6,464 research outputs found
Micromechanics of high temperature hydrogen attack
Get PDFHydrogen attack is a material degradation process that occurs at elevated temperatures in hydrogen-rich environments, such as found in petrochemical installations. Weldments in components such as reactor vessels are particularly susceptible to hydrogen attack. This paper discusses a multi-scale micromechanics modelling approach in which the chemico-mechanical damage processes at sub-micron scale are coupled to the macroscopic behaviour through a series of size-scale transitions. A simulation of hydrogen attack in a welded reactor serves as an illustration of the approach.
Relaxation of thermal stress by dislocation motion in passivated metal interconnects
Get PDFThe development and relaxation of stress in metal interconnects strained by their surroundings (substrate and passivation layers) is predicted by a discrete dislocation analysis. The model is based on a two-dimensional plane strain formulation, with deformation fully constrained in the line direction. Plastic deformation occurs by glide of edge dislocations on three slip systems in the single crystal line. The substrate and passivation layers are treated as elastic materials, and therefore impenetrable for the dislocations. Results of the simulations show the dependence of the stress evolution and of the effectiveness of plastic relaxation on the geometry of the line. The dependence of stress development on line aspect ratio, line size, slip plane orientation, pitch length and passivation layer thickness are explored.
Hydrogen Attack - Micromechanical modeling on three length scales
No full textMechanical Maritime and Materials Engineerin
Neurobiological aspects of obesity: dopamine, serotonin, and imaging
Get PDFOvereten is een belangrijke oorzaak van obesitas. De hersenen spelen een rol bij de regulatie van eetgedrag. Gedacht wordt dat het minder goed functioneren van het beloningscentrum in de hersenen gerelateerd kan zijn aan overeten. Dopamine is een belangrijke signaalstof in dit beloningssysteem. Bij zwaarlijvigen is er een disbalans in het dopaminesysteem. Deze disbalans is mogelijk gerelateerd aan het dieet. Daarnaast lijkt er binnen de obese populatie een subgroep te zijn van te zware mensen met eetbuien, die wat impulsiever zijn en daardoor mogelijk geneigd zijn meer te eten. Dit komt naar voren uit het onderzoek van Elsmarieke van de Giessen. In haar onderzoek maakte Van de Giessen gebruik van hersenscans
Plastic Ploughing of a Sinusoidal Asperity on a Rough Surface
Get PDFPart of the friction between two rough surfaces is due to the interlocking between asperities on opposite surfaces. In order for the surfaces to slide relative to each other, these interlocking asperities have to deform plastically. Here, we study the unit process of plastic ploughing of a single micrometer-scale asperity by means of two-dimensional dislocation dynamics simulations. Plastic deformation is described through the generation, motion, and annihilation of edge dislocations inside the asperity as well as in the subsurface. We find that the force required to plough an asperity at different ploughing depths follows a Gaussian distribution. For self-similar asperities, the friction stress is found to increase with the inverse of size. Comparison of the friction stress is made with other two contact models to show that interlocking asperities that are larger than similar to 2 mu m are easier to shear off plastically than asperities with a flat contact
A Generalized Finite Element Method for polycrystals with discontinuous grain boundaries
No full textWe present a Generalized Finite Element Method for the analysis of polycrystals with explicit treatment of grain boundaries. Grain boundaries and junctions, understood as loci of possible displacement discontinuity, are inserted into finite elements by exploiting the partition of unity property of finite element shape functions. Consequently, the finite element mesh does not need to conform to the polycrystal topology. The formulation is outlined and a numerical example is presented to demonstrate the potential and accuracy of the approach. The proposed methodology can also be used for branched and intersecting cohesive cracks, and comparisons are made to a related approach (Int. J. Numer. Meth. Engng. 2000; 48:1741)
Effect of defect energy on strain-gradient predictions of confined single-crystal plasticity
Get PDFGurtin recently proposed a strain-gradient theory for crystal plasticity in which the gradient effect originates from a defect energy that characterizes energy storage due to the presence of a net Burgers vector. Here we consider a number of different possibilities for this energy: specifically, working within a simple two-dimensional framework, we compare predictions of the theory with results of discrete-dislocation simulations of stress relaxation in thin films. Our objective is to investigate which specific defect energies are capable of capturing the size-dependent response of such systems for different crystal orientations.
Nonlocal Modeling of Size-Dependent Response of Thin Films
Get PDFA closed-form solution is presented for the plastic response of a single crystal thin film strained by its substrate during thermal loading, according to Gurtin’s strain gradient theory. The results are compared with those of discrete dislocation simulations.
Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study
No full textDiscrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.
Two hardening mechanisms in single crystal thin films studied by discrete dislocation plasticity
Get PDFTwo-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate are used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling is analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density is found close to the impenetrable film-substrate interface. There is a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state are independent of film thickness. In such films the stress outside the boundary layer is on average very low, so that the film-thickness-independent boundary layer is responsible for the size effect. A larger size effect is found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreases while the stress in the film increases. The size dependence is only described by a Hall–Petch type relation for films thicker than the threshold value.
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