIC) technique. From scanning electron microscopic (SEM) observations and electron back-scattered diffraction (EBSD) crystal orientation measurements performed on an austenitic-ferritic stainless steel test specimen, a quasi-2D numerical polycrystalline microstructure was constructed. Mechanical behaviors of austenitic (FCC) and ferritic (BCC) grains were modeled by two crystal plasticity laws, based on crystallographic slips and dislocation densities. FE mechanical fields were calculated using two types of boundary conditions: the displacements measured by DIC, and the average of the displacements measured by DIC. A better description of intraphase and intragranular strain heterogeneities is obtained using as boundary conditions the displacements measured instead of the average of the displacements measured.

The low-cycle fatigue (LCF) damage micromechanisms are studied in a duplex stainless steel at room temperature using complementary analysis techniques. Surface damage is observed in real-time with an in-situ microscopic device during a low-cycle fatigue test. Slip systems activated in each grain in each phase are identified from SEM photographs and EBSD measurements. The surface relief appeared at the end of the test is measured with an interferometric profilometer. Displacement and strain fields on the microstructural scale are calculated using DIC technique from surface images taken during cycling. Observations were combined to analyse damage mechanisms from slip marking appearance to strain localisation and crack initiation.

International audience; This paper focuses on the identification of activated slip systems in low-cycle fatigue (Δεt/2 = 5 × 10−3) in a duplex stainless steel. From electron backscattered diffraction measurements and scanning electron microscopy observations, the slip systems and their associated Schmid factor are analyzed in both constitutive phases. In austenitic grains, one or two slip systems are activated with Schmid factors greater than 0.25. While in the ferritic grains, several slip systems are activated, with a variety of Schmid factors.

The cyclic behavior of type DIN 1.4460 duplex stainless steels in as-received and aged conditions was investigated at room temperature and at 500°C. At room temperature, the aged samples showed embrittlement effects such as loss of toughness and reduction of fatigue life. At 500°C, beneficial effects of the synergy between temperature and strain cycling were observed. It is proposed that at high temperature in the ferrite the strain cycling can decompose the chemical composition fluctuations, promoting a demodulation of the spinodal decomposition formed by aging.

The correlation between the low-cycle fatigue behavior and microstructure of SAF 2507 duplex stainless steel has been studied. Thecyclic behavior has been analyzed through the cyclic flow stress components, i.e. the effective and the internal ones, as well as the acti-vation volume parameter. A change in the deformation mechanism occurs at De p = 1.5 · 10 3 , where the accommodation of the plasticstrain changes from the reversible to the irreversible bow-out of dislocations giving rise to a continuous increase of the internal stress. Theanalysis of the effective stress has shown that the interaction between nitrogen atoms and dislocations can be explained through a modelfor weak obstacles. Finally, above De p P 6.0 · 10 3 , dislocations on two {111} planes in the austenite can interact and form a well-developed cell structure, which increases the hardening of the phase. Consequently, the typical softening process characteristic of duplexsteels diminishes with an increase in plastic strain amplitude. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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.

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 tests performed in the temperature range 300773 K on duplex stainless steel DIN 1.4460 show that the cyclic stressstrain behaviour of this steel is strongly temperature dependent. At 300 and 473 K an almost constant peak tensile stress stage, is followed by a slight softening that continues up to failure in the case of 300 K, but by a secondary hardening at 473 K. Pronounced initial cyclic hardening followed by secondary hardening was the main feature of the temperature range between 573 and 723 K. At 773 K, after a weak hardening stage, a strong softening continues up to failure. The mechanical behaviour and the evolution of the microstructure were analysed, and the internal and the effective stresses were studied. It was found that the internal stress is responsible for the strong hardening that occurs in the intermediate temperature range and for the softening at 773 K.

Stress–strain hysteresis loops of a ferritic stainless steel during cyclic straining with increasing strain amplitudes were recorded and the shapes of the hysteresis loops were analyzed. An extra strain around zero stress as a result of magnetoelastic effect was identified. Its effect on the determination of the effective and internal stresses in cyclic straining is discussed.

The low-cycle fatigue behaviour of a cast metal matrix composite has been studied experimentally and numerically. Tension–compression cyclic tests have been carried out in situ in a scanning electron microscope to observe damage mechanisms of a AS10U3NG alloy based composite reinforced with 10 vol.% SiCp. Micro-cracks always initiate on the specimen surface, on the biggest casting defects. Then the small cracks mainly propagate in the eutectic zone by precipitate fractures or by either particle/dendrite or particle/precipitate debondings. One of these cracks becomes the main crack. When the main crack reaches a ‘critical length', it tends to grow in mode I through the dendrites and by particle fracture in front of the crack. As far as the numerical study is concerned, a finite element analysis models the composite behaviour under tension–compression cyclic loading, before crack initiation, until a stabilised cycle is reached. Numerical and experimental stress–strain hysteresis loops are compared. Numerical and experimental cyclic stress–strain curves are in a good agreement.

International audience; As other metallic materials, in low-cycle fatigue, duplex stainless steels (DSS) exhibit a cyclic hardening, followed by a cyclic softening, before stabilization of the stress. In order to simulate the cyclic hardening/softening curves in low-cycle fatigue of an austenitic–ferritic or duplex stainless steel (DSS), a new polycrystalline model is proposed. The polycrystalline model developed by (Cailletaud, 1992) and (Pilvin, 1990) and modified by Hoc and Forest (2001) was previously extended in Evrard et al. (2008) in order to take into account the bi-phased character of the DSS. This model correctly accounts for the cyclic hardening, but it is not able to simulate the cyclic softening, consequently, stresses at the stabilized state are overestimated. TEM observations of the dislocation structures built during a cyclic uniaxial tension/compression test show that, during the cyclic hardening, planar arrangements are observed in austenitic grains and no significant evolution is observed during the subsequent cyclic softening and stabilization stage. On the contrary, in ferritic grains, dislocations are homogeneously distributed during cyclic hardening, and the microstructure evolves during the subsequent cyclic softening and stabilization stage. Dislocation structures build progressively, consisting of hard zones or walls, separated by soft zones or channels. We propose to model the cyclic softening through dislocation structure evolution within ferritic grains. The single crystal law used by Hoc and Forest (2001) is modified in order to take into account the heterogeneous distribution of dislocations in the ferrite. Numerical simulations are compared with experimental data. A good agreement is observed between experimental and calculated hardening/softening curves and stabilized hysteresis loops.

International audience; The present paper proposes to modify the single crystal law [Hoc, T., Forest, S., 2001. Polycrystal modelling of IF-TI steel under complex loading path. Int. J. Plast. 27, 65–85] used in polycrystalline model developed by [Pilvin, P., 1990. Approches multiéchelles pour la prévision du comportement anélastique des métaux. Ph.D. Thesis, Université Pierre et Marie Curie; Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. Int. J. Plast. 8, 55–73], in order to account for the cyclic softening observed in some metallic materials during fully reversed low-cycle fatigue tests under total strain control. Experimental observations [Mayama, T., Sasaki, K., 2006. Investigation of subsequent viscoplastic deformation of austenitic stainless steel subjected to cyclic preloading. Int. J. Plast. 22, 374–390] show that the cyclic softening is partially due to a rearrangement of dislocations in hard and soft zones within the grains during cyclic straining. We propose thus to modify the initial single crystal law by including a composite model of hard and soft zones on each slip system within a grain. The softening is modeled through the evolution of soft zones. The model is applied to a FCC polycrystal. The self-consistent scheme is used in order to obtain the polycrystalline macroscopic behavior. The results show that the new model correctly describes an initial cyclic hardening, a subsequent cyclic softening and finally a stabilized state.

The aim of the present paper is to propose a polycrystalline approach in order to model the elastic–plastic behavior of an austenitic–ferritic stainless steel. In order to take into account the specific character of the steel, the multi-scale polycrystalline approach proposed by Cailletaud–Pilvin [Pilvin, P., 1990. Approches multiechelles pour la prevision du comportement anelastique des metaux, PhD Thesis, Universite Pierre et Marie Curie; Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. International Journal of Plasticity 8, 55–73; Cailletaud, G., Pilvin, P., 1994. Utilisation des modeles polycristallins pour lecalcul par elements finis. Revue Europeenne des Elements Finis 3 (4), 515–541] is extended to bi-phased material. In particular, two interaction laws and two local behaviors, based on the crystallographic slip and the dislocation density evolution, are simultaneously considered. After an identification of the model parameters on simple tests (monotonous tension, tension-compression), we propose an evaluation of the predictive capabilities of the multi-scale approach for non-proportional loading paths. A good agreement is observed between simulation and experimental data.

Strain field measurements by digital image correlation today offer new possibilities for analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites.

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.

The present work studies the effect of the microstructure and of the stress gradient on the early fatigue crack growth in a two-phase austenitic-ferritic stainless steel. Fatigue tests were performed on two geometries of double edge notched specimens. Direct optical observations of the surface are used to measure the surface crack growth rates. The results are presented and the effects of stress and strain gradients on the crack propagation kinetics are discussed.

Bauschinger effect in an AISI 316L austenitic stainless steel at room temperature was studied as a function of the plastic prestrain. Typical uni-axial Bauschinger tests in tension–compression and in compression–tension were carried out. Internal and effective stresses were evaluated from the experimental stress–strain curves using the method proposed by Kuhlmann-Wilsdorf and Laird. Four constitutive relations derived from a thermodynamical formulation were identified using an inverse identification method. The modelled stress–strain curves were then compared to the experimental ones. Bauschinger effect was characterised using Bauschinger indicators proposed in the literature in terms of strain, stress and energy, respectively. These indicators were evaluated from the experimental stress–strain curves on the one hand, from the modelled ones on the other hand. Their variations as functions of prestrain were then compared. The best constitutive relation uses two non-linear kinematic work-hardening terms.

A thermal fatigue device––called SPLASH––similar to the facility developed by Marsh [Fatigue crack initiation and propagation in stainless steels subjected to thermal cycling, International Conference on Mechanical Behaviour and Nuclear Applications of Stainless Steels at Elevated Temperature, 1981] has been built in CEA/SRMA in 1985. Since then, it was used mostly on austenitic stainless steels to assess the initiation and growth of thermal fatigue crack networks. In 1998, a leak appeared in an auxiliary loop of the primary circuit of a pressurized water nuclear plant in Civaux (France). Thermal fatigue was suspected and studies began on AISI 304 L type austenitic stainless steel. They were eventually compared to results obtained earlier on AISI 316 L(N). First, the initiation conditions were determined and the damage before initiation was qualitatively observed. Then, some crack networks parameters were chosen and quantitatively determined by image analysis. This part of the study was done at the surface, during crack growth, and at the end of the tests, in depth. Finally, the stability of the crack networks obtained by thermal fatigue was tested under isothermal load controlled four point bending fatigue test, and some conclusions were drawn on the mechanisms of propagating crack selection.

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.

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.

Unidirectional and multilayer composites are usually subjected to bending, and often fail in fatigue by interlaminar shearing. This work studies new experimental conditions in fatigue, using a three-point-bending test with predominant shearing, while tensile/compressive stresses are significant. The experimental device geometry is validated under monotonic loading by comparison between finite element results and experimental observations. The deflection amplitude vs. fatigue life curve is used as a pseudo-Wöhler curve. An approximate drawing of maximum deformation vs. fatigue life curve is proposed using Fournier's model. Experimental results in fatigue are well accounted for by this model.

Tensile and low-cycle fatigue tests were performed on a 316LN austenitic stainless steel at 300 and 77 K. The tensile and low-cycle fatigue properties were obtained and analysed in terms of influence of temperature on the plastic deformation process, and particularly on the strain-induced martensite formation. The martensite content was measured by a magnetic-at-saturation method. No martensite was detected at 300 K. On the contrary, strain-induced martensite transformation is responsible for the higher tensile elongation at 77 K and for the secondary hardening observed on softening-hardening curves in low-cycle fatigue at 77 K. The induced martensite content in tensile tests is a function of the strain according to Angel's model, and in low-cycle fatigue it is a function of the strain level and of the accumulated plastic strain.