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

Ultrafast absorption kinetics of NADH in folded and unfolded conformations

The non-radiative energy transfer is shown to occur on a ~3ps time scale for NADH in the folded form in H2O. Addition of methanol thermodynamically favours the open form, for which energy transfer does not occur

Characterization of the Proton-Transporting Photocycle of Pharaonis Halorhodopsin

AbstractThe photocycle of pharaonis halorhodopsin was investigated in the presence of 100mM NaN3 and 1M Na2SO4. Recent observations established that the replacement of the chloride ion with azide transforms the photocycle from a chloride-transporting one into a proton-transporting one. Kinetic analysis proves that the photocycle is very similar to that of bacteriorhodopsin. After K and L, intermediate M appears, which is missing from the chloride-transporting photocycle. In this intermediate the retinal Schiff base deprotonates. The rise of M in halorhodopsin is in the microsecond range, but occurs later than in bacteriorhodopsin, and its decay is more accentuated multiphasic. Intermediate N cannot be detected, but a large amount of O accumulates. The multiphasic character of the last step of the photocycle could be explained by the existence of a HR′ state, as in the chloride photocycle. Upon replacement of chloride ion with azide, the fast electric signal changes its sign from positive to negative, and becomes similar to that detected in bacteriorhodopsin. The photocycle is enthalpy-driven, as is the chloride photocycle of halorhodopsin. These observations suggest that, while the basic charge translocation steps become identical to those in bacteriorhodopsin, the storage and utilization of energy during the photocycle remains unchanged by exchanging chloride with azide

The probability distribution of internal stresses in externally loaded 2D dislocation systems

The distribution of internal shear stresses in a 2D dislocation system is investigated when external shear stress is applied. This problem serves as a natural continuation of the previous work of Csikor and Groma (Csikor F F and Groma I 2004 Phys. Rev. B 58 2969), where analytical result was given for the stress distribution function at zero applied stress. First, the internal stress distribution generated by a set of randomly positioned ideal dislocation dipoles is studied. Analytical calculations are carried out for this case. The theoretical predictions are checked by numerical simulations showing perfect agreement. It is found that for real relaxed dislocation configurations the role of dislocation multipoles cannot be neglected, but the theory presented can still be applied.Comment: 27 pages, 20 figure

Variational approach in dislocation theory

A variational approach is presented to calculate the stress field generated by a system of dislocations. It is shown that in the simplest case, when the material containing the dislocations obeys Hooke's law the variational framework gives the same field equations as Kr\"oner's theory. However, the variational method proposed allows to study many other problems like dislocation core regularisation, role of elastic anharmonicity and dislocation--solute atom interaction. The aim of the paper is to demonstrate that these problems can be handled on a systematic manner.Comment: 16 pages, 4 figures. Minor changes in the text and few numerical corrections. Few references also adde