Moe Khaleel

Size and boundary effects in discrete dislocation dynamics: coupling with continuum finite element

In this work, we develop a framework coupling continuum elasto-viscoplasticity with three-dimensional discrete dislocation dynamics (micro3d). The main problem is to carry out rigorous analyses to simulate the deformation of single crystal metals (fcc and bcc) of finite domains. While the overall macroscopic response of the crystal is based on the continuum theory, the constitutive response is determined by discrete dislocation dynamics analyses using micro3d. Size effects are investigated by considering two boundary value problems: (1) uniaxial loading of a single crystal cube, and (2) bending of a single crystal micro-beam. It is shown that boundary conditions and the size of the computational cell have significant effect on the results due to image stresses from free-boundaries. The investigation shows that surface effects cannot be ignored regardless of the cell size, and may result in errors as much as 10%. Preliminary results pertaining to dislocation structures under bending conditions are also given.

The displacement, and strain–stress fields of a general circular Volterra dislocation loop

Abstract A closed-form analytical solution for the displacement, and strain–stress fields of a circular Volterra dislocation loop having a glide and prismatic components is obtained. Assuming linear elasticity and infinite isotropic material, the displacement field is found by integrating the Burgers displacement equation for a circular dislocation loop. The strain field is subsequently obtained and stresses follow from Hookes law. The field equations are expressed in terms of complete elliptic integrals of the first, second, and/or third elliptic integrals. The general loop solution is, from the principle of superposition, the additive sum of the prismatic and glide solutions.

Muitiscale modeling of irradiation induced hardening in a-Fe, Fe-Cr and Fe-Ni systems

Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation-defects (such as voids, helium bubbles and prismatic loops) interactions is obtained from MD simulations. Then this information is used by DD to simulate large systems with high dislocation and defect densities.

Microstructure sensitive design and quantitative prediction of effective conductivity in fuel cell design

Statistical continuum approach is used to predict effective conductivity of anisotropic random porous heterogeneous media using two-point correlation functions. Probability functions play a critical role in describing the statistical distribution of different constituents in a heterogeneous media. In this study a 3-dimensional two-point correlation function is utilized to characterize the anisotropic porous media of a Cathode materials to incorporate all the details of the microstructure. These correlation functions are then linked to the effective properties using homogenization relations. An anisotropioc Green’s function solution is used to solve the set of field equations. Examples in this study demonstrated how the model captured the anisotropy in effective conductivity of the random heterogeneous media. Predicted results showed the influence of microstructure on the effective conductivity tensor.

Reliability Analysis of Low Alloy Ferritic Piping Materials

The aim of this study is to improving microstructure and mechanical properties of the weldable gas pipeline steel using laboratory mill. To achieve the required microstructure and mechanical properties of thermo mechanically processed HSLA steels, it is necessary to have an idea about the role of composition and process parameters. The large numbers of parameters obtained during the production process in the plant were systematically changed to optimize the strength and toughness properties. The optimized parameters were used for the production of the API X60/X70 steel. However, the controlled cooling after rolling should result in transformed products that provide excellent combination of strength and toughness. The coiling at an appropriate temperature have the advantage of the precipitation strengthening, giving further rise to the high yield strength and also improvement in toughness of the steel. The coiling temperature is a decisive parameter because it determines the beginning of the formation of fine precipitations. Therefore, four different laboratory cooling systems were used, in this study to simulate the rolling conditions of a real industrial Thermomechanically controlled process, as close as possible and to check the possibilities of improving the mechanical properties of the welded pipeline steel.

A Multidiscipline and Multi-rate Modeling Framework for Planar Solid-oxide-fuel-cell based Power-Conditioning System for Vehicular APU

A numerical modeling framework for planar solid-oxide fuel cell (PSOFC) based vehicular auxiliary power unit (APU) is developed. The power-conditioning system (PCS) model comprises the comprehensive transient models of PSOFC, balance-of-plant and power-electronics subsystems (BOPS and PES, respectively) and application load (AL). It can be used for resolving the interactions among PSOFC, BOPS, PES and AL, control design and system optimization and studying fuel-cell durability. The PCS model has several key properties including: (i) it can simultaneously predict spatial as well as temporal dynamics; (ii) it has two levels of abstraction: comprehensive (for detailed dynamics) and reduced-order (for fast simulation); and (iii) the fast-simulation model can be implemented completely in Simulink/Matlab environment, thereby significantly reducing the cost as well as time and provides the avenue for real-time simulation and integration with vehicular power-train models employing the widely used ADVISOR. The computational overhead and accuracy of the fast-simulation and comprehensive models are compared. Significant savings in time compared to using the former were obtained, without compromising accuracy.

THERMOMAGNETIC PROPERTIES OF RARE-EARTH REPLACEMENT CRITICAL MAGNETIC MATERIALS FROM DFT CALCULATION: MnBi AND MnSb

MnBi has gained much attention as a replacement for critical rare earth magnetic material not only due to its strong magnetization and coercive power, but also because of its capability to retain magnetization at elevated temperatures while most other compounds decline. To investigate the origin of this temperature dependence, we have performed a series of first principles electronic structure calculations on the thermomagnetic properties of MnBi and compared it with MnSb, another ferromagnetic material with a strong magnetic energy product, same crystal structure at room temperature and similar Curie temperature. Three structural phases were considered in this study: NiAs-type (B81), MnP-type (B31) and a zincblende-type (B3) structures. Calculated magnetizations demonstrated structural effects on temperature dependent magnetization. For the same NiAs-type structure, MnBi has a monotonic increase in magnetization with increasing temperature while MnSb decreases. In the other two structures, magnetization in MnBi and MnSb are much less sensitive to temperature. Results from this study suggest a structural design rule for the development of new MnBi related materials.