Craig D. Foster

Bimodal Phonon Scattering in Graphene Grain Boundaries

Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs.

Finite element modelling of cornea mechanics: a review

The cornea is a transparent tissue in front of the eye that refracts light and facilitates vision. A slight change in the geometry of the cornea remarkably affects the optical power. Because of this sensitivity, biomechanical study of the cornea can reveal much about its performance and function. In vivo and in vitro studies have been conducted to investigate the mechanics of the cornea and determine its characteristics. Numerical techniques such as the finite element method (FEM) have been extensively implemented as effective and noninvasive methods for analyzing corneal mechanics and possible disorders. This article reviews the use of FEM for assessing the mechanical behavior of the cornea. Different applications of FEM in corneal disease studies, surgical predictions, impact simulations, and clinical applications have been reviewed. Some suggestions for the future of this type of modeling in the area of corneal mechanics are also discussed.

Verification of Masonry Building Code to Flexural Behavior of Cement-Stabilized Soil Block

AbstractMost studies involving cement-stabilized soil blocks (CSSB) concern material properties, such as the characteristics of erosion and strength and how the composition of the block affects these properties. Moreover, research has been conducted on the performance of various mortars, investigating their material properties and the tensile bond strength between CSSB units and mortar. In contrast, very little is currently known about CSSB masonry structural behavior. Because structural design codes of traditional masonry buildings were well developed over the past century, many of the same principles may be applicable to CSSB masonry buildings. This paper details the topic of flexural behavior of CSSB masonry walls and whether the Masonry Standards Joint Committee (MSJC) code can be applied to this material for improved safety of such buildings.

Coupled finite element and multibody system dynamics modeling of a three-dimensional railroad system

During the last two centuries, railroad vehicles have been an important means of transportation of both people and cargo, due to their economic and comfort advantages. Railroad vehicles are a highly economical means of transporting large quantities of cargo over long distances, and also provide a safe and comfortable means of passenger transport. Over the last 30 years or so, the finite element method (FEM) has become more widely used to model railroad systems including the rails, sleepers and substructure. Multibody system dynamics (MBS) software programs are used to model the contact between the wheels and the rails in an effort to study the contact forces and the general dynamics of railroad vehicles. Coupling both the FEM and MBS is a very useful technique to build a reliable model that includes the advantages of both methods. In this work, a full three-dimensional finite element model is created that uses beam, solid and spring elements to model the rails, fasteners, sleepers and substructure. The model treats the rails and the substructure as deformable bodies. Mode shapes of the finite element model are extracted for use in a MBS code to analyze the deformation of the track and substructure under dynamic loading conditions. The results of this new model agree well with results published in the literature.

Soil Models and Vehicle System Dynamics

Abstract : The mechanical behavior of soils may be approximated using different models that depend on particular soil characteristics and simplifying assumptions. For this reason, researchers have proposed and expounded upon a large number of constitutive models and approaches that describe various aspects of soil behavior. However, there are few material models capable of predicting the behavior of soils for engineering applications and are at the same time appropriate for implementation into finite element (FE) and multibody system (MBS) algorithms. This paper presents a survey of some of the commonly used continuum-based soil models. The aim is to provide a summary of continuum-based soil models and examine their suitability for integration with the large-displacement FE absolute nodal coordinate formulation (ANCF) and MBS algorithms. Special emphasis is placed on the formulation of soils used in conjunction with vehicle dynamics models. The implementation of these soil models in MBS algorithms used in the analysis of complex vehicle systems is also discussed. Because semi-empirical terramechanics soil models are currently the most widely used to study vehicle/soil interaction, a review of classical terramechanics models is presented in order to be able to explain the modes of displacements that are not captured by these simpler models. Other methods such as the particle-based and mesh-free models are also briefly reviewed. A Cam-Clay soil model is used in this paper to explain how such continuum-mechanics based soil models can be implemented in FE/MBS algorithms.

Promoting convergence: The Phi spiral in abduction of mouse corneal behaviors

Why do mouse corneal epithelial cells display spiraling patterns? We want to provide an explanation for this curious phenomenon by applying an idealized problem solving process. Specifically, we applied complementary line-fitting methods to measure transgenic epithelial reporter expression arrangements displayed on three mature, live enucleated globes to clarify the problem. Two prominent logarithmic curves were discovered, one of which displayed the ϕ ratio, an indicator of an optimal configuration in phyllotactic systems. We then utilized two different computational approaches to expose our current understanding of the behavior. In one procedure, which involved an isotropic mechanics-based finite element method, we successfully produced logarithmic spiral curves of maximum shear strain based pathlines but computed dimensions displayed pitch angles of 35° (ϕ spiral is ∼17°), which was altered when we fitted the model with published measurements of coarse collagen orientations. We then used model-based reasoning in context of Peircean abduction to select a working hypothesis. Our work serves as a concise example of applying a scientific habit of mind and illustrates nuances of executing a common method to doing integrative science.

Mechanics and spiral formation in the rat cornea

During the maturation of some mammals such as mice and rats, corneal epithelial cells tend to develop into patterns such as spirals over time. A better understanding of these patterns can help to understand how the organ develops and may give insight into some of the diseases affecting corneal development. In this paper, a framework for explaining the development of the epithelial cells forming spiral patterns due to the effect of tensile and shear strains is proposed. Using chimeric animals, made by combining embryonic cells from genetically distinguishable strains, we can observe the development of patterns in the cornea. Aggregates of cell progeny from one strain or the other called patches form as organs and tissue develop. The boundaries of these patches are fitted with logarithmic spirals on confocal images of adult rat corneas. To compare with observed patterns, we develop a three-dimensional large strain finite element model for the rat cornea under intraocular pressure to examine the strain distribution on the cornea surface. The model includes the effects of oriented and dispersed fibrils families throughout the cornea and a nearly incompressible matrix. Tracing the directions of critical strain vectors on the cornea surface leads to spiral-like curves that are compared to the observed logarithmic spirals. Good agreement between the observed and numerical curves supports the proposed assumption that shear and tensile strains facilitate sliding of epithelial cells to develop spiral patterns.

EMBEDDED COLLAGEN DEFORMATION MODELS FOR COMPUTATIONAL MODELING OF HEALTHY, KERATOCONIC, AND CROSSLINKED CORNEAS

Collagen plays an extremely important role in carrying forces and maintaining the shape of the cornea. In keratoconus, the cornea shape can become distorted to the extent that normal vision is impossible, and the amount crosslinking between collagen fibrils are generally lower than in healthy eyes. In contrast, riboflavin-induced crosslinks can strengthen and stiffen the cornea. This article examined quantitatively how the extent of crosslinking in collagen fibrils influences the overall mechanical behavior of corneal tissue. Three models for the stress–strain behavior of the fibrils were examined, which is a function of the crosslink density within the fibrils. These models were then embedded in a matrix model, and tensile tests of cornea strips were examined using a finite element program. Results were compared with experiments from the literature for both normal and crosslinked corneas.

Soil models survey and vehicle system dynamics

The mechanical behavior of soils may be approximated using different models that depend on particular soil characteristics and simplifying assumptions. For this reason, researchers have proposed and expounded upon a large number of constitutive models and approaches that describe various aspects of soil behavior. However, there are few material models capable of predicting the behavior of soils for engineering applications and are at the same time appropriate for implementation into finite element (FE) and multibody system (MBS) algorithms. This paper presents a survey of different commonly used terramechanics and continuum-based soil models. The aim is to provide a summary of soil models, compare them, and examine their suitability for integration with large-displacement FE absolute nodal coordinate formulation (ANCF) and MBS algorithms. Special emphasis is placed on the formulations of soils used in conjunction with vehicle dynamic models. A brief review of computer software used for soil modeling is provided and the implementation of these soil models in MBS algorithms used in the analysis of complex vehicle systems is discussed.Copyright

Detailed experimental review of flexural behavior of cement stabilized soil block masonry

Cement stabilized soil block (CSSB) is a modern earthen building material that is used in the construction of masonry structures. Most studies involving CSSB have focused on how its composition affects strength characteristics and material properties such as erosion. In contrast, very little is currently known about CSSB structural behavior. This study set forth to address the absence of data in this research area, focusing on flexural panel testing, including separate horizontal and vertical bending tests. It was found that CSSB follows traditional masonry reactions; also, through experimental observation, two separate failure modes were identified. A linear elastic analysis confirmed these failure modes and predicted the cracking moment in the specimens analyzed