Erin McGarrity

Expanded chain dimensions in polymer melts with nanoparticle fillers

We apply the self-consistent polymer reference interaction site model (SC/PRISM) to liquid state calculations of the chain dimensions in polymer melts with added nanoparticle fillers. The nanoparticles are assumed to be smaller than the polymer radius of gyration and are attracted to the polymer so that they are miscible. We find that the nanoparticles perturb the chain dimensions, causing an increase in the radius of gyration with increasing nanoparticle volume fractions, assuming reasonable interaction energies between the various components. The magnitude of the expansion is in qualitative agreement with recent neutron scattering results and suggests that the SC/PRISM approach is reasonable when dealing with these apparent nonlinear phenomena present in nanocomposites in the protein limit.

A method for calibrating see-through head-mounted displays for AR

In order to have a working augmented reality (AR) system, the see-through system must be calibrated such that the internal models of objects match their physical counterparts. By match, we mean they should have the same position, orientation, and size information as well as any intrinsic parameters (such as focal lengths in the case of cameras) that their physical counterparts have. To this end, a procedure must be developed which estimates the parameters of these internal models. This calibration method must be both accurate and simple to use. This paper reports on our efforts to implement a calibration method for a see-through head-mounted display. We use a dynamic system in which a user interactively modifies the camera parameters until the image of a calibration object matches the image of a corresponding physical object. The calibration method is dynamic in the sense that we do not require the users head to be immobilized.

A new system for online quantitative evaluation of optical see-through augmentation

A crucial aspect in the implementation of an augmented reality (AR) system is determining its accuracy. The accuracy of a system determines the applications it can be used for. The aim of our research is measuring the overall accuracy of an arbitrary AR system. Once measurements of a system are made, they can be analyzed for determining the structure and sources of errors. From the analysis it may also be possible to improve the methods used to calibrate and register the virtual to the real. This paper describes an online system for measuring the registration accuracy of optical see-through augmentation. By online, we mean that the user can measure the registration error they are experiencing while they are using the system. We overcome the difficulty of not having retinal access by having the user indicate the projection of a perceived object on a planar measurement device. Our method provides information which can be used to analyze the structure of the system error in two or three dimensions. The results of the application of our method to two monocular optical see-through AR systems are shown.

Effects of grain boundary constraint on properties of polycrystalline materials

Grain boundary networks are engineered by increasing the fraction of boundaries which exhibit improved properties. Favourable boundaries have either low grain boundary misorientation or they are special boundaries, such as coincident site lattice boundaries. Significant improvement in properties such as corrosion resistance, critical current in superconductors and mechanical strength and toughness occur, provided percolating grain or grain boundary structures can be engineered. We develop computational models for grain boundary engineered polycrystals and demonstrate that grain boundary constraints modify the behaviour near the percolation threshold. We postulate that this is due to an enhanced clustering of weak boundaries induced by grain boundary constraints. In random grain structures the fraction of strong grain boundaries may be measured in two ways, either the length fraction, c, or the edge fraction ce. We find that grain boundary constraint shifts the length fraction threshold, c*, of Potts model polycrystals to higher values, while the edge fraction, ce*, remains almost the same in both correlated and uncorrelated grain structures.

Critical Manifolds in Polycrystalline Grain Structures

With the development of new, fully three-dimensional metallographic techniques, there is considerable interest in characterizing three-dimensional microstructures in ways that go beyond twodimensional stereology. One characteristic of grain structures is the surface of lowest energy across the microstructure, termed the critical manifold (CM). When the grain boundaries are sufficiently weak, the CM lies entirely on grain boundaries, while when the grain boundaries are strong, cleavage occurs. A scaling theory for the cleavage to intergranular transition of CMs is developed. We find that a critical length scale exists, so that on short length scales cleavage is observed, while at long length scales the CM is rough. CMs for realistic polycrystalline grain structures, determined by a network optimization algorithm, are used to verify the analysis.

Network Algorithms and Critical Manifolds in Disordered Systems

Random manifolds in the form of critical paths or surfaces, are observed in a wide variety of processes in materials physics. Spectacular examples are critical manifolds on which failure occurs, for example strain localisation and fracture of materials under stress and electric current localisation in dielectric failure. In these cases localisation of the applied load on a low dimensional manifold acts as a precursor to the failure instability. Failure is an irreversible process, however critical manifolds also emerge in reversible nonlinear processes such as the onset of voltage in an ideal superconductor and the onset of current in a varistor. We shall show that it is possible to study these latter cases in great detail using network optimization algorithms.