Acoustic wave propagation along cladded fibers of trigonal crystalsymmetry

An approximate method is proposed to analyze the acoustic wave propagation characteristics in nonpiezoelectric cladded LiNbO₃ fibers of trigonal crystal symmetry. The fiber axis coincides with the crystalline Z axis and the cladding thickness is infinite. This method utilizes coupled mode equations and exact solutions for corresponding cladded fibers of hexagonal crystal symmetry are used as the approximation base. The analysis concentrates on the trigonal materials of 32, 3 m, and ¯3 m groups. Dispersion curves of flexural F₁₁, F₂₁ and F₁₂ modes in such a fiber are presented for the first time with a focus on the fundamental F₁₁ mode. Acoustic modal birefringence phenomenon (or polarization dispersion) has been observed, which means that the dispersion relation of a mode depends on the relative transverse polarizations of the corresponding coupled modes. It was also found that in trigonal cladded fibers there are generally no pure torsional and radial-axial modes, which agrees with results previously obtained for trigonal rods     

Propagation of electroacoustic axial shear waves in a piezoelectric medium reinforced by continuous fibers

The propagation of electroacoustic axial shear waves in a fiber reinforced piezocomposites is studied in which matrix and fibers consist of piezoelectric transversely isotropic materials with symmetry axes parallel to the fiber axes. The effective medium method self-consistent variant as developed by Sabina and Willis is used to obtain explicit equations for the complex wave vector and it is solved numerically. Its real part determines the effective wave velocity and the imaginary part the attenuation factor. Integral equations expressed via dynamic Green’s function kernels are set up. The central problem of the method is the axial shear electroacoustic wave scattering on one isolated fiber in the medium having the effective piezoelectric properties. It is solved approximately by the Galerkin type method. The obtained expressions for the effective wave velocity and attenuation factor cover not only the long-wave region but the intermediate wave and it is valid for long wavelenghts up to the diameter of the inclusion. Wave velocity and attenuation coefficient coincide with ones obtained earlier in some other way. Some numerical examples are presented for real materials.     

Transparent conductive film having sandwich structure of gallium-indium-oxide/silver/gallium-indium-oxide

New transparent conductive films having the sandwich structure of gallium-indium-oxide/silver/gallium-indium-oxide (GIO/Ag/GIO) were prepared by conventional magnetron sputtering method at ambient substrate temperature. The electrical and optical properties of the films were compared with those of conventional indium-tin-oxide (ITO) films and ITO/Ag/ITO sandwich films. The GIO/Ag/GIO (40 nm/8 nm/40 nm) sandwich films, in which the GIO film was deposited using a GIO ceramic target with In content [In/(Ga + In)] of 10 at.%, exhibited a low sheet resistance of 11.3 Ω/sq and a large average transmittance of over 92.9% in the visible region (400-800 nm). This GIO/Ag/GIO films also exhibited a novel characteristic of transparency in the ultraviolet region; they showed high transmittance of 82.2% at the wavelength of 330 nm and 40.8% at the wavelength of 280 nm, which was not shown in the ITO films and the ITO/Ag/ITO sandwich films. The GIO/Ag/GIO sandwich films are useful as transparent electrode for emitting devices of ultraviolet radiation because of both their high conductivity and high transparency in the ultraviolet region.     

On the Problem of Magnetic Wave Propagation and Absorption with Regard for Relaxation Processes

To describe the interaction of electromagnetic waves with a polarized medium, electric and magnetic relaxation times are introduced. A solution to the problem of periodic boundary regime propagation with regard for the hereditary characteristics of the absorbing medium, which qualitatively agrees with the experimental data, has been obtained.     

Fatigue growth of surface cracks in bending

A simple procedure for calculating the fatigue crack propagation lifetimes of surface-initiated part-through cracks in bending is discussed and agreement is obtained with previously reported experimental data. It is shown that a realistic fatigue analysis of these cracks must include an explicit determination of crack depth as a function of surface length and that, in general, the depth-to-length ratio varies with crack length, crack nucleus geometry and with the stress intensity factor exponent in the Paris growth rate relationship. Numerical results are presented for some typical situations.     

Spectrum-sliced Fourier-domain low-coherence interferometry for measuring the chromatic dispersion of an optical fiber

We present a novel spectrum-slicing method for measuring the chromatic dispersion of an optical fiber in Fourier-domain low-coherence interferometry. Broadband spectral interference data obtained from a low-coherence interferometer is sliced with Gaussian window functions. Each sliced spectral datum is used to calculate a relative group delay with Fourier transformation at the peak wavelength of a narrow window function. We have demonstrated that our proposed method is very powerful and simple for measuring chromatic dispersion and second-order dispersion in optical fibers and optical devices. Comparison of the proposed method with a conventional measurement method agrees within 0.5%.     

Gelatin stabilized iron oxide nanoparticles as a three dimensional template for the hydroxyapatite crystal nucleation and growth

Inspired by the structural similarity of gelatin (and collagen) linked to a mineral phase based on Ca-phosphates compounds with natural bone and increasing application of magnetic iron oxides in hyperthermia, gelatin coated iron oxide (GIO) was synthesized and hydroxyapatite (HAp) crystal nucleation and growth in the nanoparticles was explored. A series of GIO/HAp nanocomposites with various amount of GIO were synthesized by co-precipitation technique using calcium hydroxide and phosphoric acid as precursor. Various physico-chemical analysis showed that the HAp crystal nucleation and growth occurred at acidic group of gelatin, while magnetic iron oxide nanoparticles (< 8nm) were bound to the amide groups of the gelatin chain. Moreover, the growth of HAp nanocrystals in aq. GIO solution was highly influenced by the GIO contents in the solution. The mineralized composite with magnetic properties could have great scope in biomedical field as a thermoseed to kill the cancerous cell in bone side by side for the bone reinforcement.     

Vector description of higher-order modes in photonic crystal fibers

We extensively study the propagation features of higher-order modes in a photonic crystal fiber (PCF). Our analysis is based on a full-vector modal technique specially adapted to accurately describe light propagation in PCF's. Unlike conventional fibers, PCF's exhibit a somewhat unusual mechanism for the generation of higher-order modes. Accordingly, PCF's are characterized by the constancy of the number of modes below a wavelength threshold. An explicit verification of this property is given through a complete analysis of the dispersion relations of higher-order modes in terms of the structural parameters of this kind of fiber. The transverse irradiance distributions for some of these higher-order modes are also presented, showing an excellent agreement with recent experimental measurements. In the same way, the full-vector nature of our approach allows us to analyze the rich polarization structure of the PCF mode spectrum.     

Geometric propagation of an axially symmetric optical wavefront in a homogeneous medium

We obtain a Taylor series neighbor expansion that allows the geometric propagation of an axially symmetric wavefront in such a way that the change in shape of the wavefront is determined as an explicit function of coordinates. We then apply the expansion to the propagation of conicoids in particular. The theory is applied to the propagation of a general conicoidal, a spherical, and a paraboloidal wavefront. Application of the theory to the general conicoid leads to a proof of the invariance of the product of radius of curvature and conic constant for short propagation distances in homogeneous media. For the case of the spherical wavefront, the intuitive result is obtained (i.e. the wavefront remains spherical).     

Consistent diagonal mass matrices and finite element equations for one-dimensional problems

The conventional dynamic variational approach and finite element base functions lead to non-diagonal consistent mass matrics which are inappropriate for use with an explicit time integration scheme. In this work, it is shown that if orthogonal base function are used with a mixed variational formulation, then consistant diagonal mass matrices and corresponding sets of spatially discretized field equations are obtained. Although the approach is quite general, the theory is purposely illustrated by a detailed development for one set of base functions. Central difference time integration is incorporated for applications to one-dimensional wave propagation and to Euler-Bernoulli beams. Numerical examples are provided for elastic and elastic-plastic materials.     

Intralaminar fracture mechanism in unidirectional CFRP composites — part II: analysis

Three kinds of representative carbon fiber reinforced unidirectional composite materials are used, and their intralaminar fracture behavior is investigated by using the double-cantilever beam (DCB) specimen with a simultaneous acoustic emission measuring. In Part I, the experimental results on the crack propagation, the bridging fibers, the intralaminar fracture toughness acoustic emission characteristics and microscope observations were obtained. Here, we use a bridging fiber model to analyze the debonding force acting on a bridging fiber and try to estimate the number of bridging fibers during the crack propagating process. At the same time, the intralaminar fracture toughness is calculated by both the adhesive force model and the finite element analysis. As a result, it is found that the intralaminar fracture toughness without the bridging fibers will have a constant value during the crack propagation, but it increases greatly when bridging fibers exist. It is clear that the bridging fibers play an important role in the intralaminar fracture toughness. The debonding forces acting on the bridging fibers and the number of bridging fibers are obtained. Furthermore, the quantitative estimation of the increment of the intralaminar fracture toughness contributed by bridging fibers is made according to the adhesive force model and it is comparable with the results obtained by the finite element analysis.     

The velocity of streamer propagation toward anode and cathode in He,Xe, N<Subscript>2</Subscript>, and SF<Subscript>6</Subscript>

A simple mechanism of ionization spreading in a dense gas, based on the background electron multiplication wave in an inhomogeneous electric field, is considered. This process is independent of the sign of the field projection onto the direction of propagation. An analytical expression for the velocity of the ionization front propagation is obtained, which agrees well with the results of numerical calculations performed using both the simple background electron multiplication model and a detailed diffusion-drift theory. The ionization wave front velocity as a function of the field strength at the streamer boundary has been tabulated for He, Xe, N₂, and SF₆. Some peculiarities observed in the motion of streamers, such as the velocity jerks, can be related to the recently discovered nonmonotonic dependence of the ionization rate on the field strength.     

Mass sensitivity of thin rod acoustic wave sensors

Theoretical and experimental investigations of mass sensitivities of thin rod acoustic wave sensor are presented. From the low-frequency approximation of the dispersion equations, explicit forms of the relation describing the mass sensitivity are derived with the consideration of the effects due to elasticity and inertia of the loading layer. The three lowest thin rod acoustic modes are presented. Mass sensing experiments are based on the electrodeposition of loading material on a thin metallic fiber (the thin rod). Copper has been used to load the propagation of acoustic waves in gold fibers. The mass of copper deposited and the phase shift of the acoustical thin rod delay line were monitored simultaneously by a computer. Mass response curves showing the variation in phase due to the mass deposited per unit surface area were then obtained in order to determine the mass sensitivity. Both flexural and extensional wave modes have been excited. Theoretical and experimental results were found to be consistent in both sign and magnitude     

Simulation of the rate of turbulent homogeneous combustion using the “quasi-laminar” approach

The possibility is considered of using a bulk (“quasi-laminar”) model of combustion for describing the rate of propagation of flame front in a turbulent flow of homogeneous fuel-oxidizer mixture. It is demonstrated that the analytical relation for the rate of propagation of flame front, obtained using a bulk model of combustion, agrees well with the available experimental data. Bulk and front models of combustion, as well as an original reduced model of kinetics of methane-air mixture, are used for calculating a V-flame. Two-dimensional calculations of an axisymmetric low-emission combustion chamber are performed using the bulk model of combustion and two reduced kinetic schemes. Comparison is made of calculation and experimental data.     

A closed-form solution for wave propagation in optical waveguides of longitudinally invariant structures without using beam propagation algorithm

In this paper, we propose a different method to study wave propagation in longitudinally invariant waveguides with arbitrary index profile. In our method, both the electric field and the refractive index profile are expanded into two Fourier cosine series. With these series substituted into the wave equation, a differential matrix equation can then be obtained. We show here that such a matrix equation can be solved and an explicit expression for the wave field at any longitudinal position along an optical waveguide can be obtained. The solution proposed in this method can thus exclude the use of the beam propagation algorithm. This study demonstrates that our approach yields the same results as those obtained by using commercial softwares in which a beam propagation method with the Padé approximation is used.     

Rayleigh surface waves problem in linear thermoviscoelasticity with voids

In this paper, we study the propagation of the Rayleigh surface waves in a half-space filled by an exponentially functionally graded thermoviscoelastic material with voids. We take into consideration the dissipative character of the porous thermoviscoelastic models upon the propagation waves and study the damped in time wave solutions. The propagation condition is established in the form of an algebraic equation of tenth degree whose coefficients are complex numbers. The eigensolutions of the dynamical system are explicitly obtained in terms of the characteristic solutions. The concerned solution of the Rayleigh surface wave problem is expressed as a linear combination of the five analytical solutions, while the secular equation is established in an implicit form. The explicit secular equation is obtained for an isotropic and homogeneous thermoviscoelastic porous half-space, and some numerical simulations are given for a specific material.     

Effects of a liquid layer on thickness-shear vibrations of rectangular AT-cut quartz plates

Thickness-shear vibrations of rectangular AT-cut quartz with one face in contact with a layer of Newtonian (linearly viscous and compressible) fluid are studied. The two-dimensional (2D) governing equations for vibrations of piezoelectric crystal plates given previously are used in the present study. The solutions for 1D shear wave and compressional wave in a liquid layer are obtained, and the stresses at the bottom of the liquid layer are used as approximations to the stresses exerted on the crystal surface in the plate equations. Closed form solutions are obtained for both free and piezoelectrically forced thickness-shear vibrations of a finite, rectangular AT-cut quartz plate in contact with a liquid layer of finite thickness. From the present solutions, a simple and explicit formula is deduced for the resonance frequency of the fundamental thickness-shear mode, which includes the effects of both shear and compressional waves in the liquid layer and the effect of the thickness-to-length ratio of the crystal plate. The formula reduces to the widely used frequency equation obtained by many previous investigators for infinite plates. The resonance frequency of a rectangular AT-cut quartz, computed as a function of the thickness of the adjacent liquid layer, agrees closely with the experimental data measured by Schneider and Martin (Anal. Chem., vol. 67, pp. 3324-3335, 1995)     

Propagation of a large amplitude longitudinal wave in a semi-infinite elastic rod

A general solution is given to the problem of large amplitude wave propagation in a semi-infinite elastic thin rod. The solution is obtained by the method of characteristics. An explicit form of the solution is given in the case where an end velocity, which is a step function of time, is prescribed. More general types of end velocity can be accommodated provided that the variation with time is monotonically increasing; otherwise shocks will form and the present solution is no longer valid.     

PROCESSING AND FRACTURE TOUGHNESS OF SINTERED FIBER REINFORCED CERAMIC MATRIX COMPOSITES

ABSTRACT

The paper discusses the processing and the resulting mechanical properties of sintered fiber reinforced ceramic matrix composites. In situ observations of the sintering process revealed that stresses which develop due to the differential shrinkage between the fibers and the matrix initiate already during the heating cycle and are of sufficient magnitude and duration that crack like damage forms. Successful methods were employed for reducing and avoiding these stresses during the sintering process. Coarse grained alumina coatings deposited onto the fibers with a coating thickness of up to 10 µm delayed and reduced the stress development. Polymer coated fibers produced fully dense composites on which fracture toughness measurements were performed. Crack propagation and crack/fiber interaction was observed in situ inside a scanning electron microscope. The importance of studying both the crack front and the crack wake phenomena in fiber reinforced composites is illustrated. In specimens where the cracks are already bridged by 10% area fraction of fibers a toughness of 7 MPa √m was obtained. However, in samples where the cracks are not bridged yet by fibers, the crack becomes unstable before reaching the fiber positions and the fibers had no effect in resisting the crack propagation.