James A. DiLellio

Temporal evolution of shear band thickness

Boundary layer methods are used to derive expressions for the jump discontinuities in velocity, stress gradient and temperature gradient across a shear band. This analysis also yields an expression for the temporal evolution of the thermal boundary layer width, which can be interpreted as a measure of shear band thickness. The jump discontinuities are used to formulate a simplified version of the boundary value problem for the half-slab geometry. Calculated results for an illustrative example reveal a shear band thickness which evolves by initially narrowing and then later widening as diffusion becomes significant.

Numerical solution of shear localization in Johnson-Cook materials

Abstract To compute shear localization, a recently developed numerical scheme is adapted for application to certain types of materials described by a Johnson–Cook model. In the context of a slab geometry, the numerical scheme requires discretization only in the time variable to yield the stress and temperature in a shear band. Numerical results for several materials are presented.

An asymptotic model of the mold region in a continuous steel caster

A model is developed to simulate the solidification of the steel shell in the mold region of the continuous casting process. Conduction-dominated temperature fields in the mold, mold flux, steel shell, and molten steel regions are determined through the development of an evolution equation for the solidifying front. This equation is derived in the limit of small aspect ratio, mold width to height, using asymptotic methods. These results are coupled with a lubrication-theory model for the mold flux region. This model assumes a temperature-dependent viscosity for the mold flux and allows for solidification of the flux at temperatures below a critical value. System response to changing casting speeds, superheat, mold wall temperatures, and mold flux properties is investigated.

Shear band formation due to a thermal flux inhomogeneity

The one-dimensional model for the unidirectional shearing of a slab is investigated for the situation in which localized plastic deformation is induced by an inhomogeneity in the thermal flux. The problem is treated as a perturbation of the elastic solution, which persists until plastic deformation becomes significant. The analysis utilizes singular perturbation techniques, with the thin zone of shear localization identified as a boundary layer. It is found that the evolution of the temperature and stress perturbations are governed by only one parameter, which involves the various material constants. Numerical results indicate that the magnitude of this parameter plays an essential role in the growth of the perturbations and hence in the formation of a shear band.

Using VIX Entropy Indicators for Style Rotation Timing

In this article, the authors examine the feasibility of market timing between large-capitalization value and growth portfolios with the use of entropy measures as compared with previously tested methods of market timing using stock market volatility (using the CBOE’s Volatility Index, VIX). Including transaction fees, style rotations using entropy measures appear to provide superior risk-adjusted returns and may offer a desirable alternative strategy for risk-averse investors seeking equity exposure.

Optimal Strategies for Traditional versus Roth IRA/401(k) Consumption During Retirement

We establish an algorithm that produces an optimal strategy for retirees to withdraw funds between their tax-deferred accounts (TDAs), like traditional IRA/401(k) accounts, and their Roth IRA/401(k) accounts, in the context of a financial model based on American tax law. This optimal strategy follows a geometrically simple, intuitive approach that can be used to maximize the size of a retirees bequest to an heir or, alternatively, to maximize a retirees portfolio longevity. We give examples where retirees following the approach currently implemented by major investment firms, like Fidelity and Vanguard, will reduce their bequests by approximately 10% or lose 18 months of portfolio longevity compared to our optimal approach. Further, our strategy and algorithm can be extended to many cases where the retiree has additional, known yearly sources of money, such as income from part-time work, taxable investment accounts, and Social Security.

Income Falsification on Mortgage Applications During the Housing Bubble

We develop new measures to detect income falsification on mortgage applications during the housing bubble. We find that regulators failed to prevent income falsification. Additionally, regulatory requirements imposed on Fannie Mae and Freddie Mac (the “GSEs�?) to promote lending in regulator-defined “underserved areas,�? encouraged securitizers to purchase mortgages with falsified income from those areas, for resale to the GSEs. There were also regulations that required lenders that wished to merge across state lines to demonstrate that they were serving “community needs.�? These regulations are found to have a similar effect. They encouraged lenders to make loans with falsified income to low-income areas, and may have also caused lenders to buy poor quality securitized loans. These findings provide a systemic explanation for the rise in poor quality subprime mortgages, particularly securitized mortgages.

A Hybrid GNSS Integrity Design Leveraging a Priori Signal Noise Characteristics

The objective of this paper is to explore a hybrid Global Navigation Satellite System (GNSS) architecture that efficiently meets the stringent needs of safety of life systems. An architecture is proposed that allocates error bounding and alerting functionality between the space, ground and user segments based on refining the assumptions of the leading-order fault free error sources expected in the near future from developing GNSS technologies. By revisiting the first principles used to derive standard RAIM fault detection, a modified detection algorithm is developed to more accurately accommodate these new fault-free error distributions while supporting timely user alerts. The results of the analysis and simulation indicate that this optimized receiver algorithm and associated architecture can provide significant development and operational benefit for navigation users requiring high levels of integrity.

The Use of Global Navigation Systems in the Aviation Industry

The GPS navigation system, initially tested as Block Iin the 1970s and early 1980s, was developed as an operational system in the 1980s, and became fully operational in the 1990s. Today, modernization is underway to provide additional, more powerful, more robust, and more flexible civil signals for navigation. As we look forward to a combined GNSS featuring Galileo and modernized GPS, opportunities to enhance civil aircraft navigation emerge. These opportunities require mutual cooperation between the aviation authorities which operate the augmentation systems and the GNSS designers who are developing the next generation of space-based navigation systems. The benefits of cooperation range from reduced total ownership costs to a seamless, interoperable global navigation solution. This paper begins to identify the challenges facing government policy makers, while demonstrating the mutual benefits of global cooperation on space-based navigation.