Wave reflection at a free interface in an anisotropic pyroelectric medium with nonclassical thermoelasticity

In this paper, the well-established two-dimensional mathematical model for linear pyroelectric materials is employed to investigate the reflection of waves at the boundary between a vacuum and an elastic, transversely isotropic, pyroelectric material. A comparative study between the solutions of (a) classical thermoelasticity, (b) Cattaneo–Lord–Shulman theory and (c) Green–Lindsay theory equations, characterised by none, one and two relaxation times, respectively, is presented. Suitable boundary conditions are considered in order to determine the reflection coefficients when incident elasto–electro–thermal waves impinge the free interface. It is established that, in the quasi-electrostatic approximation, three different classes of waves: (1) two principally elastic waves, namely a quasi-longitudinal Primary (qP) wave and a quasi-transverse Secondary (qS) wave; and (2) a mainly thermal (qT) wave. The observed electrical effects are, on the other hand, a direct consequence of mechanical and thermal phenomena due to pyroelectric coupling. The computed reflection coefficients of plane qP waves are found to depend upon the angle of incidence, the elastic, electric and thermal parameters of the medium, as well as the thermal relaxation times. The special cases of normal and grazing incidence are also derived and discussed. Finally, the reflection coefficients are computed for cadmium selenide observing the influence of (1) the anisotropy of the material, (2) the electrical potential and (3) temperature variations and (4) the thermal relaxation times on the reflection coefficients

A mathematical model of shear wave propagation in the incompressible transversely isotropic thermoelastic half-spaces

This article deals with the problem of reflection and transmission of shear waves at a plane interface between two dissimilar incompressible transversely isotropic thermoelastic half-spaces. Two coupled quasi-shear waves are found to propagate due to the incompressibility of such materials. Applying appropriate boundary conditions at the plane interface, amplitude ratios of the reflected and transmitted quasi-shear waves are obtained. It has been observed that these ratios are functions of the angle of incidence, elastic and thermal parameters of the materials. These ratios are computed numerically for a particular model to see the effects of specific heat and thermal expansion on quasi-shear waves in incompressible transversely isotropic thermoelastic materials. The results are also presented graphically

Mathematical modelling of Stoneley wave in a transversely isotropic thermoelastic media

This paper is concerned with the study of propagation of Stoneley waves at the interface of two dissimilar transversely isotropic thermoelastic solids without energy dissipation and with two temperatures. The secular equation of Stoneley waves is derived in the form of the determinant by using appropriate boundary conditions i.e. the stresses components, the displacement components, and temperature at the boundary surface between the two media are considered to be continuous at all times and positions . The dispersion curves giving the Stoneley wave velocity and Attenuation coefficients with wave number are computed numerically. Numerical simulated results are depicted graphically to show the effect of two temperature and anisotropy on resulting quantities. Copper material has been chosen for the medium and magnesium for the medium Some special cases are also deduced from the present investigation

Influence of rotation and initial stress on Propagation of Rayleigh waves in fiber-reinforced solidanisotropic magneto-thermo-viscoelastic media.

This paper is concerned with giving a mathematical model on the propagation of Rayleigh waves in a homogeneous magneto-thermo-viscoelastic,pre-stressed elastic half – space subjected to theinitial stress and rotation. The dispersion equation has been derived for a half-space, when both media are considered as pre-stressed and the effect of rotation and initial stressshown in earlier investigators.Numerical results have been obtained  in the physical domain. Numerical simulated results are depicted graphically to show the effect of rotation and magnetic field and initial stressonRayleigh wave velocity. Comparison was made with the results obtained in the presence and absence of the rotation , initial stressand magnetic field. The study shows that there is a variational effect of magneto-elasticityand rotation, initial stress on the Rayleigh wave velocity

Menhaden distribution as mediated by feeding (filter-feeding, phytoplankton)

Distribution patterns of the Atlantic menhaden Brevoortia tyrannus) have been evaluated in respect to the feeding ecology of the species. Feeding experiments, functional morphology, and field distribution studies with young postmetamorphic fish were conducted to differentiate the feeding dynamics of different size menhaden, and to define the relationship between feeding and distribution. Feeding experiments were conducted with 138 mm fork length menhaden to determine their particle size-specific feeding abilities. The minimum size of particles filtered, the minimum size threshold, was 7 to 9 (mu)m. Phytoplankton larger than the minimum size threshold and smaller than 20 (mu)m upper limit for nanoplankton, were filtered at efficiencies averaging 21% (n = 24). Prey particles exceeding the size limits of nanoplankton, were filtered at average efficiencies ranging from 22% to 84%. The mean filtration efficiency for Artemia sp. nauplii of 36% (n = 7) was lower than for smaller phytoplankton prey. as menhaden grown, their feeding repertoire shifts to larger planktonic organism. Surface ultrastructure, epithelial organization, and gross morphology of the branchial basket in menhaden were analyzed with respect to food particle capture, transport of food from the point of capture to point of ingestion, and potential gustatory reception. Branchiospinules, sites of small particle capture, lacked mucous cells, suggesting that food is captured primarily by mechanical sieving. Taste buds on the glossohyal are thought to be mechanoreceptors, whereas taste buds on the crest of the fifth branchial arch are thought to be chemoreceptors. Concurrent synoptic observation of the relative abundance of menhaden and parameters relevant to characterizing primary production along transects in estuarine creek ecosystems have been used to interpret the factors governing the fishes\u27 local distribution. The strongest associations were between catch and chlorophyll-a, catch and microflagellates, and catch and diatoms. Fish often distributed with a gradient of one phytoplankton taxa over another based on selectivity for large phytoplankton cell size. Menhaden are optimal foragers displaying kinesis selecting for areas of optimally sized prey, chemosensory preference for plant versus detrital particles, and possibly taxa specific avoidance. Comparison of latitudinal distribution patterns of menhaden with the latitudinal trends in plankton community size frequency suggest that fish stratify by size at latitude to maximize the efficiency with which they filter-feed

Generalized Magneto-thermo-microstretch Response of a Half-space with Temperature-dependent Properties During Thermal Shock

Abstract The generalized magneto-thermoelastic problem of an infinite homogeneous isotropic microstretch half-space with temperature

Theoretical and experimental study of generation mechanisms for laser ultrasound in woven graphite /epoxy composites with translaminar stitching

The aerospace industry is beginning to use advanced composite materials for primary load bearing structures and their failure mechanisms must be better understood to predict their behavior in service. The Combined Loads Tests (COLTS) facility is being constructed at the NASA Langley Research Center to characterize these failure mechanisms. Laser based ultrasonic NDE can monitor the samples during dynamic loading without interfering with the structural tests. However, the constraints of implementing laser ultrasound in a structures laboratory reduces the efficiency of the technique. The system has to be eye-safe because many people will be present during the structural tests. Consequently, laser light has to be delivered through fiber optics and a significant amount of light is lost. Also, the nature of the composite materials makes laser ultrasonic inspection difficult. The composites of interest are formed from woven layers that are stitched through the laminate thickness and bound in a resin matrix. These materials attenuate ultrasound strongly and exhibit a high degree of scattering.;Generation mechanisms in laser based ultrasound must be better understood to improve generation efficiency and consequently improve the signal-to-noise ratio. Although some experimental and theoretical studies have been conducted to characterize generation mechanisms, more extensive work is needed for composite materials. Specifically, we are concerned with generation mechanisms in thick, stitched composite structures. We describe a theoretical and experimental investigation of laser generated ultrasound in complex composite materials. We first develop a mathematical model describing the thermoelastic generation of ultrasound in a general anisotropic material. We then present a wide range of experimental data investigating the effects of laser and material parameters on the generated ultrasound. We specifically consider the relationship between laser pulse width, laser wavelength, and material composition. Finally, we compare this data to our mathematical model

Theory of thermoelasticity

The development of the theory of thermoelasticity, which examines the interactions between the deformation of elastic media and the thermal field, is traced and the fundamental problems of the theory are presented. Results of recent studies on the subject are presented. Emphasis is primarily on media with generalized anisotropy, or isotropy media. Thermomechanical problems and mathematical formulations and resolutions are included

A continual model of a damaged medium used for analyzing fatigue life of polycrystalline structural alloys under thermal-mechanical loading

International audienceThe main physical laws of thermal–plastic deformation and fatigue damage accumulation processes in polycrystalline structural alloys under various regimes of cyclic thermal–mechanical loading are considered. Within the framework of mechanics of damaged media, a mathematical model is developed that describes thermal–plastic deformation and fatigue damage accumulation processes under low-cycle loading. The model consists of three interrelated parts: relations defining plastic behavior of the material, accounting for its dependence on the failure process; evolutionary equations describing damage accumulation kinetics; a strength criterion of the damaged material. The plasticity model based on the notion of yield surface and the principle of orthogonality of the plastic strain vector to the yield surface is used as defining relations. This version of defining equations of plasticity describes the main effects of the deformation process under monotone cyclic, proportional and nonproportional loading regimes. The version of kinetic equations of damage accumulation is based on introducing a scalar parameter of damage degree and energy principles, and account for the main effects of nucleation, growth and merging of microdefects under arbitrary regimes of low-cycle loading. The strength criterion of the damaged material is based on reaching a critical value of the damage degree. The results of numerically modeling cyclic thermal–plastic deformation and fatigue damage accumulation in heat-resistant alloys (Nimonic 80A, Haynes 188) under combined thermal–mechanical loading are presented. Special attention is paid to the issues of modeling the processes of cyclic thermal–plastic deformation and fatigue damage accumulation for complex deformation processes accompanied by the rotation of the main stress and strain tensor areas. It is shown that the present damaged medium model accurately enough for engineering purposes describes the processes of cyclic isothermal and nonisothermal deformation and fatigue damage accumulation under combined thermal–mechanical loading and makes it possible to evaluate low-cycle fatigue life of heat-resistant alloys under arbitrary deformation trajectories

Investigation into the applicability of AVO techniques to coal mine exploration

AVO (for Amplitude Versus Offset) is a seismic technology known in the oil industry for its ability to differentiate gas sands (over oil producing reservoirs) from wet sands (over non-producing reservoirs) in the Gulf of Mexico. Its applicability to a broad range of exploration fields has led to its potential application within the ADEMA (Advances in Exploration Methods and Applications) project being considered. Within the frame of this project, this PhD thesis assesses the ability of AVO to detect a difference between Mudstone and Sandstone roofs over a 6 meters thick coal seam, in the region of Daw Mill (UK). It moreover presents a modification of a triaxial cell aimed at calibrating AVO in the laboratory, and thereby improving the quality of AVO analysis in general. The analysis presented in this PhD thesis leads to the conclusion that in this region, the particular rock properties can lead to a signal difference detectable by AVO. However, analysis of seismic data confirmed that tuning effect interferes with these distinct signals, and prevents AVO technology to differentiate mudstone from sandstone roofs. Finally, this thesis proposes different technical recommendations for successfully running a test with the modified triaxial cell in future