Modelling of the scatter in short fatigue cracks growth kinetics in relation with the polycrystalline microstructure

International audienceFinite element computations of the stresses ahead of a tortuous microcrack in a polycrystal (including crack flanks friction) are coupled with simulations of crystallographic crack growth based on discrete dislocations dynamics. An incubation period for crack growth beyond grain boundaries is introduced. The model reproduces the shape of experimental crack growth curves obtained on 316LN stainless steel and the decrease in arrest periods at grain boundaries as the crack grows. It predicts a large scatter in growth rates related to the variety of local textures. It also describes the fact that overloads allowing arrested cracks to cross the grain boundaries can make small cycles damaging

Homogeneisation du comportement polycristallin considerer la plasticite a l'echelle des grains ou des bandes de glissement ?

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Identification of crystalline behavior on macroscopic response and local strain field analysis: application to alpha zirconium alloys

The purpose of this paper is to present an identification method of the crystalline behavior of a material from a mechanical test performed on a polycrystalline sample. Because of the lack of knowledge about its crystalline behavior, this method is applied to a Zirconium alloy. This identification is based on a finite element modeling of the microstructure, and the results are compared to both the macroscopic and the microscopic experimental results. On the microscopic scale, the plastic strains are obtained using a micro-extensometry technique and the crystalline orientation using an EBSD technique. In order to validate the method, an identification is performed with only two free parameters: the evolutions of the macroscopic and microscopic errors appear to be regular and exhibit a well-defined minimum so that the parameters can be clearly identified

High temperature mechanical strength and microstructural stability of advanced 9-12%Cr steels and ODS steels

International audienceThe present study proposes a comparison between several martensitic and ferritic steels in terms of creep strength and cyclic softening effect. The damage mechanisms are identified using fractographic observations and the microstructural evolutions are observed by TEM. The effects of a modified chemical composition on the high temperature mechanical behaviour are studied

Microstructure-sensitive estimation of small fatigue crack growth in bridge steel welds

A probabilistic finite element model is implemented to estimate microstructurally small fatigue crack growth in bridge steel welds. Simulations are based on a microstructure-sensitive crystal plasticity model to quantify fatigue indicator parameters (FIPs) at the slip system level and a fatigue model that relates FIPs to fatigue lives of individual grains. Microstructures from three weld zones, namely, fusion zone (FZ), heat affected zone (HAZ), and base metal (BM), are constructed based on their microstructural attributes such as grain morphology, size, and orientation. Statistical volume elements (SVEs) are generated and meshed independently for the three welding zones. Each grain within the SVEs is divided into several slip bands parallel to crystallographic planes. During the loading process, cracks nucleate at the slip bands (SBs) with the largest FIP next to the free surface. The crack extension path is assumed to be transgranular along SBs and the number of cycles required to crack the neighbor grain is calculated by the corresponding FIP-based crack growth rate equation. The simulation process is carried out using ABAQUS with a user defined subroutine UMAT for crystal plasticity. After the calibration of the constitutive model and irreversibility parameters, numerical simulations for small crack growth in three zones are presented. The crack length vs. the predicted fatigue resistance shows significant differences in the mean values and variability among the three weld zones

Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

Modélisation des contraintes cinématiques dues aux microstructures de dislocations obtenues par déformation

Si la déformation appliquée à un métal ou un alliage à structure Cubique à Face Centrée est suffisamment forte, des microstructures de déformation apparaissent dans le matériau. Ces microstructures sont composées de deux phases, une phase "molle" à faible densité de dislocations (intérieurs des cellules, canaux) et une phase "dure" à forte densité de dislocations (murs). De nombreuses expériences ont montré que ces microstructures induisaient des contraintes cinématiques à l'échelle du monocristal. Une application de la loi de localisation du modèle autocohérent de Berveiller-Zaoui est proposée dans cet article en vue d'évaluer ces contraintes cinématiques. Les solutions du problème d'inclusion de Eshelby sont utilisées ainsi que le coef-ficient d'accommodation proposé par Berveiller-Zaoui de manière à tenir compte de déformations plastiques importantes. Le modèle est validé à l'échelle du monocristal pour lequel seules les contraintes cinématiques intragranulaires sont présentes. La validation utilise de nombreuses déterminations expérimentales des contraintes issues d'essais monotones ou cycliques sur monocristaux CFC. Le modèle n'a recours à l'identification d'aucun paramètre propre

Analytical modelling of intragranular backstresses due to deformation induced dislocation microstructures

International audienceDeformation induced dislocation microstructures appear in Face-Centred Cubic metals and alloys if applying large enough tensile/cyclic strain. These microstructures are composed of a soft phase with a low dislocation density (cell interiors, channels.. .) and a hard phase with a high dislo-cation density (walls). It is well known that these dislocation microstructures induce backstresses, which give kinematic hardening at the macroscopic scale. A simple two-phase localization rule is applied for computing these intragranular backstresses. This is based on Eshelby's inclusion problem and the Berveiller-Zaoui approach. It takes into account an accommodation factor. Close-form for-mulae are given and permit the straightforward computation of reasonable backstress values even for large plastic strains. Predicted backstress values are compared to a number of backstress experimental measurements on single crystals. The agreement of the model with experiments is encouraging. This physical intragranular kinematic hardening model can easily be implemented in a polycrystalline homogenization code or in a crystalline finite element code. Finally, the model is discussed with respect to the possible plastic glide in walls and the use of enhanced three phase localization models

Multiscale simulation of microcrack nucleation induced by slip localization at the surface of polycrystals

International audienceCrack initiation along surface persistent slip bands (PSBs) has been widely observed and modelled. Nevertheless, from our knowledge, no physically-based fracture modelling has been proposed and validated with respect to the numerous recent experimental data showing the strong relationship between extrusion and microcrack initiation. The whole FE modelling accounts for- localized plastic slip in PSBs;- production and annihilation of vacancies induced by cyclic slip. If temperature is high enough, point defects may diffuse in the surrounding matrix due to large concentration gradients, allowing continuous extrusion growth in agreement with Polak's model. At each cycle, the additional atoms diffusing from the matrix are taken into account by imposing an incremental free dilatation;- brittle fracture at the interfaces between PSBs and their surrounding matrix which is simulated using cohesive zone modelling.Any inverse fitting of parameter is avoided. Only experimental single crystal data are used such as hysteresis loops and resistivity values. Two fracture parameters are required the {111} surface energy which depends on environment and the cleavage stress which is predicted by the universal binding energy relationship. The predicted extrusion growth curves agree rather well with the experimental data published for copper and the 316L steel. A linear dependence with respect to PSB length, thickness and slip plane angle is predicted in agreement with recent AFM measurement results. Crack initiation simulations predict fairly well the effects of PSB length and environment for copper single and poly- crystals

Etude par éléments finis de la propagation de fissures microstructurellement courtes : prise en compte de l'élasticité et de la plasticité cristallines

Dans le régime de la fatigue à grand nombre de cycles, la part la plus importante de la durée de vie d'une éprouvette consiste en la propagation de fissures dont la longueur est de l'ordre de quelques grains. Ces fissures subissent des déflections importantes lorsqu'elles passent d'un plan de glissement d'un grain à celui du grain voisin. Pour quantifier l'influence de ces déflections sur la vitesse de propagation dans le cas des aciers austénitiques, nous utilisons tout d'abord les concepts de la mécanique linéaire élastique de la rupture. Cette approche élastique est insuffisante car les variations de vitesse de propagation ainsi calculées sont beaucoup plus faibles que celles obtenues expérimentalement. Compte tenu des limites de cette approche, nous proposons de simuler le comportement élasto-plastique de monocristaux et polycristaux de 316L. L'évolution de la contrainte interne sur chaque système de glissement suit une loi à écrouissage cinématique non linéaire identifiée sur monocristal orienté en glissement simple. Cette loi est utilisée pour simuler le comportement cyclique d'un polycristal de 316