Search

The dynamic thermomechanical responses of polycrystalline materials under shock loading are often dominated by the interaction of defects and interfaces. For example, within metals, a prescribed deformation associated with a shock wave may be accommodated by crystallographic slip, provided a sufficient population of mobile dislocations is available. However, if the deformation rate is large enough, there may be an insufficient number of freely mobile dislocations. In this case, additional dislocations may be nucleated, or alternate mechanisms (e.g. twinning, damage) activated in order to accommodate the deformation. Direct numerical simulation at the mesoscale offers insight into these physical processes that can be invaluable to the development of macroscale constitutive theories, if the mesoscale models adequately represent the anisotropic nonlinear thermomechanical response of individual crystals and their interfaces. The paper briefly outlines a continuum mesoscale modeling framework founded upon a nonlocal dislocation-density based crystal plasticity theory. The nonlocal theory couples continuum dislocation transport with an otherwise local single-crystal model employing nonlinear thermoelasticity and crystallographic plasticity. Dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. The configuration of geometrically necessary dislocation density gives rise to a back-stress that inhibits or accentuates the flow of dislocations. In particular, this paper emphasizes recent implementation of the coupled nonlocal model into a 3D shock hydrocode and simulation results for the dynamic response of polycrystalline copper in two and three dimensions. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the mechanical strength of a semiconductor structure based on gallium nitride in terms of light-emitting diode crystal nearby a dislocation. The role of thermoplastic deformation in the expansion of defect areas during formation of new defects in the heterostructure was estimated. According to the calculation, the local stresses caused by local overheating are insufficient to form new defects, but these stresses can lead to the movement and accumulation of existing dislocations, which can lead to the formation of dislocation clusters, additional overheating and failure of the light-emitting diode. [ABSTRACT FROM AUTHOR]

This paper presents the study of the evolution in situ of distribution of local deformations in near-surface layers in pads made of carbon lamellas under strain deformation. The specimens were produced from two face-to-face docked steel pads 120×40×4 mm and two carbon lamellas FibARM Lamel 120×40×1,2 mm joint using the adhesion "FibARM Resin Laminate+". The strain tests were performed using the "INSTRON 3386" test bed with a maximum tension of 100 κN (10,19 TS). Optical measurement system VIC-3D was used to identify the evolution of deformations distribution in the near-surface layers. The VIC-3D system allowed obtaining the images that reflect the evolution of distribution of relative deformations iso-fields under different loads. In situ experimental research was conducted focused on strain deformations in specimens made of a layered composite: metal/adhesive/carbon lamella. A crack in the specimen was modeled, which allowed determining that with a growth of the overall deformation there develops an evolution of special structural elements from chaotic (along the whole lamella area) to localized ones with higher deformation values in the crack region. It was established that at higher levels of overall deformation in the crack region on the carbon lamella surface, the maximum deformation region with one extremum is divided by a narrow line of low deformation into two parts along the joint of steel plates. A limit deformation is identified. [ABSTRACT FROM AUTHOR]

A large strain elastic-plastic single crystal constitutive law, based on dislocation annihilation and storage, is implemented in a new self-consistent scheme, leading to a multiscale model which achieves, for each grain, the calculation of plastic slip activity, with help of regularized formulation drawn from visco-plasticity, and dislocation microstructure evolution. This paper focuses on the relationship between the deformation history of a BCC grain and induced microstructure during monotonic and two-stage strain paths. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper, we propose a multi-time scale modeling technique for analyzing cyclic plastic deformation in crystalline solids subject to periodic loading. An asymptotic expansion of the variables in the time domain, together with homogenization forms the basis of multi-time scaling. In the macroscopic scale analysis, the oscillatory behavior of the load is averaged out and neglected, and the rate of averaged material behavior is quite slow in cyclic deformation. Implicitly, this means that the periodicity of some variables may be assumed for the oscillatory potion of the material behavior may be approximated. In this formulation, the governing equations are divided into two initial-boundary value problems with two different time scale: one is long time scale problem for describing the smooth averaged solution (global problem) and the other is for the remaining oscillatory potion (local problem). For the global problem, long time increments, which are longer than one cycle period, can be used and this multi-time scaling becomes an effective integrator. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper, we propose a multi-time scale modeling technique for analyzing cyclic plastic deformation in crystalline solids subject to periodic loading. An asymptotic expansion of the variables in the time domain, together with homogenization forms the basis of multi-time scaling. In the macroscopic scale analysis, the oscillatory behavior of the load is averaged out and neglected, and the rate of averaged material behavior is quite slow in cyclic deformation. Implicitly, this means that the periodicity of some variables may be assumed for the oscillatory potion of the material behavior may be approximated. In this formulation, the governing equations are divided into two initial-boundary value problems with two different time scale: one is long time scale problem for describing the smooth averaged solution (global problem) and the other is for the remaining oscillatory potion (local problem). For the global problem, long time increments, which are longer than one cycle period, can be used and this multi-time scaling becomes an effective integrator. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]