The hardening and the embrittlement under neutron irradiation of an A508 type RPV steel considering three different microstructures (bainite, bainite-martensite and martensite)have been investigated These microstructures were obtained by quenching after autenitization at 1100 degrees C. The irradiation induced hardening appears to depend on microstructure and is correlated to the yield stress before irradiation. The irradiation induced embrittlement shows a more complex dependence. Martensite bearing microstructures are more sensitive to non hardening embrittlement than pure bainite. This enhanced sensitivity is associated with the development of intergranular brittle facture after irradiation; the pure martensite being more affected than the bainite-martensite. It is of interest to note that this mixed microstructure appears to be more embrittled than the pure bainitic or martensitic phases in terms of temperature transition shift. This behaviour which could emerge from the synergy of the embrittlement mechanisms of the two phases needs further investigations. However, the role of microstructure on brittle intergranular fracture development appears to be qualitatively similar under neutron irradiation and thermal ageing. (C) 2015 Elsevier B.V. All rights reserved.

A comparative experimental and FEM study has been carried out, in order to investigate dynamic and constraint aspects of the Charpy test. Standard V-notch Charpy specimens were tested under dynamic and static loading conditions. 2-D plane strain and 3-D models were employed in numerical analysis. In order to incorporate strain-rate effects, an elastic-viscoplastic constitutive equation has been applied, based on actual test data obtained for a low-alloy structural steel. Fully dynamic analysis clearly indicated inertial effects. Modal analysis enabled the confirmation of the origin of the oscillations on the load-displacement curve as beam vibration of the specimen resulting from interaction with the elastic striker. (C) 1999 Elsevier Science S.A. All rights reserved.

Ferritic-martensitic steels are prime candidate materials for future reactors. We present here the results of a study on the effects of helium implantation on the fracture behavior of 9Cr (T91) martensitic steels. Three-points static bending tests were performed at room temperature on implanted specimens and at −170°C on un-implanted material. All these tests led to brittle fracture. Based on a mechanical analysis of the tests results using Finite Element calculations, we have proposed that the mechanism of brittle fracture is controlled by a double criterion depending on implantation temperature and helium content. Furthermore, by applying the Beremin model, the toughness of helium implanted steel has been evaluated.

An experimental characterization was conducted of helium effects on the mechanical properties of a 9Cr martensitic steel. Six sub-size Charpy samples were implanted in the notch region at 250°C with 0.25at.% helium and subsequently tested in 3-point bending at room temperature. Brittle fracture mode (cleavage and intergranular fracture) was systematically observed in the implanted zones of the samples. Finite element calculations of the tests, using as input the tensile properties measured on a helium loaded sample, were performed in order to determine the fracture stress at the onset of brittle crack propagation. Preliminary TEM investigations of the implantation-induced microstructure revealed a high density of small helium bubbles.

In order to investigate helium effects on the fracture properties of martensitic mod 9Cr–1Mo (T91) steel, miniature Charpy specimens were implanted at 250°C in the notch region to 0.25at.% helium using a degraded 34MeV 3He ion beam and subsequently submitted to static bending tests at room temperature. For the six implanted specimens, a ‘pop-in’ phenomenon, which is an arrested unstable crack extension, was systematically recorded during testing. In the implanted zones of the samples, the fracture mode was fully brittle with both intergranular and cleavage fracture, whereas for unimplanted samples tested at −170°C, the fracture mode was found to be 100% cleavage. Finite element simulations of the tests performed on unimplanted and implanted specimens were also carried out to determine stress and strain fields at the onset of crack propagation. Based on these computations, the fracture toughness of implanted T91 was tentatively evaluated using the Beremin model of the local approach to brittle fracture.

In this work the residual stress field in cracked fatigue specimens was determined by neutron-diffraction technique in order to quantify the influence of the different loading parameters on the fatigue crack growth. At the same time, some microstructural parameters, such as the average size of coherently diffracting blocks and the mean-square microstrain, were estimated by combining neutron and X-ray (synchrotron radiation) diffraction techniques.

Cleavage initiation in CVN (Charpy V-notch) and CT (compact tension) specimens made of a bainitic pressure vessel steel was investigated, in the ductile-to-brittle transition region. The fracture surfaces were examined with a scanning electron microscope (SEM), in order to measure the coordinates of the cleavage initiation sites, and to identify the fractographic and/or microstructural features suspected to have triggered cleavage. Cleavage initiation was found to be promoted by the same microstructural features for both CVN and CT specimens. However, the types of cleavage initiation sites differ over the range of temperature investigated. At low temperatures, small spherical and large elongated inclusions are equally important for cleavage initiation. As the temperature increases, the role of the large MnS inclusions also increases. In 80% of the specimens tested, over the temperature range investigated, manganese sulfides play a major role in cleavage initiation.

Mechanical tests on compact tensile (CT) specimens were carried out at 0 degrees C in order to investigate the fracture behaviour in the ductile to brittle transition of a low alloy ferritic steel (A 508 Cl 3). At this temperature, the fracture toughness exhibits an important scatter (200 < Kj < 500 MPa root m) associated with a large scatter in the stable crack growth behaviour before cleavage (0 < Delta a <8 mm)An original statistical local approach, assuming that cleavage fracture is triggered by the local stress magnification due to the manganese sulphides clusters (MnS) was used in conjunction with finite elements calculations describing stable crack growth (Rousselier model). Assuming a temperature dependence of the parameter related to the local cleavage fracture stress, the model predicts satisfactorily the experimental behaviour in the ductile to brittle transition.

Fracture toughness testing of irradiated or aged materials is often limited due to the lack of available material. Subsize Charpy, small CT specimens or small punch test is generally used to overcome this limitation. In this paper, a method based on small notched tensile specimen testing is presented and used for evaluation of fracture toughness properties of a C-Mn steel with a large inclusions content. Specimens are cut in the radial and circumferential directions from a C-Mn steel pipe and tested at 100 degrees C. The diameter of the smallest specimens is 3.6 mm. The average strain at fracture and finite elements simulation of the notched specimens are used to determine the critical values of a local ductile fracture criterion. These values are subsequently used in conjunction with finite element simulations to predict the fracture toughness of the material in different orientations. The predictions are compared with fracture toughness measured on 1/2 T, 1 T and 2 T-CT specimens.

316LN stainless steel (SS) and Glidcop AL-25 (DS-Cu) are primary candidates as structural materials for international thermonuclear experimental reactor (ITER) in-vessel components. Their joining is currently being studied at CEA/CEREM. This paper summarises recent progress on structural materials joining using a solid hot isostatic pressing (HIP) technique. The properties of the materials used and the effect of HIP cycles on the metallurgical quality of the SS are reported. A new specification for the DS-Cu is proposed. Different materials and surfaces preparation, the use of interlayers and different HIP conditions are presented and analysed in relation to the mechanical properties of the joints. Characterisation of the bi-metallic joints such as SS/DS-Cu is discussed and further developments with various mode loadings are briefly reported. Some preliminary modelling results regarding stress/strain distribution arising from manufacturing and thermal load conditions are presented. A visco-plastic material database is currently being developed for that purpose.

La comprehension des mecanismes d'endommagement, notamment intergranulaires des polycristaux passe par la connaissance precise des champs mecaniques (contraintes, deformations) a l'echelle mesoscopique. Le but de cette etude est de les determiner par simulation numerique. Pour cela nous considererons un motif multicristallin inclus dans un milieu homogene equivalent. Ce milieu obeit au comportement experimental du materiau (ici du 316 L(N) a chaud) a l'echelle macroscopique; le comportement cristallin microscopique est lui aussi connu. Une simulation par un code de calcul par elements finis prenant en compte l'aspect cristallin du motif et celui continu de la matrice nous donnera les champs mecaniques dans le motif. Nous definirons le cadre theorique de ce mesoscope numerique, en insistant sur le concept de relocalisation et sur sa position vis a vis des modeles de passage micro-macro. Puis, nous aborderons les difficultes du choix du motif multicristallin et de la coherence entre la loi de comportement du milieu homogene equivalent et celle des cristaux. Nous donnerons ensuite les resultats de l'identification des parametres de la loi macroscopique. Enfin, le traitement numerique par elements finis du probleme sera aborde, et nous discuterons des premiers resultats. Nous conclurons sur les evolutions et les possibilites de ce nouvel outil, notamment pour la description du comportement des joints de grains.

Tensile tests were performed on homothetic different sized axisymmetrical notched tensile specimens (NT), at 100°C. A string size effect was observed on the ductility: The average strain at fracture, as well as the scatter of results, decrease with the size of the specimen. Two statistical methods are proposed to model this size effect. The first modelling is a continuous approach based on mechanics of porous material, on a distribution of Manganese Sulphide volume fraction in a representative volume elements (R.V.E.) and on the weakest link theory. The second one is a discrete approach based on a Monte Carlo randomisation of critical growth rate of voids in finite element calculation. These two models are in agreement with the experimentation. In both cases, the size and scatter effects are obtained by the dispersion of the chosen intrinsic variable, as well as by the relative dimensions of the R.V.E. and of the whole specimen, The discrete approach shows a larger variation of the scatter than the continuous one. In particular, the saturation for large volume seems to be better described by the discrete approach. However, in the continuous approach, the introduction of an upper bound in the statistical description of the Manganese Sulphide fraction leads to an accurate description of the saturation effect.

Mechanical tests on axisymmetrically notched specimens and on CT type specimens were carried out at various temperatures (from -150 degrees C to 0 degrees C), in order to investigate the ductile-brittle transition of a low alloy ferritic steel. The results on notched specimens were used to determine the parameters of the local approach based on Weibull statistics. These parameters in conjunction with finite element calculations were used to interpret the variations of fracture toughness with temperature both in the regime where fracture takes place without ductile crack extension and in the regime corresponding to significant ductile crack growth before cleavage fracture.

Tensile tests on axisymmetrically notched specimens were performed at various temperatures in the ductile-brittle transition region to study the initiation of fracture. Fracture toughness tests on CT specimens were also carried out in the same region to investigate the critical values of stable crack growth occurring before unstable cleavage fracture. Probabilistic modelling is made by coupling the Beremin model for cleavage crack initiation and a model derived from the Gurson potential. In an elementary volume the probability for fracture depends on the probability of both modes of failure. In a larger volume the probability to is expressed in terms of elementary probabilities of fracture through the weakest link theory. Modelling is applied to axisymmetrically notched bars and to CT specimens. The analysis of results shows that the ductile-brittle transition of ferritic steels can be interpreted quantitatively as a competition between both types of damage.

Today there is a strong demand from different industries to use local fracture criteria in a simplified way. This paper presents the main results of a two years joint research project where CEA, EDF, School of Mines and IRSID were associated. A simplified method based on the Beremin models for cleavage and ductile fracture has been developed. This procedure can be used to evaluate the risk of fracture of industrial structures or to allow a rapid development of new materials. Finite element simulations were systematically performed for axisymmetrically notched tensile and precracked specimens using a parametric description of stress-strain laws. Results were presented in the form of graphs that can be used directly for measuring local fracture parameters from tests on axisymmetrically notched specimens and to predict fracture toughness. An additional parameter measuring the level of plastic constraint has been taken into account. This parameter can be easily evaluated on industrial structures. This allows a simple way of using local criteria to evaluate the safety of such structures.

Austenitic stainless steel 316L is used extensively in the field of nuclear industry, and more specifically in the primary circuit of fast breeder reactors. As the normal operation temperature of the latter is about 650°C, it is very important to determine the role of residual stresses in the deformation and the fracture process in order to estimate the component's lifetimes. The plastic deformation is also an important parameter related to the residual stress relaxation and its redistribution after fatigue loading. The aim of this work was to determine the residual stress field in cracked fatigue specimens of austenitic stainless steel 316L by neutron diffraction techniques in order to use this data for quantifying the influence of the different loading parameters on the fatigue crack growth. On the other hand, some microstructural parameters, such as the average size of coherently diffracting blocks D and the mean- square microstrain <ε2>1/2, were estimated by combining neutron and X-ray (synchrotron radiation) diffraction techniques. In fact, the shape and the broadening of diffraction profiles are directly correlated with the evolution and redistribution of microstructural defects . Non-destructive neutron diffraction technique is nowadays extensively used for the determination of internal elastic stresses in polycrystalline materials