Discrete dislocation dynamics analysis of the effect of lattice orientation on void growth in single crystals

The micromechanisms of plastic deformation and void growth were analyzed using discrete dislocation dynamics in an isolated FCC single crystal deformed in-plane strain in the plane. Three different stress states (uniaxial tension, uniaxial deformation and biaxial deformation) were considered for crystals oriented in different directions and with a different number of active slip systems. It was found that strain hardening and void growth rates depended on lattice orientation in uniaxial tension because of anisotropic stress state. Crystal orientation did not influence, however, hardening and void growth when the crystals were loaded under uniaxial or biaxial deformation because the stress state was more homogeneous, although both (hardening and void growth rates) were much higher than under uniaxial tension. In addition, the number of active slip systems did not substantially modify the mechanical behavior and the void growth rate if plastic deformation along the available slip systems was compatible with overall crystal deformation prescribed by the boundary conditions. Otherwise, the incompatibility between plastic deformation and boundary conditions led to the development of large hydrostatic elastic stresses, which increased the strain hardening rate and reduced the void growth rate

Simulation of the deformation of polycrystalline nanostructured Ti by computational homogenization

Computational homogenization by means of the finite element analysis of a representative volume element of the microstructure is used to simulate the deformation of nanostructured Ti. The behavior of each grain is taken into account using a single crystal elasto-viscoplastic model which includes the microscopic mechanisms of plastic deformation by slip along basal, prismatic and pyramidal systems. Two different representations of the polycrystal were used. Each grain was modeled with one cubic finite element in the first one while many cubic elements were used to represent each grain in the second one, leading to a model which includes the effect of grain shape and size in a limited number of grains due to the computational cost. Both representations were used to simulate the tensile deformation of nanostructured Ti processed by ECAP-C as well as the drawing process of nanostructured Ti billets. It was found that the first representation based in one finite element per grain led to a stiffer response in tension and was not able to predict the texture evolution during drawing because the strain gradient within each grain could not be captured. On the contrary, the second representation of the polycrystal microstructure with many finite elements per grain was able to predict accurately the deformation of nanostructured Ti

Micromecánica computacional de materiales compuestos reforzados con partículas

La capacidad de cálculo actual de los ordenadores digitales permite resolver problemas que antes solo podían abordarse mediante grandes simplificaciones. Esta tesis doctoral presenta una contribución en esta línea al relacionar el comportamiento mecánico de los materiales compuestos con su estructura a nivel microscópico. La investigación partió de una representación de la microestructura de los materiales compuestos reforzados con partículas a partir de volúmenes representativos tridimensionales, denominados celdas multipartícula, que estaban formados por distribuciones aleatorias de esferas cuyas posiciones se generaron mediante diferentes algoritmos desarrollados a este efecto. Las celdas se discretizaron y analizaron mediante el método de los elementos finitos y el comportamiento mecánico del material compuesto se determinó promediando los resultados de varias dispersiones diferentes por cada microestructura simulada. Las simulaciones numéricas permitieron obtener en primer lugar una solución \emph{exacta} de un problema clásico en Mecánica de Sólidos: las constantes elásticas de una distribución aleatoría y estadísticamente homogénea de esferas rígidas o huecos esféricos embebidos en una matriz continua. Este resultado permitió evaluar la precisión de los principales modelos analíticos desarrollados durante los últimos cuarenta años y se llegó a la conclusión de que los mejores resultados se logran con la aproximación de tercer orden de Torquato. La simulación numérica de las celdas multipartícula también proporcionó una solución cercana a la exacta para el comportamiento elasto-plástico de un material compuesto formado por una distribución aleatoria y estadísticamente homogénea de esferas elásticas dispersas en una matriz elasto-plástica. Los resultados de las simulaciones numéricas se compararon con las soluciones obtenidas a partir de los modelos de campo medio secantes --- que incluyen la deformación plástica de la matriz a partir de la teoría de la plasticidad en deformaciones totales --- y se demostró que estos modelos proporcionan mejores resultados cuando utilizan como tensión de referencia para determinar la deformación plástica de la matriz la tensión equivalente calculada a partir de momento de segundo orden del tensor de tensiones en la matriz. Sin embargo, los modelos de campo medio no son capaces de reproducir adecuadamente la localización de la deformación en la matriz que aparece al comienzo de la deformación plástica. El análisis de los microcampos de tensiones en la matriz proporcionados por las simulaciones de celdas multipartícula señaló el origen de esta limitación e indicó la líneas a seguir para desarrollar nuevas aproximaciones que incluyan el efecto de la localización. Las nuevas técnicas de simulación también permitieron abordar de manera rigurosa el efecto de la distribución espacial de las partículas de refuerzo en las propiedades mecánicas de los materiales compuestos. Se generaron microestructuras inhomogéneas donde las partículas de refuerzo estaban concentradas en regiones esféricas con una fracción volumétrica local superior al valor medio. Las resultados numéricos mostraron que la inhomogeneidad en la distribución del refuerzo sólo incrementaba ligeramente la rigidez del material compuesto a nivel global, aunque las distribuciones de tensiones presentaban grandes diferencias a nivel local: los valores máximos de la tensión y la deformación en ambas fases eran mucho más elevados en lo materiales con microestructura inhomogénea. La evidencia experimental ha demostrado que la rotura de los materiales compuestos es precedida por tres mecanismos de daño: fractura de las partículas de refuerzo, de la intercara matriz/refuerzo y de la matriz. Los dos primeros mecanismos se introdujeron en las simulaciones mediante un elemento finito de intercara cuyo comportamiento mecánico venía dictado por una ley cohesiva mientras que la fractura dúctil de la matriz se incluyó a través del modelo de Gurson. Los resultados numéricos reprodujeron con precisión los patrones de daño observados experimentalmente, y mostraron que la presencia de estos mecanismos de daño reduce sensiblemente la capacidad de endurecimiento por deformación de los materiales compuestos. Esta reducción fue mucho más acusada en las microestructuras inhomogéneas donde el daño se inició rápidamente en las regiones con una fracción volumétrica de refuerzo muy elevada como consecuencia de las concentraciones de tensiones generadas durante la deformación, y las microestructuras inhomogéneas presentaron valores mucho menores de la tensión de rotura y la ductilidad. Finalmente, las simulaciones numéricas se validaron experimentalmente comparando con los resultados obtenidos sobre un material compuesto ``modelo'' fabricado para esta investigación, y donde el mecanismo de daño dominante era la decohesión entre la matriz de aluminio y las esferas de refuerzo de WC

Dislocation Dynamics in Non-conves Domains using Finite Elements with Embedded Discontinuities

The standard strategy developed by Van der Giessen and Needleman (1995 Modelling Simul. Mater. Sci. Eng. 3 689) to simulate dislocation dynamics in two-dimensional finite domains was modified to account for the effect of dislocations leaving the crystal through a free surface in the case of arbitrary non-convex domains. The new approach incorporates the displacement jumps across the slip segments of the dislocations that have exited the crystal within the finite element analysis carried out to compute the image stresses on the dislocations due to the finite boundaries. This is done in a simple computationally efficient way by embedding the discontinuities in the finite element solution, a strategy often used in the numerical simulation of crack propagation in solids. Two academic examples are presented to validate and demonstrate the extended model and its implementation within a finite element program is detailed in the appendix

Finite Deformation of Porous Elastomers: A Computational Micromechanics Approach

Finite Deformation of Porous Elastomers: A Computational Micromechanics Approac

Latent hardening size effect in small-scale plasticity

We aim at understanding the multislip behaviour of metals subject to irreversible deformations at small-scales. By focusing on the simple shear of a constrained single-crystal strip, we show that discrete Dislocation Dynamics (DD) simulations predict a strong latent hardening size effect, with smaller being stronger in the range [1.5 µm, 6 µm] for the strip height. We attempt to represent the DD pseudo-experimental results by developing a flow theory of Strain Gradient Crystal Plasticity (SGCP), involving both energetic and dissipative higher-order terms and, as a main novelty, a strain gradient extension of the conventional latent hardening. In order to discuss the capability of the SGCP theory proposed, we implement it into a Finite Element (FE) code and set its material parameters on the basis of the DD results. The SGCP FE code is specifically developed for the boundary value problem under study so that we can implement a fully implicit (Backward Euler) consistent algorithm. Special emphasis is placed on the discussion of the role of the material length scales involved in the SGCP model, from both the mechanical and numerical points of view

An inverse optimization strategy to determine single crystal mechanical behavior from polycrystal tests: Application to AZ31 Mg alloy

An inverse optimization strategy was developed to determine the single crystal properties from experimental results of the mechanical behavior of polycrystals. The polycrystal behavior was obtained by means of the finite element simulation of a representative volume element of the microstructure in which the dominant slip and twinning systems were included in the constitutive equation of each grain. The inverse problem was solved by means of the Levenberg-Marquardt method, which provided an excellent fit to the experimental results. The iterative optimization process followed a hierarchical scheme in which simple representative volume elements were initially used, followed by more realistic ones to reach the final optimum solution, leading to important reductions in computer time. The new strategy was applied to identify the initial and saturation critical resolved shear stresses and the hardening modulus of the active slip systems and extension twinning in a textured AZ31 Mg alloy. The results were in general agreement with the data in the literature but also showed some differences. They were partially explained because of the higher accuracy of the new optimization strategy but it was also shown that the number of independent experimental stress-strain curves used as input is critical to reach an accurate solution to the inverse optimization problem. It was concluded that at least three independent stress-strain curves are necessary to determine the single crystal behavior from polycrystal tests in the case of highly textured Mg alloys

Integrated computational materials engineering in solar plants: the virtual materials design project

The final publication is available at Springer via http://dx.doi.org/10.1007/s11837-018-2970-5The high temperatures required for efficient operation of solar thermal power plants constitutes one of the major challenges of this technology. Gaining insight into materials behavior at very high temperatures is critical to improve their techno-economic feasibility. Standard material characterization approaches become inefficient, as extensive testing campaigns are required. We propose a multiscale–multiphysical approach that accounts for materials composition to (1) predict the behavior of both Inconel 625 and new solar salts, and (2) assess the thermomechanical performance of key components. We carried out a complete thermoelastic multiscale analysis that spans six time and length scales in a single simulation platform, combining discrete and continuum tools (from quantum to continuum mechanics). These applications show the substantial economic benefits that may be achieved by an ICME approach in the energy sector, reducing the cost of prototypes while decreasing development times and maintenance costs due to a better understanding of materials behavior.Peer ReviewedPostprint (author's final draft

Computational study of atomic mobility for bcc phase in Ti-Al-Fe system

Experimental diffusion data were critically assessed to develop the atomic mobility for the bcc phase of the Ti–Al–Fe system by using the DICTRA software. Good agreements were obtained from comprehensive comparisons made between the calculated and the experimental diffusion coefficients. The developed atomic mobility was then validated by well predicting the interdiffusion behavior observed from the diffusion-couple experiments in available literature

Micropillar compression of LiF [111] single crystals: effect of size, ion irradiation and misorientation

The mechanical response under compression of LiF single crystal micropillars oriented in the [111] direction was studied. Micropillars of different diameter (in the range 1–5 lm) were obtained by etching the matrix in directionally-solidified NaCl–LiF and KCl–LiF eutectic compounds. Selected micropillars were exposed to high-energy Ga+ ions to ascertain the effect of ion irradiation on the mechanical response. Ion irradiation led to an increase of approximately 30% in the yield strength and the maximum compressive strength but no effect of the micropillar diameter on flow stress was found in either the as-grown or the ion irradiated pillars. The dominant deformation micromechanisms were analyzed by means of crystal plasticity finite element simulations of the compression test, which explained the strong effect of micropillar misorientation on the mechanical response. Finally, the lack of size effect on the flow stress was discussed to the light of previous studies in LiF and other materials which show high lattice resistance to dislocation motion