Fatigue (physiologie) | EPFL Graph Search

La fatigue est un état résultant de contraintes physiologiques ou psychologiques aboutissant à une diminution des performances physiques ou cognitives. La fatigue physiologique est réversible avec la mise au repos, qui restaure un niveau normal de performances. La fatigue psychique se manifeste par une baisse de l’attention et de la concentration. Lorsque associée à une pathologie, on parle plutôt d'asthénie. Au niveau sociologique, l’accélération des rythmes de vie inhérents aux sociétés modernes accentue ce phénomène de fatigue. Ainsi, la fatigue peut être vue comme une désadaptation à l’environnement. Au niveau historique, on peut faire l'histoire des différentes formes de fatigue : « langueur », « dépérissement », « pénibilité », « stress », « burn out », « charge mentale », etc.. Physiologie La fatigue fait son apparition de façon physiologique en fin de journée ou à l’issue d’une contrainte. Ces contraintes peuvent être physiques (bruit, vibration

In-situ Laue diffraction to follow dislocation patterning during fatigue

Ainara Irastorza Landa

The macroscopic strength of metals is determined by the dislocation arrangements that are formed when dislocations slip in the crystal lattice in response to the applied stress. Despite the extensive research carried out, the transition from uniform to non-uniform dislocation structures is not yet fully understood. This information is however essential to support the development of computational models that aim to predict dislocation patterning. Lattice rotation caused by the presence of dislocations is, for instance, a parameter that is often not taken into account in computational models although it plays a role in the development of dislocation ensembles. Experimentally following in-situ dislocation patterning including involved lattice rotations at lengthscales comparable to those in simulations is not straightforward. In fact, the current available techniques that have the necessary spatial and angular resolution are either destructive (3D-EBSD) or very time consuming (3D X-ray Laue diffraction). In this dissertation we present a new experimental approach that allows following lattice rotation and dislocation ensembles time-resolved during deformation. The technique is based on X-ray Laue diffraction scanning in transmission mode, which provides a sub-micron spatial resolution in 2D and statistical information on orientation spread in the third dimension. The in-situ mechanical tests are performed at the microXAS beamline of the Swiss Light Source Synchrotron. The method has been applied to understand dislocation pattering of copper single crystals occurring in the earlier phases of low cycle fatigue. Samples with different crystal orientations (single crystal, coplanar and collinear) have been cyclically deformed up to a maximum of 120 cycles. A dedicated miniaturized shear-fatigue device compatible with Laue diffraction has been constructed for that purpose and a sample preparation based on picosecond laser ablation has been developed. The developed procedure analyzes the evolving dislocation microstructures in terms of lattice rotation, lattice curvature and geometrically necessary dislocation densities at lengthscales similar to those addressable in computational models. It sacrifices on the fully 3D aspect but it brings the time resolution required to validate ongoing simulations. It has been shown that the error made by integration depends on the dislocation network expected. For instance, the technique can be used to validate 3DDD models or density based dislocation dynamics models when applied in deformation geometries where the formation of 2D dislocation patterns are expected (e.g. vein-channel structure in single slip oriented fatigued samples). The greatest advantage of this technique is that the evolving regions are easy to follow due to the high rotational sensitivity and time resolution of the method. What is more, the influence of the initial microstructure can be evaluated and quantitative values can be provided. On the other hand, the principal drawbacks are related to the gauge thickness of the samples. Even if the technique can estimate the orientation spread along the third direction, the approach cannot physically resolve the integrated signal along the thickness. This is source of uncertainty when providing actual values of lattice curvatures and geometrically necessary dislocation densities. Another shortcoming is the sensitivity of the method to the energy distribution of the X-ray beam provided in the beamline, which determines the collected signal - the principal basis of the subsequent analysis and interpretation.

EPFL2016

Physics-based constitutive modelling for crystal plasticity finite element computation of cyclic plasticity in fatigue

Nicolò Grilli

Metal fatigue during cyclic loading puts an endurance limit on most of today's technology. It impacts the reliability of metallic components used for transportation, electronic devices and energy production because fatigue failure can occur without any apparent forewarning. This phenomenon limits the lifetime of many industrially manufactured items, whose periodic replacement affects the cost of everyday products. The problem of predicting fatigue failure in metals has been addressed using the finite element method, which can provide the stress and strain state of the material in complex geometries. Engineering models describing the material behaviour have been used to predict the damage accumulation, but microstructure and material design require constitutive models at the micrometre length scale, incorporating the physical processes in the material. This involves the study of dislocation dynamics and the formation of dislocation structures, which have been recognized as the key phenomena affecting the macroscopic fatigue behaviour of metals. This problem is computationally challenging, first because of the large number of state variables required to describe the dislocation behaviour, and second because the formation of dislocation structures takes place only after many deformation cycles. In this project a continuum dislocation-based model, specific for cyclic fatigue at the micrometre length scale, is developed. The main novelty is the prediction of 3D dislocation structures starting from a random initial dislocation distribution. In single slip deformation, the characteristic length scale and shape of dislocation structures are predicted using only physical parameters, such as the stacking fault energy, and without any fitting procedure. The model is implemented in a crystal plasticity finite element solver, describing all the slip systems. Therefore, it is possible to model polycrystalline structures and to study the orientation of multiple slip dislocation structures with respect to the loading direction. Compared with existing models, the implementation of the dislocation junction formation mechanism in a continuum framework is a step forward. A collaboration with another PhD student, carrying out electron channeling contrast imaging experiments on fatigued 316 stainless steel, has provided a validation of the model by direct comparison of experimental and simulated dislocation structures close to the specimen surface. The crystal lattice rotation is another variable that can be extracted from finite element simulations. The developed model correlates the forming dislocation structures with the lattice rotation around the coordinate axes. This allows the comparison with Laue microdiffraction experiments carried out on copper single crystal specimens by another PhD student at the SLS, the synchrotron at Paul Scherrer Institut. The dislocation-based model can calculate experimental observables, such as the rotation components and rotation gradients, as a function of the simulated dislocation density in the specimen, which is not directly observable. This provides a validation of the constitutive equations at the micrometre length scale.

EPFL2016

Addressing the need to monitor concrete fatigue with Non Destructive Testing: results of Infrastar European project

Antoine Bassil, Imane Bayane, Eugen Brühwiler, Xiao Wang

Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to fatigue phenomena that could lead to premature failure of certain structural elements. To date, fatigue of reinforced concrete has been given little focus. Knowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate. Meanwhile, this topic has got much more attention as time-variant loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, and for wind turbines for renewable energy production, e.g. for offshore wind turbine support structures affected by wind and waves. The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing concrete structures. First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.

2018

In-situ Laue diffraction to follow dislocation patterning during fatigue

Ainara Irastorza Landa

EPFL2016

Addressing the need to monitor concrete fatigue with Non Destructive Testing: results of Infrastar European project

Antoine Bassil, Imane Bayane, Eugen Brühwiler, Xiao Wang

2018

Physics-based constitutive modelling for crystal plasticity finite element computation of cyclic plasticity in fatigue

Nicolò Grilli

EPFL2016