Discrete dislocation modeling

Strain rate and temperature dependent elastic limit of mild steel is investigated by developing a dislocation incipient motion-based proportionality limit stress model. Temperature effect on strain energy of an edge dislocation is modeled by using unified mechanics theory. Unified mechanics theory-based index, called thermodynamic state index, is used to model thermally assisted degradation of strain energy. Kinetic energy due to thermal vibrations is added to the kinetic energy of an accelerating dislocation.

Contact: Dr. Siddiq Qidwai, siddiq.qidwai@nrl.navy.mil

Postdoc Information: http://hroffice.nrl.navy.mil/jobs/postdoc.htm. Note the substantial stipend and benefits.

In the context of a European project involving partners in France, Germany and Belgium, we are seeking a motivated post-doc to implement a saddle-point search method in a discrete dislocation dynamics code to study thermally-activated dislocation processes, such as cross-slip, in complex microstructures. The start date is October 2014 and the post-doc will be based in Lyon (Institut Lumière Matière) but with strong interactions with the Laboratoire d'Etude des Microstructures, CNRS/ONERA near Paris.

The Multifunctional Materials Branch of the Materials Science & Technology Division at the US Naval Research Laboratory in Washington DC has an interest in postdoctoral associate candidates for several areas in computational mechanics and multiphysics.

Topic of interest include: a) biomechanics of injury at high-rate of deformation at multiple scales, b) multiscale dynamics of flexible armor, c) crystal plasticity and dislocation dynamics in electrically assisted deformation of metals, and d) multiphysics modeling of corrosion.

This is a paper we recently published in JMPS on a study of the mechanical properties on thin films comparing experimental results with discrete dislocation simulations. It provides insight in the strengthening that occurs in thin metal films when surface or interface effects become important.

The abstract is below; the full paper can be downloaded from here

Abstract - Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress–strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank–Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.