Dislocation transport and line length increase in averaged descriptions of dislocations

Crystal plasticity is the result of the motion and interaction of dislocations. There is, however, still a major gap between microscopic and mesoscopic simulations and continuum crystal plasticity models. Only recently a higher dimensional dislocation density tensor was defined which overcomes some drawbacks of earlier dislocation density measures. The evolution equation for this tensor can be considered as a continuum version of dislocation dynamics. We use this evolution equation to develop evolution equations for the total dislocation density and an average curvature which together govern a faithful representation of the dislocation kinematics without having to use extra dimensions

Continuum modeling of dislocation plasticity: Theory, numerical implementation, and validation by discrete dislocation simulations

Miniaturization of components and devices calls for an increased effort on physically motivated continuum theories, which can predict size-dependent plasticity by accounting for length scales associated with the dislocation microstructure. An important recent development has been the formulation of a Continuum Dislocation Dynamics theory (CDD) that provides a kinematically consistent continuum description of the dynamics of curved dislocation systems [T. Hochrainer, et al., Philos. Mag. 87, 1261 (2007)]. In this work, we present a brief overview of dislocation-based continuum plasticity models. We illustrate the implementation of CDD by a numerical example, bending of a thin film, and compare with results obtained by three-dimensional discrete dislocation dynamics (DDD) simulation

Dislocation multiplication by cross-slip and glissile reaction in a dislocation based continuum formulation of crystal plasticity

Modeling dislocation multiplication due to interaction and reactions on a mesoscopic scale is an important task for the physically meaningful description of stage II hardening in face centered cubic crystalline materials. In recent Discrete Dislocation Dynamics simulations it is observed that dislocation multiplication is exclusively the result of mechanisms, which involve dislocation reactions between different slip systems. These findings contradict multiplication models in dislocation based continuum theories, in which density increase is related to plastic slip on the same slip system. An application of these models for the density evolution on individual slip systems results in self-replication of dislocation density. We introduce a formulation of dislocation multiplication in a dislocation based continuum formulation of plasticity derived from a mechanism-based homogenization of cross-slip and glissile reactions in three-dimensional face-centered cubic systems. As a key feature, the presented model includes the generation of dislocations based on an interplay of dislocation density on different slip systems. This particularly includes slip systems with vanishing shear stress. The results show, that the proposed dislocation multiplication formulation allows for a physically meaningful microstructural evolution without self-replication of dislocations density. The results are discussed in comparison to discrete dislocation dynamics simulations exposing the coupling of different slip systems as the central characteristic for the increase of dislocation density on active and inactive slip systems. (C) 2019 Elsevier Ltd. All rights reserved

A semi-implicit method for thrombus formation in haemodynamic fluid-structure interaction

Aortic flows with thrombus formation represent a challenging application of fluidstructure interaction (FSI) in biomechanics where blood flow, thrombus, and vessel wall are strongly coupled. Considering patient-specific FSI and thrombus formation on identical time scales remains unfeasible. To resolve this issue, we propose incorporating the dynamics-based thrombus formation model of Menichini et al. [1] into our recently presented semi-implicit, splitstep partitioned FSI scheme for non-Newtonian fluids [2, 3]. Herein, we formulate the basic split-step scheme and present the first promising results, merely coupling the fluid pressure and structure displacement iteratively at each time step

Characterization of dislocation ensembles: measures and complexity

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Cosmic Bell Test using Random Measurement Settings from High-Redshift Quasars

In this Letter, we present a cosmic Bell experiment with polarization-entangled photons, in which measurement settings were determined based on real-time measurements of the wavelength of photons from high-redshift quasars, whose light was emitted billions of years ago, the experiment simultaneously ensures locality. Assuming fair sampling for all detected photons and that the wavelength of the quasar photons had not been selectively altered or previewed between emission and detection, we observe statistically significant violation of Bell's inequality by $9.3$ standard deviations, corresponding to an estimated $p$ value of $\lesssim 7.4 \times 10^{-21}$. This experiment pushes back to at least $\sim 7.8$ Gyr ago the most recent time by which any local-realist influences could have exploited the "freedom-of-choice" loophole to engineer the observed Bell violation, excluding any such mechanism from $96\%$ of the space-time volume of the past light cone of our experiment, extending from the big bang to today.Comment: 9 pages, 4 figures, plus Supplemental Material (16 pages, 8 figures). Matches version to be published in Physical Review Letter