Thermomechanical Framework for the Constitutive Modeling of Asphalt Concrete

This study is concerned with the constitutive modeling of asphalt concrete. Unlike most constitutive models for asphalt concrete that do not take into account the evolution of the microstructure of the material, this study incorporates the evolution of the microstructure by using a framework that recognizes that a body’s natural configurations can evolve as the microstructure changes. The general framework, on which this study is based, is cast within a full thermomechanical setting. In this paper, we develop models within the context of a mechanical framework that stems from the general framework for models based on the full thermodynamic framework and the resulting equations represent a nonlinear rate type viscoelastic model. The creep and stress relaxation experiments of Monismith and Secor are used for validating the efficacy of the model, and it is found that the predictions of the theory agree very well with the available experimental results. The advantages of using such a framework are many, especially when one wants to model the diverse mechanical and thermodynamic response characteristics of asphalt and asphalt concrete.     

A Comparative Study of Explicit Frequency and Conventional Signal Representations for Speech Recognition

Krishnan, S., and Rao, P. V. S., A Comparative Study of Explicit Frequency and Conventional Signal Representations for Speech Recognition,Digital Signal Processing6(1996), 249–264.The performance of a speech recognition system depends to a large extent on the signal representation strategy. It is therefore important to evaluate various signal representations, with a view to comparing their relative performance on speech recognition tasks. Various comparative studies have been reported earlier in the literature in this respect. However, these studies are limited to comparing some subsets of representations on different kinds of recognition tasks. In this sense, they preclude a fair comparison of the representations on the same task. In contrast, we attempt here a systematic and fairly comprehensive comparison of signal representations along various dimensions (frequency and amplitude scales, speaker normalization, and two statistical classifiers) on a standard (TIMIT) speech database. This study shows that the line spectrum pair frequency representation augmented with spectral amplitudes yields the best recognition performance.     

Quantum size effect in thin films

When the thickness of a film is of the order of the de Broglie wavelength of the electron, size effects appear in the electrical properties of the material. In this paper we derive the current in the presence of a magnetic field perpendicular to the plane of the film. We use the quantum mechanical density-matrix formalism. Unlike previous works we obtain an exact expression for the current. It is found that the current has a non-linear behavior with the electric field. The current also oscillates with the thickness of the film.     

A Technique for Optimal Sequential Maintenance Scheduling

This paper presents the mathematical development of, and a simple solution technique for, an optimal sequential maintenance scheduling problem. The model is shown to satisfy the Kuhn-Tucker conditions, the necessary and sufficient conditions for the existence of an optimal solution. Real-world systems consisting of mixtures of constant and increasing failure rate devices are considered in the model. Sequential preventive maintenance schedules are developed for groups of identical items with increasing failure rates. Provision is made for the corrective maintenance of these groups if failures occur in between the preventive maintenance schedules. Also, constant failure rate devices are accorded corrective maintenance when failures occur. Optimality is achieved by minimizing the total annual maintenance cost, subject to constraints on the system availability, number of maintenance personnel and intervals of preventive maintenance. The model is applied to a coal mine power system example.     

8 MeV electron irradiation effects in silicon photo-detectors

Results of investigations on the electrical properties of n⁺-p-p⁺ silicon (Si) photo-detectors irradiated with 8 MeV electrons are presented. The photo-detectors were irradiated with electrons of doses up to 100 kGy. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics under dark conditions were measured as a function of dose. A significant change in the diffusion component of the saturation current is observed after irradiation, while the generation-recombination component of the saturation current remains almost unchanged. The series resistance is found to increase with increasing dose while the shunt resistance and carrier concentration decrease with dose. Optoelectronic properties, namely short circuit current I_sc, open circuit voltage V_oc under air mass zero illumination and spectral response, were measured at various doses. From the spectral responses of the devices, the minority carrier diffusion length was estimated.     

Non-linear properties of conditional returns under scale mixtures

This article provides a framework for analyzing multifactor financial returns that violate the Gaussian distributional assumption. Analytical expressions are provided for the non-linear regression equation and its prediction error (heteroscedasticity) by modeling the returns of financial assets as scale mixtures of the multivariate normal distribution. The expressions involve conditional moments of the mixing variable. These conditional moments are explicitly derived when the mixing variable belongs to the generalized inverse Gaussian family, of which gamma, inverse gamma and the inverse Gaussian distributions are distinguished members. The derived expressions are non-linear in the parameters and involve the modified Bessel function of the third kind. The effects of the non-linear model, in terms of both the regression equation and heteroscedasticity against the corresponding values for the standard linear regression model, are captured through simulations for the gamma, inverse gamma and inverse Gaussian distributions. The proposed scale mixture models extend the well-known arbitrage pricing theory (APT) in financial modeling to non-Gaussian cases. The methodology is applied to analyze the intra-day log returns quarterly data for DELL and COKE regressed against S&P 500 for the years 1998-2000.     

off-State Degradation in Drain-Extended NMOS Transistors: Interface Damage and Correlation to Dielectric Breakdown

Off-state degradation in drain-extended NMOS transistors is studied. Carefully designed experiments and well-calibrated simulations show that hot carriers, which are generated by impact ionization of surface band-to-band tunneling current, are responsible for interface damage during off-state stress. Classical on-state hot carrier degradation has historically been associated with broken equivSi-H bonds at the interface. In contrast, the off-state degradation in drain-extended devices is shown to be due to broken equivSi-O- bonds. The resultant degradation is universal, which enables a long-term extrapolation of device degradation at operating bias conditions based on short-term stress data. Time evolution of degradation due to broken equivSi-O- bonds and the resultant universal behavior is explained by a bond-dispersion model. Finally, we show that, under off-state stress conditions, the interface damage that is measured by charge-pumping technique is correlated with dielectric breakdown time, as both of them are driven by broken equivSi-O- bonds.     

A note on the computation of large-change multiparameter sensitivities

In this article a method of computing multiparameter finite change sensitivities is derived. The method is conceptually simpler than existing techniques and, for certain applications, computationally more efficient.