Comparing rutting of airfield pavements to simulations using Pavement Analysis Using Nonlinear Damage Approach (PANDA)

This study presents the rutting performance results of full-scale pavement test sections subjected to F-15E and C-17 aircraft wheels at two different temperatures. Pavement structures for the tests were constructed under shelter in the U.S. Army Engineer Research and Development Center's (ERDC) pavement test facility. The full-scale test results are used to validate viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in the Pavement Analysis Using Nonlinear Damage Approach (PANDA) model. PANDA is a mechanistic-based model which incorporates nonlinear viscoelastic, viscoplastic, hardening-relaxation, viscodamage, moisture-induced damage and ageing constitutive relationships. Results of dynamic modulus and different repeated creep-recovery laboratory tests are analysed to extract the parameters associated with viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in PANDA. Once calibrated, PANDA is used to predict the rutting performance observed in full-scale pavement test sections. The simulation results illustrate that PANDA is capable of predicting the rutting of airfield pavements subjected to heavy aircraft wheel loads at intermediate and high temperatures. It is shown that PANDA successfully predicts the effect of shear flow and upheaval at the edges of the wheel. The data from simulation suggested that PANDA, once calibrated, can provide insight into the critical locations of tensile and compressive stresses within the pavement structure. PANDA simulations not only provide a tool for evaluating existing structures, but also can be used in designing more sustainable pavement structures and materials.     

32—THE WEATHERING OF COTTON,NYLON, AND TERYLENE FABRICS IN THE UNITED KINGDOM

It is well known that textile fabrics lose strength when subjected to unprotected outdoor exposure, and the amount of strength lost for a given period of time, or for a given number of sun-hours as measured by the Campbell-Stokes apparatus, may vary considerably according to the nature of the fibre and to the climatic conditions prevailing at the exposure site. From data recorded at eight exposure sites in the United Kingdom during 1961 and 1962, it is possible to draw certain conclusions. Thus, in rural areas, such as Pontypool (Mon.), Invergowrie (Perth), and Carlton (Durham), where ‘clean’ or unpolluted atmospheres might be expected to exist, an 11-oz/yd² cotton duck was found to be more resistant than either a 10½-oz/yd² nylon or a 14-oz/yd² Terylene fabric. In semi-rural areas, such as Lanark and Farnborough (Hants.), 11-oz/yd-²; cotton duck appeared to be as resistant as Terylene. It was only in urban areas, such as Didsbury and Droylsden (Lanes.) and Bradford (Yorkshire), that cotton was found to be inferior to Terylene and to a less extent to nylon. The 16-oz/yd² cotton fabric was found to be more resistant than the 11-oz/yd² one.     

Effective harvesting,detection, and conversion of IR radiation due to quantum dots with built-in charge

We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.     

Bayesian internal dosimetry calculations using Markov Chain Monte Carlo

A new numerical method for solving the inverse problem of internal dosimetry is described. The new method uses Markov Chain Monte Carlo and the Metropolis algorithm. Multiple intake amounts, biokinetic types, and times of intake are determined from bioassay data by integrating over the Bayesian posterior distribution. The method appears definitive, but its application requires a large amount of computing time.     

Vision-based global localization and mapping for mobile robots

We have previously developed a mobile robot system which uses scale-invariant visual landmarks to localize and simultaneously build three-dimensional (3-D) maps of unmodified environments. In this paper, we examine global localization, where the robot localizes itself globally, without any prior location estimate. This is achieved by matching distinctive visual landmarks in the current frame to a database map. A Hough transform approach and a RANSAC approach for global localization are compared, showing that RANSAC is much more efficient for matching specific features, but much worse for matching nonspecific features. Moreover, robust global localization can be achieved by matching a small submap of the local region built from multiple frames. This submap alignment algorithm for global localization can be applied to map building, which can be regarded as alignment of multiple 3-D submaps. A global minimization procedure is carried out using the loop closure constraint to avoid the effects of slippage and drift accumulation. Landmark uncertainty is taken into account in the submap alignment and the global minimization process. Experiments show that global localization can be achieved accurately using the scale-invariant landmarks. Our approach of pairwise submap alignment with backward correction in a consistent manner produces a better global 3-D map.     

Finite-Element Analysis of Hot Mix Asphalt Microstructure Using Effective Local Material Properties and Strain Gradient Elasticity

This paper presents the development of a methodology for microstructure analysis and modeling of hot mix asphalt (HMA). This methodology relies on using effective local material properties and strain gradient theory in the finite element analysis of HMA microstructure. The effective local properties are calculated using an analytical micromechanical model that captures the influence of percent of particles on the microscopic response of HMA. Strain gradient elasticity is used in order to account for the effect of particle size in the finite element analysis. The autocorrelation function and the moving window technique are used to determine the microstructure characteristic length scales that are used in strain gradient elasticity. A number of asphalt mixes with different aggregate types and size distributions are analyzed in this paper.     

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.     

Using Real-Time Stereo Vision for Mobile Robot Navigation

This paper describes a working vision-based mobile robot that navigates and autonomously explores its environment while building occupancy grid maps of the environment. We present a method for reducing stereo vision disparity images to two-dimensional map information. Stereo vision has several attributes that set it apart from other sensors more commonly used for occupancy grid mapping. We discuss these attributes, the errors that some of them create, and how to overcome them. We reduce errors by segmenting disparity images based on continuous disparity surfaces to reject spikes caused by stereo mismatches. Stereo vision processing and map updates are done at 5 Hz and the robot moves at speeds of 300 cm/s.     

Air/Water Oxygen Transfer in a Biological Aerated Filter

The oxygen-transfer characteristics of an upflow biological aerated filter filled with angular clay media were determined over a wide range of gas and liquid flow rates. Liquid-side, oxygen-transfer coefficients (KLa) were measured using a nitrogen gas stripping method under abiotic conditions and were found to increase as both gas and liquid superficial velocity increases, with values ranging from 12 to 110?h?1 based on empty bed volume. The effect of gas and liquid velocity, wastewater to clean water ratio, and temperature dependence was correlated to within ±20% of the experimental KLa value. Stagnant gas holdup is roughly double in wastewater compared to clean water, but the dynamic gas holdup is the same. The oxygen-transfer coefficient is directly proportional to the dynamic gas holdup. Stagnant gas holdup does not influence the rate of oxygen transfer. The results suggest that dynamic gas holdup largely determines the specific interfacial area (a), whereas the interstitial liquid velocity largely controls the oxygen-transfer coefficient (KL).     

A multiple-scale analysis of grating-assisted couplers

Optical directional couplers with longitudinal periodic perturbations or gratings are analyzed by a multiple-scale solution to the coupled-mode equations. The use of two length scales in the analysis becomes the key to obtaining globally valid analytic solutions, which are shown to be in excellent agreement with the exact numerical solutions. Because of the nonorthogonality of the coupled modes in the structure, two different coupling lengths, a maximum power transfer length and a zero crosstalk length, are predicted for the power exchange. The spectral properties of the coupler are also studied through a generalized multiple scale method