Déformation d'un matériau | EPFL Graph Search

La déformation des matériaux est une science qui caractérise la manière dont réagit un matériau donné quand il est soumis à des sollicitations mécaniques. Cette notion est primordiale dans la conception (aptitude de la pièce à réaliser sa fonction), la fabrication (mise en forme de la pièce), et le dimensionnement mécanique (calcul de la marge de sécurité d'un dispositif pour éviter une rupture). La capacité d'une pièce à se déformer et à résister aux efforts dépend de trois paramètres :

la forme de la pièce ;
la nature du matériau ;
des processus de fabrication : traitement thermique , traitement de surface, etc.

L'influence de la forme de la pièce est étudiée en mécanique des milieux continus (MMC). L'influence du matériau est décrite très succinctement ici. Caractérisation On caractérise mécaniquement un matériau par sept propriétés identifiables et mesurables. Chacune de ces caractéristiques fait l'objet de tests ou de mesures normalisés et bien définis

Long-term behaviour and ductility analysis of standard and fibre-reinforced concrete beams

Michele Albertini

Long-term concrete behaviour are two subjects of current interest in many studies published over the last decade. This thesis project is dedicated to the study of both the above mentioned topics, developing a comprehensive work that can further bring a higher degree of knowledge of the subject and the state of the art today. The first part of the work is devoted to the study of creep phenomena on reinforced concrete beams, creating a model that describes the real behavior of the structure, evaluating strains and long-term deformations, which cause phenomena of stress redistribution and delayed breakage over time. In this regard, a numerical model will be presented to evaluate the effect of the phenomena of increased deformations linked to time, such as linear creep, non-linear creep and shrinkage. This model was then applied to a case study, which allowed the evaluation of the degree of reliability of the proposed model. The second part of the work, instead, is dedicated to the study of the phenomenon of ductile breakage of beams subjected to bending. In particular the study concerned the influence of metal fibers on the degree of ductility of the structure. A test campaign was carried out, which made it possible, through an analysis of the results obtained, to assess the effect of the fibres on the breaking of the compressed area of the beams. The laboratory tests on fibre-reinforced concrete beams conducted as part of this study have shown that the presence of metal fibres within the concrete matrix increases the degree of ductility of the structure and also the displacement capacity after breakage.

2020

Mechanical properties of tailored nanostructured alloys produced by electrodeposition

Juri Aljoscha Wehrs

The studies presented in this thesis aim to extend the current knowledge and understanding of the mechanical behavior of nanocrystalline materials with respect to temperature- and time dependence. Free standing electrodeposited nanocrystalline nickel specimen with mean grain sizes in the order of 40nm are synthesized and investigated. A miniaturized in situ test rig is developed, capable of performing uniaxial tensile tests on dedicated miniaturized specimen. A novel displacement controlled in situ high temperature indentation system is presented, capable of performing sharp tip indentation and uniaxial micropillar compression experiments up to 600°C. Elevated temperature micropillar strain rate jump tests are performed to extract the strain rate sensitivity factor, apparent activation volume and activation energy. The results suggest grain boundary diffusion to be the rate controlling deformation mechanism. Post deformation imaging also indicates the activity of dislocation based mechansims. To compare the results to established techniques, micro specimen are tested under three different load cases: uniaxial tension, uniaxial compression and hydrostatic pressure. The results compare well to the previous, suggesting the rate controlling mechanism to be the same for all three load cases. A transient load relaxation tests is developed to assess time dependent plasticity on short and intermediate time scales. The results suggest grain boundary mediated processes to be rate controlling. To enhance the identification of specific deformation mechanisms, elevated temperature micropillar strain rate jump- and load relaxation tests are also conducted on an inert gas condensated nanocrystalline palladium-gold model alloy. Distinct features of this material are extremely high chemical purity and small, homogeneous grain sizes in the order of ~10nm. Shear transformation mediated plasticity is observed, similar to the deformation of metallic glasses. The demonstrated methods are relevant for the Swiss watch industry. A series of mechanical tests on standard watch materials are compared. An approach for the electrodeposition of a nickel-tungsten alloy for watch applications with improved thermal stability and less pronounced time dependent plastic behavior is presented.

EPFL2017

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