Tariq Khraishi

Model studies of the flow in abdominal aortic aneurysms during resting and exercise conditions.

Pulsatile flow in abdominal aortic aneurysm (AAA) models has been examined in order to understand the hemodynamics that may contribute to growth of an AAA. The model studies were conducted by experiments (flow visualization and laser Doppler velocimetry) and by numerical simulation using physiologically realistic resting and exercise flow conditions. We characterize the flow for two AAA model shapes and sizes emulating early AAA development through moderate AAA growth (mean and peak Reynolds numbers of 362 < Re(mean) < 1053 and 3308 < Re(peak) < 5696 with Womersley parameter 16.4 < alpha < 21.2). The results of our investigation indicate that AAA flow can be divided into three flow regimes: (i) Attached flow over the entire cycle in small AAAs at resting conditions, (ii) vortex formation and translation in moderate size AAAs at resting conditions, and (iii) vortex formation, translation and turbulence in moderate size AAAs under exercise conditions. The second two regimes are classified in the medical literature as disturbed flow conditions that have been correlated with atherogenesis as well as thrombogenesis. Thus, AAA disturbed hemodynamics may be a contributing factor to AAA growth by accelerating the degeneration of the arterial wall. Our investigation also concluded that vortex development is considerably weaker in an asymmetric AAA. Furthermore, turbulence was not observed in the asymmetric model. Finally, our investigation suggests a new mode of transition to turbulence: vortex ring instability and bursting to turbulence. The transition process depends on a combination of the pulsatile flow conditions and the tube cross-sectional area change.

Mechanical property degradation in irradiated materials: A multiscale modeling approach

Abstract High-energy particle irradiation of low stacking fault energy, face centered cubic (fcc) metals produces significant degradation of mechanical properties, as evidenced in tensile tests performed at or near room temperature. Post-irradiation microstructural examination reveals that approximately 90% of the radiation-induced defects in copper are stacking fault tetrahedra (SFT). Radiation damage is an inherently multiscale phenomenon involving processes spanning a wide range of length and time scales. Here we present a multiscale modeling methodology to study the formation and evolution of defect microstructure and the corresponding mechanical property changes under irradiation. At the atomic scale, molecular dynamics (MD) simulations have been used to study the evolution of high energy displacement cascades, SFT formation from vacancy rich regions of displacement cascades, and the interaction of SFTs with moving dislocations. Defect accumulation under irradiation is modeled over diffusional length and time scales by kinetic Monte Carlo (KMC), utilizing a database of displacement cascades generated by MD. The mechanical property changes of the irradiated material are modeled using three-dimensional dislocation dynamics (DD). Key input into the DD includes the spatial distribution of defects produced under irradiation, obtained from KMC, and the fate of dislocation interactions with SFTs, obtained from MD.

A parametric-experimental study of void growth in superplastic deformation

Abstract Substantial void growth in metals constitutes a problem in many industrial operations that utilize superplastic deformation. This is because of the likelihood of material failure due to such growth. Hence, there is a need to study void growth mechanisms in an effort to understand the parameters governing it. In this work, numerical and experimental studies of void growth, and the parameters that affect it, in a superplastically deforming (SPD) metal have been performed. In the numerical studies, using the finite-element method, a 1×2 sized thin plate (i.e. plane stress conditions) of a viscoplastic material with pre-existing holes has been subjected to a constant extension rate. The experimental studies were performed under similar conditions to the numerical ones and provided for qualitative comparison. The parameters affecting void growth in SPD are: m (the strain-rate sensitivity), void size (i.e. diameter) and the number (density) of existing voids. The results showed that increased m values produced strengthening and decreased the rate of void growth. In addition, larger initial void size (or, equivalently, a larger initial void fraction) had the effect of weakening the specimen through causing accelerated void growth. Finally, multiple holes had the effect of increasing the metal ductility by reducing the extent of necking and its onset. This was realized through diffusing the plastic deformation at the different hole sites and reducing the stress concentration. The numerical results were in good qualitative agreement with the experiment and suggested the need to refine existing phenomenological void growth models to include the dependence on the void fraction.

Free-Surface Effects in 3D Dislocation Dynamics: Formulation and Modeling

Recent advances in 3-D dislocation dynamics include the proper treatment of free surfaces in the simulations. Dislocation interaction and slip is treated as a boundary-value problem for which a zero-traction condition is enforced at the external surfaces of the simulation box. Here, a new rigorous method is presented to handle such a treatment. The method is semi-analytical/numerical in nature in which we enforce a zero traction condition at select collocation points on a surface. The accuracy can be improved by increasing the number of collocation points. In this method, the image stress-field of a subsurface dislocation segment near a free surface is obtained by an image segment and by a distribution of prismatic rectangular dislocation loops padding the surface. The loop centers are chosen to be the collocation points of the problem. The image segment, with proper selection of its Burgers vector components, annuls the undesired shear stresses on the surface. The distributed loops annul the undesired normal stress component at the collocation points, and in the process create no undesirable shear stresses. The method derives from crack theory and falls under generalized image stress analysis whereby a distribution of dislocation geometries or entities (in this case closed rectangular loops), and not just simple mirror images, are used to satisfy the problems boundary conditions (BCs). Such BCs can, in a very general treatment, concern either stress traction or displacements.

Comparison of Plate-Screw Systems Used in Mandibular Fracture Reduction: Finite Element Analysis

A finite element model of the human dentate mandible has been developed to provide a comparison of fixation systems used currently for fracture reduction. Volume domains for cortical bone, cancellous bone, and teeth were created and meshed in ANSYS 8.0 based on IGES curves created from computerized tomography data. A unilateral molar clench was loaded on the model with a fracture gap simulated along the symphysis. Results based on Von Mises stress in cortical and cancellous bone surrounding the screws, and on fracture surface spatial fixation, show some relative differences between different screw-plate systems, yet all were judged to be appropriate in their reduction potential.

The stress field of a general circular Volterra dislocation loop: Analytical and numerical approaches

A closed-form analytical solution for the stress field of a circular Volterra dislocation loop, having glide and prismatic components, is obtained. Assuming linear elasticity and infinite isotropic material, the stress field is found by line integration of the Peach-Koehler equation for a circular dislocation loop. The field equations are expressed in terms of complete elliptic integrals of the first and second kinds. The general loop solution is, from the principle of superposition, the additive sum of the prismatic and glide solutions. Finally, the obtained stress solution is compared with the stress calculation results from segmented loops (six to 24 segments) having the same radius. Such comparisons are useful as a benchmarking measure for newly emerging dislocation dynamics codes which discretize a curved dislocation line in some form or another.

Strain partitioning in coherent compliant heterostructures

A self-consistent set of equations describing strain partitioning in planar bilayers is developed using a typical definition of strain, the assumption of a coherent interface and a mechanical equilibrium criterion. This approach eliminates the need for the concepts of lattice mismatch and compatibility of deformation, leading to a general solution for the strains and in-plane lattice parameter in bilayer structures. Using the strain equations, the strain energies in the system are calculated as a function of the epilayer to substrate thickness ratio. It was found that for a given substrate thickness, the epilayer strain energy contains a maximum at a layer thickness ratio of ∼1. The peak epilayer strain energy is only ∼25% of the maximum possible in the system. A criterion based on energy considerations is proposed for determining whether to use the epilayer or substrate dislocation formation energy when calculating the epilayer critical thickness. This criterion is applied to the GexSi1−x/Si(100) materia...

Dislocation dynamics simulations of the interaction between a short rigid fiber and a glide circular dislocation pile-up

Abstract This paper presents dislocation dynamics simulations of the interaction of a circular dislocation pileup with a short rigid fiber as occurs, for example, in metal–matrix composites. The pile-up is composed of glide dislocation loops surrounding the fiber. Here, this problem is treated as a boundary value problem within the context of dislocation dynamics. The proper boundary condition to be enforced is that of zero displacements at the fiber–matrix interface. Such a condition is satisfied by a distribution of image rectangular dislocation loops meshing the fibers surface. This treatment is similar to crack modeling using distributed dislocations and falls under the category of “generalized image stress analysis”. The unknown in this problem is the Burgers vectors of the surface loops. Once those are found, the Peach–Koehler (PK) force acting on the circular dislocation loops, and emulating the fiber–dislocation interaction, can be determined and the dynamical arrangement of the circular pile-up evolves naturally from conventional dislocation dynamics analysis.

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.

Biomechanical Analysis of Mandibular Angle Fractures

BACKGROUND Clinical evidence has suggested that minimal fixation can reduce complications of mandibular angle fractures, though no detailed biomechanical model has yet explored this unique and somewhat unexpected finding. METHODS The current study uses finite element analysis to biomechanically evaluate different fixation schemes used to fixate mandibular angle fractures. Three fixation scenarios were considered: a single tension band at the superior mandibular border, a single bicortical angle compression plate at the inferior border and the tension band and bicortical plate used together. RESULTS The dual plate model incurred the lowest von Mises stresses in the plates and the lowest principal strain in the callus. The tension band model observed the highest plate and screw von Mises stresses, but had fracture-site callus strain near to that of the dual plate model. The bicortical angle compression plate model observed the highest fracture-site callus strain. CONCLUSION The results from this study support the use of the single tension band configuration as a less invasive fixation approach to fractures of the mandibular angle. This is the first known study to explore and confirm clinical observations of angle fracture fixation outcomes with a detailed biomechanical modeling methodology.