James D. Hogan

The Spanwise Dependence of Vortex-Shedding From Yawed Circular Cylinders

Simultaneous measurements of the fluctuating wall pressure along the cylinder span were used to examine the spanwise characteristics of the vortex-shedding for yaw angles varying from α = 60 deg to α = 90 deg. The Reynolds number based on the diameter of the cylinder was 56,100. The results indicate that yawing the cylinder to the mean flow direction causes the vortex-shedding in the wake to become more disorderly. This disorder is initiated at the upstream end of the cylinder and results in a rapid decrease in correlation length, from 3.3D for a = 90 deg to 1.1D for a = 60 deg. The commonly used independence principle was shown to predict the vortex-shedding frequency reasonably well along the entire cylinder span for a > 70 deg, but did not work as well for α = 60 deg.

Growth and Characterization of Metastable Hexagonal Nickel Thin Films via Plasma-Enhanced Atomic Layer Deposition

There is a great interest in various branches of the advanced materials industry for the development of novel methods (and improvements to existing ones) for the deposition of conformal ultrathin metallic films. In most of these applications, like enhanced solar absorbers and microelectronics, achieving the capacity to deposit a conformal thin film on a three-dimensional structure is an important condition. Plasma-enhanced atomic layer deposition (ALD) is known for its potential for growth of conformal thin films with a precise control over the thickness and its capability for deposition at relatively low temperatures (below 500 °C). This study evaluates the potential of plasma-enhanced ALD for growth of conformal nickel thin films, using bis(ethylcyclopentadienyl)nickel and nitrogen/hydrogen plasma as precursors. A comprehensive analysis of the structure, composition, and physical properties of the films was performed. The results indicate that conformal nickel films with low levels of impurity were successfully deposited on sapphire. The films had a roughness of Ra = 1.5 nm and were seen to be under strain. The deposited nickel had a hexagonal crystal structure, with a random in-plane orientation of the grains, while the grains had their c-axes oriented along the normal to the interface. These results pave the way for conformal low-temperature deposition of high-quality nickel thin films on three-dimensional structures.

The Use of a Railgun Facility for Dynamic Fracture of Brittle Materials

Linear electromagnetic acceleration has a wide range of applications. Among them are applications in the field of materials science where relatively small, inexpensive systems can be of high value. For instance, it was shown recently at the French-German Institute of Saint Louis (ISL), France, that the symmetric Taylor test - a method to investigate the deformation behaviour of specimens at high deformation rates- can be realized with higher precision than attainable with other acceleration methods by using a railgun with velocity control. In this work, we present another example for an application of a railgun in the field of materials science, namely the study of impact phenomena and terminal ballistics. One of the major advantages of railgun technology is that the acceleration profile can be well defined at velocity ranges from very low speeds (<; 10 m/s) up to more than 2000 m/s - for one and the same launcher. Moreover, the geometry of a railgun projectile can be round, rectangular or, as in the case discussed here, hexagonal. Finally, electromagnetic acceleration does not require the use of propellants, which in the case of impact experiments could lead to complications - at least for the experimental application presented. The quest is to study the ejecta field of fractured brittle materials at moderate impact velocities (<; 500 m/s). It transpires that a railgun can be a valuable tool for such investigations. Using high-speed cameras and novel data processing methods ejecta distribution-velocity relations are explored. Our contribution describes the experimental setup used and will introduce some of the major results obtained so far.

Changes in Neurofilament and Microtubule Distribution following Focal Axon Compression

Although a number of cytoskeletal derangements have been described in the setting of traumatic axonal injury (TAI), little is known of early structural changes that may serve to initiate a cascade of further axonal degeneration. Recent work by the authors has examined conformational changes in cytoskeletal constituents of neuronal axons undergoing traumatic axonal injury (TAI) following focal compression through confocal imaging data taken in vitro and in situ. The present study uses electron microscopy to understand and quantify in vitro alterations in the ultrastructural composition of microtubules and neurofilaments within neuronal axons of rats following focal compression. Standard transmission electron microscopy processing methods are used to identify microtubules, while neurofilament identification is performed using antibody labeling through gold nanoparticles. The number, density, and spacing of microtubules and neurofilaments are quantified for specimens in sham Control and Crushed groups with fixation at <1min following load. Our results indicate that the axon caliber dependency known to exist for microtubule and neurofilament metrics extends to axons undergoing TAI, with the exception of neurofilament spacing, which appears to remain constant across all Crushed axon diameters. Confidence interval comparisons between Control and Crushed cytoskeletal measures suggests early changes in the neurofilament spatial distributions within axons undergoing TAI may precede microtubule changes in response to applied loads. This may serve as a trigger for further secondary damage to the axon, representing a key insight into the temporal aspects of cytoskeletal degeneration at the component level, and suggests the rapid removal of neurofilament sidearms as one possible mechanism.

Vortex Shedding in a Tandem Circular Cylinder System With a Yawed Downstream Cylinder

This investigation examines the flow produced by a tandem cylinder system with the downstream cylinder yawed to the mean flow direction. The yaw angle was varied from α=90deg (two parallel tandem cylinders) to α=60deg; this has the effect of varying the local spacing ratio between the cylinders. Fluctuating pressure and hot-wire measurements were used to determine the vortex-shedding frequencies and flow regimes produced by this previously uninvestigated flow. The results showed that the frequency and magnitude of the vortex shedding varies along the cylinder span depending on the local spacing ratio between the cylinders. In all cases the vortex-shedding frequency observed on the front cylinder had the same shedding frequency as the rear cylinder. In general, at small local spacing ratios the cylinders behaved as a single large body with the shear layers separating from the upstream cylinder and attaching on the downstream cylinder, this caused a correspondingly large, low frequency wake. At other positions where the local span of the tandem cylinder system was larger, small-scale vortices began to form in the gap between the cylinders, which in turn increased the vortex-shedding frequency. At the largest spacings, classical vortex shedding persisted in the gap formed between the cylinders, and both cylinders shed vortices as separate bodies with shedding frequencies typical of single cylinders. At certain local spacing ratios two distinct vortex-shedding frequencies occurred indicating that there was some overlap in these flow regimes.

Dynamic Failure and Fragmentation of a Hot-Pressed Boron Carbide

This study investigates the failure and fragmentation of a hot-pressed boron carbide during high rate impact experiments. Four impact experiments are performed using a composite-backed target configuration at similar velocities, where two of the impact experiments resulted in complete target penetration and two resulted in partial penetration. This paper seeks to evaluate and understand the dynamic behavior of the ceramic that led to either the complete or partial penetration cases, focusing on: (1) surface and internal failure features of fragments using optical, scanning electron, and transmission electron microscopy, and (2) fragment size analysis using state-of-the-art particle-sizing technology that informs about the consequences of failure. Detailed characterization of the mechanical properties and the microstructure is also performed. Results indicate that transgranular fracture was the primary mode of failure in this boron carbide material, and no stress-induced amorphization features were observed. Analysis of the fragment sizes for the partial and completely penetrated experiments revealed a possible correlation between larger fragment sizes and impact performance. The results will add insight into designing improved advanced ceramics for impact protection applications.

Correlation of Microstructure to Mechanical Properties in Two Grades of Alumina

In this research, two grades of alumina, one at nominally 85% composition and the other at 99.5% were characterized. Microstructural and phase characterization was conducted using Scanning Electron Microscopy, Energy Dispersive Spectroscopy, X-ray Diffraction, and micro-computed X-ray tomography. It was determined that the Knoop hardness values were influenced by the porosity in the 85% composition. Quasi-static compressive tests and high strain rate compression experiments were conducted to determine the influence of the microstructure to the compressive properties. It was observed that the overall compressive strengths increased with strain rate. Although the Knoop hardness values were much lower in the 85% alumina due to the porosity, the compressive strength at both quasi-static and dynamic strain rates were not significantly lower than those of the 99.5% composition.

Block Diagram Model for the Simulation of an Electromagnetic Rail Accelerator System

Rail accelerators are superior to classical gas driven accelerators with regard to attainable terminal velocity, ignition delay variation, and controllability and, therefore, have received a growing scientific interest in the recent past. The behavior of such a launcher is generally described with a system of nonlinear differential equations, which can be solved in many ways. This is done in order to describe an existing launcher, to predict its performance when parameters change, or to estimate the properties of such a system in the design phase. This paper presents a simplified electro-mechanical system of differential equations in a novel form. This form enables scientists and engineers to solve the equations using the software Scilab, Matlab or a similar one. The model is robust and parameter changes have a low impact on the solving time. This makes it a reliable tool for parametric studies. The model comprises the pulsed power capacitors, the pulse forming network, cables and the launcher itself. At first, the electrical part is described with its circuit equivalent and coupled with the mechanical process. Secondly, the system is analytically transposed to a standard form. This form is then transferred into the simulation software, which solves the equations. It can be simply adapted to other launchers and modified to comprise nonlinear side effects. The simulation results are compared to experimental results from the augmented rail accelerator at the French-German Research Institute Saint-Louis, France. A comparison between simulation results and experimental measurements shows a good agreement, whereas the model is still kept simple.

Block diagram model for the simulation of an electromagnetic rail accelerator system

Rail accelerators are superior to classical gas-driven accelerators with regard to attainable terminal velocity, ignition delay variation, and controllability. The behavior is generally described with a system of nonlinear differential equations, which can be solved in many ways. This is done in order to describe an existing launcher, to predict its performance when parameters change, or to estimate the properties of such a system in the design phase. This paper presents a simplified electromechanical system of differential equations in a novel form. This form enables scientists to solve the equations using one of the following software: Scilab, MATLAB, or a similar one. The model is robust and parameter changes have a low impact on the solving time. This makes it a reliable tool for parametric studies. The model comprises pulsed power capacitors, the pulse-forming network, cables, and the launcher itself. At first, the electrical part is described with its circuit equivalent and coupled with the mechanical process. Second, the system is analytically transposed to a standard form. This form is then transferred into the simulation software, which solves the equations. It can simply be adapted to other launchers and modified to comprise nonlinear side effects. The simulation results are compared with experimental results from the augmented rail accelerator at the French-German Research Institute of Saint-Louis, France.

The use of a railgun facility for dynamic fracture of brittle materials

Linear electromagnetic acceleration has a wide range of applications. Among them are applications in the field of materials science, where relatively small, inexpensive systems can be of high value. For instance, it was shown recently at the French-German Institute of Saint Louis, France, that the symmetric Taylor test-a method to investigate the deformation behavior of specimens at high deformation rates-can be realized with higher precision than attainable with other acceleration methods using a railgun with velocity control. In this paper, we present another example for an application of a railgun in the field of materials science, namely, the study of impact phenomena and terminal ballistics. One of the major advantages of railgun technology is that the acceleration profile can be well defined at velocity ranges from very low speeds (<;10 m/s) up to more than 2000 m/s-for one and the same launcher. Moreover, the geometry of a railgun projectile can be round, rectangular or, as in the case discussed here, hexagonal. Finally, electromagnetic acceleration does not require the use of propellants, which in the case of impact experiments could lead to complications-at least for the experimental application presented. The quest is to study the ejecta field of fractured brittle materials at moderate impact velocities (<;500 m/s). It transpires that a railgun can be a valuable tool for such investigations. Using high-speed cameras and novel data processing methods ejecta distribution-velocity relations are explored. Our contribution describes the experimental setup used and will introduce some of the major results obtained so far.