Some Limitations of Dislocation Walls as Models for Plastic Boundary Layers

It has recently become popular to analyze the behavior of excess dislocations in plastic deformation under the assumption that such dislocations are arranged into walls with periodic dislocation spacing along the wall direction. This assumption is made plausible by the fact that periodic walls represent minimum energy arrangements for dislocations of the same sign, and it allows to use the analytically known short-ranged stress fields of such walls for analyzing the structure of plastic boundary layers. Here we show that unfortunately both the idea that dislocation walls are low-energy configurations and the properties of their interactions depend critically on the assumption of a periodic arrangement of dislocations within the walls. Once this assumption is replaced by a random arrangement, the properties of dislocation walls change completely.Comment: To appear in: Proceedings of the International conference on numerical analysis and applied mathematics (ICNAAM) 2011, 4 pages, to appear in APS proceeding

Abnormal subgrain growth in a dislocation-based model of recovery

Simulation of subgrain growth during recovery is carried out using two-dimensional discrete dislocation dynamics on a hexagonal crystal lattice having three symmetric slip planes. To account for elevated temperature (i) dislocation climb was allowed and (ii) a Langevin type thermal noise was added to the force acting on the dislocations. During the simulation, a random ensemble of dislocations develop into subgrains and power-law type growth kinetics are observed. The growth exponent is found to be independent of the climb mobility, but dependent on the temperature introduced by the thermal noise. The in-depth statistical analysis of the subgrain structure shows that the coarsening is abnormal, i.e. larger cells grow faster than the small ones, while the average misorientation between the adjacent subgrains remains nearly constant. During the coarsening Holt's relation is found not to be fulfilled, such that the average subgrain size is not proportional to the average dislocation spacing. These findings are consistent with recent high precision experiments on recovery.Comment: 17 pages, 11 figure

A comparison of a statistical-mechanics based plasticity model with discrete dislocation plasticity calculations

A two-dimensional nonlocal version of continuum crystal plasticity theory is proposed, which is based on a statistical-mechanics description of the collective behavior of dislocations coupled to standard small-strain crystal continuum kinematics for single slip. It involves a set of transport equations for the total dislocation density field and for the net-Burgers vector density field, which include a slip system back stress associated to the gradient of the net-Burgers vector density. The theory is applied to the problem of shearing of a two-dimensional composite material with elastic reinforcements in a crystalline matrix. The results are compared to those of discrete dislocation simulations of the same problem. The continuum theory is shown to be able to pick up the distinct dependence on the size of the reinforcing particles for one of the morphologies being studied. Also, its predictions are consistent with the discrete dislocation results during unloading, showing a pronounced Bauschinger effect. None of these features are captured by standard local plasticity theories. (C) 2003 Elsevier Ltd. All rights reserved

The role of weakest links and system size scaling in multiscale modeling of stochastic plasticity

Plastic deformation of crystalline and amorphous matter often involves intermittent local strain burst events. To understand the physical background of the phenomenon a minimal stochastic mesoscopic model was introduced, where microstructural details are represented by a fluctuating local yielding threshold. In the present paper, we propose a method for determining this yield stress distribution by lower scale discrete dislocation dynamics simulations and using a weakest link argument. The success of scale-linking is demonstrated on the stress-strain curves obtained by the resulting mesoscopic and the discrete dislocation models. As shown by various scaling relations they are statistically equivalent and behave identically in the thermodynamic limit. The proposed technique is expected to be applicable for different microstructures and amorphous materials, too.Comment: 13 pages, 12 figure

Matrix metalloproteinase 9 (MMP-9) is differently expressed in cutaneous lichen planus and lichen sclerosus

Lichen planus is a mucocutaneous inflammatory disease of unknown etiology. Hyperkeratosis, focal hypergranulosis, damage to the basal cell layer, and bandlike infiltrate are hallmarks of LP skin. Lichen sclerosus is a lymphocyte-mediated dermatosis that has a predilection for the genital skin in both sexes. Both pathologies are sharing a number of common characteristics. Previous studies have reported on involvement of matrix metalloproteinases and their capability of digesting extracellular matrix and basement membrane components. We report on the clinical examination of 11 lichen planus patients and 5 lichen sclerosus patients and the morphological evaluation of their skin biopsy samples. Clinical and routine light microscopy findings correlate with the literature data. By contrast, the expression of MMP-9 was greatly varying among clinical types of cutaneous lichen ruber planus. There was similarity in the MMP-9 expression observed between lichen planus pemphigoides and lichen sclerosus