Dynamic stability of cross-ply laminated composite cylindrical shells

The dynamic stability of thin, laminated cylindrical shells under combined static and periodic axial forces is studied using Love’s classical theory of thin shells. A normal-mode expansion of the equations of motion yields a system of Mathieu–Hill equations. Bolotin’s method is then employed to obtain the dynamic instability regions. The present study examines the dynamic stability of antisymmetric cross-ply circular, cylindrical shells of different lamination schemes. The effect of the magnitude of the axial load on the instability regions is also examined.     

A study of the plastic buckling of axially compressed cylindrical shells with a thick-shell theory

A thick shell theory is used to calculate the critical load of plastic buckling of axially compressed cylindrical shells. The buckling equations are derived with the principle of virtual work on the basis of a transverse shear deformable displacement field. The deformation theory of plasticity is used for constitutive equations. To fit the uniaxial stress–strain curve, the Ramberg–Osgood equation is used. In the numerical examples special attention is paid to the dependence of the buckling mode on the ratios of radius to thickness R/h and length to radius L/R. This dependence divides the (R/h,L/R)-plane into simply connected regions each of which corresponds to a buckling mode. These regions form a “buckling mode map”.     

Measurement of the Parameters of the Dielectric Windows of the Linear Collider TESLA Cavities

A setup for measuring the permittivity and the loss parameter of ceramic samples is described. The setup operation is based on a conventional method for measuring changes in the frequency and Q-factor values of cavities caused by insertion of experimental samples into the cavity. Cylindrical ceramic samples with the permittivity = 8–10 and loss parameter tan = (2–4) × 10^–4 were studied. A cylindrical E ₀₁₀ cavity with a cutoff waveguide attached to one of the faces of the cavity was selected, because it provided sufficient accuracy of measurement and could be effectively used for testing series of samples of various sizes and shapes at a fixed frequency of 1300 MHz. It was shown that the permittivity of various samples could be determined with an accuracy of 1.5%, whereas the loss parameter could be determined with an accuracy of 7%.     

A Self-Initiated Periodically Pulsed Light Source on Chlorine and Krypton Chloride Molecules

An electric-discharge light source, operating in the spectral range of 170–270 nm on a system of bands of Cl₂ ( = 200 and 257 nm) and KrCl ( = 222 nm) molecules is described. The radiator is pumped by a low-pressure volume discharge in a spherical anode-flat cathode system of electrodes with an interelectrode distance of 6 cm, so that the plasma has no contact with the quartz envelope of the lamp. The working mixtures are P(Kr)/P(Cl₂) = (40–640)/(40–280) Pa. When a dc voltage U 1 kV is applied to the discharge gap, a volume discharge exists only in a periodically pulsed mode (f = 0.1–50 kHz) and represents a source of short-wave radiation with a cylindrical working surface (1 cm in diameter and 6 cm long) and a mean radiation power of 3 W.     

Linear free vibration analysis of cross-ply laminated cylindrical helical springs

A linear free vibration analysis of symmetric cross-ply laminated cylindrical helical springs is performed based on the first-order shear deformation theory. Considering the rotary inertia, the shear and axial deformation effects, governing equations of symmetric laminated helical springs made of a linear, homogeneous, and orthotropic material are presented in a straightforward manner based on the classical beam theory. The free vibration equations consisting of 12 scalar ordinary differential equations are solved by the transfer matrix method. The overall transfer matrix of the helix is computed up to any desired accuracy. The soundness of the present results are verified with the reported values which were obtained theoretically and experimentally. After presenting the non-dimensional graphical forms of the free vibrational characteristics of (0°/90°/90°/0°) laminated helical spring made of graphite-epoxy material (AS4/3501-6) with fixed–fixed ends, a non-dimensional parametric study is worked out to examine the effects of the number of active turns, the shear modulus in the 1–2 plane (G₁₂), the ratio of the cylinder diameter to the thickness (D/d), and Young's moduli ratio in 1 and 2 directions (E₁/E₂) on the first six natural frequencies of a uniaxial composite helical spring with clamped-free, clamped-simple, and clamped–clamped ends.     

Determining Generation–Recombination Characteristics of a Semiconductor–Dielectric Interface from the Current Kinetics of Surface Minority Carrier Generation

Conditions for the one-to-one characterization of the generation (G _s) and surface recombination (R _s) rates of minority charge carriers (MCCs) in a metal–oxide–semiconductor (MOS) structure (in the case of strong nonequilibrium depletion) by the MCC surface generation current (I(t)) flowing in an external circuit of this structure are revealed. These conditions are the following: (1) the generation current I is independent of the time t (until the structure enters an equilibrium state) and the voltage V _{g 0} corresponding to the initial nonequilibrium depletion and (2) the duration of current steps I(V _{g 0}) = const and, consequently, the equilibrium surface charge increase with increasing V _{g 0}. The observed kinetics of the MCC generation current for the MCCs induced in an n-Si MOS structure at 293 K experimentally confirms the realization of these conditions. The values of the generation and recombination rates G _s = 2.84 × 10¹⁰ cm^–2s^–1 and R _s = 6.82 cm s^–1 obtained from current levels I(V _{g 0}) = const are typical of high-quality Si MOS structure. Additionally measured capacitance–voltage characteristics were used to determine the interface state density at the Si/SiO₂ contact near the middle of the Si gap (N _ss(E) 6.4 × 10¹⁰ cm^–2eV^–1), which allowed the estimation of the effective capture cross section of these states _eff 1.4 × 10^–16 cm².     

Postbuckling of cross-ply laminated cylindrical shells with piezoelectric actuators under complex loading conditions

A postbuckling analysis is presented for a cross-ply laminated cylindrical shell with piezoelectric actuators subjected to the combined action of mechanical, electric and thermal loads. The temperature field considered is assumed to be a uniform distribution over the shell surface and through the shell thickness and the electric field is assumed to be the transverse component E_z only. The material properties are assumed to be independent of the temperature and the electric field. The governing equations are based on the classical shell theory with a von Kármán–Donnell-type of kinematic nonlinearity. The nonlinear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A boundary layer theory of shell buckling, which includes the effects of nonlinear prebuckling deformations, large deflections in the postbuckling range, and initial geometric imperfections of the shell, is extended to the case of hybrid laminated cylindrical shells. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, cross-ply laminated cylindrical thin shells with fully covered or embedded piezoelectric actuators subjected to combined mechanical loading of external pressure and axial compression, and under different sets of thermal and electric loading conditions. The effects played by temperature rise, applied voltage, shell geometric parameter, stacking sequence, as well as initial geometric imperfections are studied.     

High-Sensitivity Magnetometers Based on HTSC Ceramics

The magnetic response of the YBa₂Cu₃O_{7 – x} superconducting ceramics to an alternating magnetic field was studied experimentally. A magnetometer with a sensitivity level of 2 × 10^–7Oe was developed on the basis of the experimental data. The ways of improving such devices are discussed.     

Thermoelastic damping in cylindrical shells with application to tubular oscillator structures

Thermoelastic vibration and damping of cylindrical shell structures is studied in this paper. The general thermoelastic coupled equations for cylindrical thin shells are presented first. Since the general governing equations are quite complicated, they are then simplified with Donnell–Mushtari–Vlasov approaches for the cylindrical shells under transverse deflection-dominated vibrations. By solving the simplified thermoelastic equations with Galerkin method, the approximate solutions for thermoelastic damping in a cylindrical shell structure are obtained. The solutions are suitable for the predictions of thermoelastic damping of tubular oscillator structures. Some numerical examples for micro- or nano-tube resonators are provided to illustrate the results.     

Temperature measurement in orthogonal metal cutting

Orthogonal cutting experiments were carried out on steel at different feedrates and cutting speeds. During these experiments the chip temperatures were measured using an infrared camera. The applied technique allows us to determine the chip temperature distribution at the free side of the chip. From this distribution the shear plane temperature at the top of the chip as well as the uniform chip temperature can be found. A finite-difference model was developed to compute the interfacial temperature between chip and tool, using the temperature distribution measured at the top of the chip.Nomenclature contact length with sticking friction behaviour [m] - c specific heat [J kg^–1 K^–1] - contact length with sliding friction behaviour [m] - F _P feed force [N] - F _V main cutting force [N] - h undeformed chip thickness [m] - h _c deformed chip thickness [m] - i,j denote nodal position - k thermal conductivity [W m^–2 K^–1] - L chip-tool contact length [m] - p defines time—space grid, Eq. (11) [s m^–2] - Q _C heat rate entering chip per unit width due to friction at the rake face [W m^–1] - Q _T total heat rate due to friction at the rake face [W m^–1] - Q _% percentage of the friction energy that enters the chip - q ₀ peak value ofq(x) [W m^–2] - q _e heat rate by radiation [W] - q(x) heat flux entering chip [W m^–2] - t time [s] - T temperature [K] - T _C uniform chip temperature [°C] - T _max maximum chip—tool temperature [°C] - T _mean mean chip—tool temperature [°C] - T _S measured shear plane temperature [°C] - x,y Cartesian coordinates [m] - V cutting speed [m s^–1] - V _C chip speed [m/s] - rake angle - ,, control volume lumped thermal diffusivity [m² s^–1] - emmittance for radiation - exponent, Eq. (3) - density [kg m^–3] - Stefan-Boltzmann constant [W m^–2 K⁴] - (x) shear stress distribution [N m^–2] - shear angle     

Improving the Accuracy in Determining the Insulator Capacitance in Metal–Insulator–Semiconductor Structures

The results of the voltage–capacitance spectroscopy of interface states in metal–insulator–semiconductor (MIS) structures are critical functions of the accuracy in determining the insulator capacitance C _i, which is typically no higher than a few fractions of a percent. This substantially limits the energy range of the observed spectrum of the interface states (E 0.5 eV for Si-based MIS structures) and the sensitivity to the density of the interface states at the spectrum edges (N _ss 1 × 10¹⁰ cm^–2 eV^–1). We propose a method for minimizing these errors that is based on a sequential variation of the initial estimate C _i C _{i 0} C _ij, j = 0, 1, 2, ... and the identification of singular points in the dependences and on C _ij, where are the mean arithmetic values of the voltage difference between the experimental and ideal voltage–capacitance characteristic and are the rms deviations of the voltage values taken in the high-accumulation (ac) and inversion (in) regions from values. The highest (10^–4%) accuracy in determining C _i is achieved in the regions of the equidistant experimental and ideal voltage–capacitance characteristic. This method, combined with the technique of _s / _s diagrams, ensures an extension of E to 0.9 eV at N _ss 1 × 10¹⁰ cm^–2 eV^–1 and the possibility of determining the sign and density of the fixed charge in the gate insulator.     

Efficiency of Pulsed Regimes for Enhancing the Breakdown Strength of Vacuum Insulation

Processing the experimental data on breakdown delay time in a vacuum was used to obtain the function K (t _p) of the relative change in the field gain factor at microscopic inhomogeneities of the cathode surface resulting from the realization of optimum pulse conditioning regimes. It is shown that over the range of pulse durations 10^–8 t _p 10^–6 s, the relative change in the state of the cathode surface corresponds to the relative changes in the breakdown strength of all-metal electrodes and to the voltage of the appearance of local flashes in a system of evaporated electrodes—a microchannel plate and the screen of an image intensifier. Applying optimum regimes for conditioning the surface with pulses of durations t _p < 10^–8 s makes it possible to achieve the limiting breakdown strength determined by the cathode mechanism of breakdown initiation.     

Paradoxical buckling behaviour of a thin cylindrical shell under axial compression

The widely accepted theory of buckling of thin cylindrical shells under axial compressive loading emphasises the sensitivity of the buckling load to the presence of initial imperfections. These imperfections are conventionally taken to be minor geometric perturbations of a shell which is initially stress-free. The original aim of the present study was to investigate the effect on the buckling load of imperfections in the form of local initial stress, which are probably more typical of practice than purely geometric ones. Experiments were performed on a vertical “melinex” cylinder of diameter 0.9 m and height 0.7 m, with radius/thickness ratio 1800. The upper and lower edges of the cylinder were clamped to end discs by means of circumferential belts — an arrangement that allowed states of self-stress to be introduced to the shell readily by means of local “uplift” at the base. The upper disc was made sufficiently heavy to buckle the shell, and it was supported by a vertical central rod under screw control. Many buckling tests were performed. Surprisingly, the buckling loads were generally at the upper end of the range of fractions of the classical buckling load that have been found in many previous experimental studies. Even when the local uplift at the base caused a local “dimple” to be formed before the shell was loaded, the buckling load was relatively high. A surface-scanning apparatus allowed the geometric form of the shell to be monitored, and the progress of such a dimple to be followed; and it was found that a dimple generally grew in size and migrated in a stable fashion up the shell as the load increased, until a point was reached when unstable buckling occurred. These unexpected and paradoxical features of the behaviour of the experimental shell may be attributed to the particular boundary conditions of the shell, which provide in effect statically determinate support conditions. This study raises some new issues in the field of shell buckling, both for the understanding of buckling phenomena and for the rational design of shells by engineers against buckling.     

A photo-thermally stimulated exoemission technique used to study the properties of HTSC ceramics based on YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

The method of photo-thermally stimulated exoelectron emission (PTSE) is used to study the processes in the surface layers of HTSC ceramics based on YBa₂Cu₃O_7–. A correlation is observed between variations of the exoemission current and the transition to the superconducting state. The experimentally detected hysteresis in the PTSE intensity of YBa₂Cu₃O_7– ceramics, when thermally cycled in the range T=80–160 K, is limited by two processes. Below T=90 K, the chemisorption has a significant effect on the physical phenomena under study, thus impeding the search for new PTSE regularities in HTSC ceramics. At the same time, exoemission properties of the ceramics considerably change in the range 80–160 K. It is also shown that the superconducting transition and the thermal absorption phenomena for an YBa₂Cu₃O_6.8 specimen are split in the studied temperature range, which allows the PTSE technique to detect the superconducting state and to observe the transition dynamics.Translated from Defektoskopiya, Vol. 40, No. 12, 2004, pp. 54–59.Original Russian Text Copyright © 2004 by Syurdo, Kortov, Milman, Slesarev, Mikhailovich.This revised version was published online in April 2005 with a corrected cover date.     

Elasticity solution for an axially compressed and encased cylindrical specimen

We present the solution of the linear elasticity equations governing the deformation of an elastic cylinder encased in a tube and subjected to uniform compression on the flat ends. The solutions for the stresses, strains, and displacements in the encased and compressed cylinder are all systematically determined from the basic solution of Lamé's classical elasticity problem of the long tube subjected to internal and external pressures. We first derive the general elastostatic analysis for an encased hollow cylinder, stress-free at the cavity, and later particularize the solution to a solid cylindrical specimen. The effective modulus E_eff of the encased sample is found to be a function of the bulk modulus k and Poisson's ratio ν of the material. E_eff differs from k except for nearly incompressible materials, where E_eff approaches the bulk modulus value. In the incompressible case, we also show how a load applied on the cylinder's flat ends is equivalent to, and can be replaced by, the same load acting on the curved surface. For compressible materials, a more general expression for E_eff is found that also accounts for the case deformation. These results explain the deformation of an axially compressed and encased cylindrical specimen tested in compressibility measuring devices such as those described by Matsuoka and Maxwell [Response of linear high polymers to hydrostatic pressure. Journal of Polymer Science 1958; 32:131–59]. The present analysis thus contributes to a better understanding of how this device works and to the interpretation of measurements taken with it.     

Optimization of Parameters of the Gamma-Absorption Method of Measuring the Layer Thickness of a Two-Layer Composition

Mathematical relations are obtained and a technique of determining the requirements for the relative measurement errors ₁ and ₂ in signals recorded for prescribed errors in measuring the composition layer thicknesses is proposed. It is shown that the density fluctuations of materials affect the thickness measurement errors. Analytic expressions determining the optimum gamma-quantum energies E ₁ and E ₂ (E ₁ < E ₂) for arbitrary values of ₁ and ₂ are derived. On the basis of these expressions, it is concluded that the optimum energies are independent of the densities of the materials and their dimensions. With reference to an Al{C composition, it is shown that for the gamma-quantumenergy ranges considered (0.04–1.25 MeV), the admissible values of E ₁ lie on a bounded interval 0.04–0.12 MeV, the optimum values of E ₂ are related to E ₁ by an approximate relation E _2opt/E ₁ 2.5–3.2, and the best characteristics with respect to the thickness measurement errors are reached as E ₁ E _1min = 0:04 MeV.     

Elimination of Systematic Measurement Inaccuracy in Voltage–Capacitance Spectroscopy of Semiconductor/Insulator Interface

An improved algorithm for measurements of quasi-static voltage–capacitance characteristics (VCC) of metal–insulator–semiconductor (MIS) structures is suggested. This algorithm includes the following stages: high-temperature depolarization of a MIS structure, its cooling at a constant depolarization voltage, measurement of the MIS current I(V _g) upon the linearly time-swept (increasing or decreasing) gate potential, and averaging of the dependences I(V _g) obtained in the modes of sweep rate _v > 0 and _v < 0 ( _v = 0). The method allows the systematic error of the VCC C(V _g) measurement caused by the relaxation polarization of the insulator, leakage current, and shift of the electrometer zero to be eliminated. As a result, the accuracy and reliability of the determination of the interface state (IS) density N _ss(E) are increased, and the energy range of the IS spectra is extended. To test the efficiency of the algorithm developed, it was used for determining the function N _ss(E) in the system SiO₂/(100)Si : P.     

Identification of yield stress and plastic hardening parameters from a spherical indentation test

Assuming plastic hardening of metals are specified by the stress–strain curve in the form , the material parameters σ₀, k and m are identified from spherical indentation tests by measuring compliance moduli in loading and unloading of the load–penetration curve. The curve P(h_p) is analytically described by a two term expression, each with different exponents. Here, ε_p and h_p denote the plastic strain and permanent penetration. The proposed identification method is illustrated by specific examples including numerical and physical identification tests.     

Friction coefficient of soft contact lenses: measurements and modeling

Tribological conditions for contact lenses have very low contact pressures in the range 3–5 kPa and sliding speeds around 12 cm/s. Using a microtribometer a series of experiments was run on commercially available contact lenses made from Etafilcon-A. These tests were run using 10–50 mN of normal load at speeds from 63 to 6280 m/s using a 1-mm radius glass sphere as a pin. The resulting contact pressures are believed to be nearly an order of magnitude larger than the targeted 3–5 kPa. It is hypothesized that the viscoelastic nature of the hydrogel, viscous shearing of the packaging solution, and interfacial shear between the glass sphere and the contact lens all contribute to the friction forces. A model that includes all three of these contributors is developed and compared to the experimental data. The experimental friction coefficients vary from = 0.025 to 0.075. The calculated fluid filmthicknesses were between 1 and 30 nm. The average surface roughness of the lens and the glass sphere are R_a=15 nm and R_a=8 nm, respectively, suggesting that the contact is not in full elastohydrodynamic lubrication. Finally, the largest contributors to the friction force in these experiments were found to be viscous dissipation within the hydrogel and interfacial shear within the contact zone.     

Determination of a Mobile-Ion Concentration in the Dielectric Films of Metal–Insulator–Semiconductor Structures

An experimental technique for determining the surface concentration N _Sof mobile ions in dielectric films of metal–insulator–semiconductor (MIS) structures is described. The technique is based on synchronous recording of the dynamic volt–ampere and low-frequency capacity–voltage characteristics of a sample under investigation. These experimental dependences are shown to ensure accurate extraction of the ion current peaks whose areas are proportional to N _S. These characteristics also allow the relaxation of the surface semiconductor potential to be found, which is needed for reconstructing the dependence of the convection ion current on the voltage drop across the insulation gap of the MIS capacitor. A comparative analysis with other known methods for determining N _Sis carried out. The proposed technique helps find a mobile-ion concentration from a 5 × 10⁹to 10¹³-cm^–2range, including the case when ion current peaks do not appear on the current–voltage characteristics.