Plasticity size effects in tension and compression of single crystals

The effect of size and loading conditions on the tension and compression stress–strain response of micron-sized planar crystals is investigated using discrete dislocation plasticity. The crystals are taken to have a single active slip system and both small-strain and finite-strain analyses are carried out. When rotation of the tensile axis is constrained, the build-up of geometrically necessary dislocations results in a weak size dependence but a strong Bauschinger effect. On the other hand, when rotation of the tensile axis is unconstrained, there is a strong size dependence, with the flow strength increasing with decreasing specimen size, and a negligible Bauschinger effect. Below a certain specimen size, the flow strength of the crystals is set by the nucleation strength of the initially present Frank–Read sources. The main features of the size dependence are the same for the small-strain and finite-strain analyses. However, the predicted hardening rates differ and the finite-strain analyses give rise to some tension–compression asymmetry.

Discrete dislocation simulations and size dependent hardening in single slip

Plastic deformation in two-dimensional monophase and composite materials is studied using a discrete dislocation dynamics method. In this method, dislocations are represented by line defects in a linear elastic medium, and their interactions with boundaries or second-phase elastic particles are incorporated through a complementary finite element solution. The formulation includes a set of simple constitutive rules to model the lattice resistance to dislocation glide, as well as the generation, annihilation and pinning of dislocations at point obstacles. The focus is on the predicted strain hardening of these materials when only a single slip system is active. When the particle morphology is such as to require geometrically necessary dislocations, hardening in the composite materials exhibits a distinct size effect. This size effect is weaker than that predicted by simple analytical estimates based on geometrically necessary dislocations.

Probing star formation and ISM properties using galaxy disk inclination. III. Evolution in dust opacity and clumpiness between redshift 0.0 < z < 0.7 constrained from UV to NIR

Attenuation by dust severely impacts our ability to obtain unbiased observations of galaxies, especially as the amount and wavelength dependence of the attenuation varies with the stellar mass M-*, inclination i, and other galaxy properties. In this study, we used the attenuation - inclination models in ultraviolet, optical, and near-infrared bands designed by Tuffs and collaborators to investigate the average global dust properties in galaxies as a function of M-*, the stellar mass surface density mu(*), the star-formation rate SFR, the specific star-formation rate sSFR, the star-formation main-sequence offset dMS, and the star-formation rate surface density Sigma(SFR) at redshifts z similar to 0 and z similar to 0.7. We used star-forming galaxies from the Sloan Digital Sky Survey (similar to 20 000) and Galaxy And Mass Assembly (similar to 2000) to form our low-z sample at 0.04 < z < 0.1 and star-forming galaxies from Cosmological Evolution Survey (similar to 2000) for the sample at 0.6 < z < 0.8. We found that galaxies at z similar to 0.7 have a higher optical depth tau(f)(B) and clumpiness F than galaxies at z similar to 0. The increase in F hints that the stars of z similar to 0.7 galaxies are less likely to escape their birth cloud, which might indicate that the birth clouds are larger. We also found that tau(f)(B) increases with M-* and mu(*), independent of the sample and therefore redshift. We found no clear trends in tau(f)(B) or F with the SFR, which could imply that the dust mass distribution is independent of the SFR. In turn, this would imply that the balance of dust formation and destruction is independent of the SFR. Based on an analysis of the inclination dependence of the Balmer decrement, we found that reproducing the Balmer line emission requires not only a completely optically thick dust component associated with star-forming regions, as in the standard model, but an extra component of an optically thin dust within the birth clouds. This new component implies the existence of dust inside H II regions that attenuates the Balmer emission before it escapes through gaps in the birth cloud and we found it is more important in high-mass galaxies. These results will inform our understanding of dust formation and dust geometry in star-forming galaxies across redshift.Australian Research Council FT140101202 639.042.611Netherlands Organization for Scientific Research (NWO)Scientific Exchanges visitor fellowship IZSEZO_202357Swiss National Science Foundation (SNSF)European Commissio

Scaling of discrete dislocation predictions for near-threshold fatigue crack growth

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small scale yielding are carried out using discrete dislocation dynamics. Plastic deformation is modelled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. Calculations are carried out with different material parameters so that values of yield strength, cohesive strength and elastic moduli varying by factors of three to four are considered. The fatigue crack growth predictions are found to be insensitive to the yield strength of the material despite the number of dislocations and the plastic zone size varying by approximately an order of magnitude. The fatigue threshold scales with the fracture toughness of the purely elastic solid, with the experimentally observed linear scaling with Young's modulus an outcome when the cohesive strength scales with Young's modulus. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Finite strain discrete dislocation plasticity

A framework for carrying out finite deformation discrete dislocation plasticity calculations is presented. The discrete dislocations are presumed to be adequately represented by the singular linear elastic fields so that the large deformations near dislocation cores are not modeled. The finite deformation effects accounted for are: (i) finite lattice rotations and (ii) shape changes due to slip. As a consequence of the nonlinearity, an iterative procedure is needed to solve boundary value problems. Elastic anisotropy together with lattice curvature is shown to lead to a polarization stress term in the rate boundary value problem. The general three-dimensional framework is specialized to plane strain. The plane strain specialization is implemented in a conventional finite element code and two numerical examples are given: plane strain tension of a single crystal strip and combined bending and tension of that strip. The capabilities and limitations of a conventional finite element framework for this class of problems are illustrated and discussed. (C) 2003 Elsevier Ltd. All rights reserved