Showing papers on "Shell (structure) published in 2000"

Subdivision surfaces: a new paradigm for thin-shell finite-element analysis

Abstract: We develop a new paradigm for thin-shell finite-element analysis based on the use of subdivision surfaces for (i) describing the geometry of the shell in its undeformed configuration, and (ii) generating smooth interpolated displacement fields possessing bounded energy within the strict framework of the Kirchhoff–Love theory of thin shells. The particular subdivision strategy adopted here is Loop's scheme, with extensions such as required to account for creases and displacement boundary conditions. The displacement fields obtained by subdivision are H2 and, consequently, have a finite Kirchhoff–Love energy. The resulting finite elements contain three nodes and element integrals are computed by a one-point quadrature. The displacement field of the shell is interpolated from nodal displacements only. In particular, no nodal rotations are used in the interpolation. The interpolation scheme induced by subdivision is non-local, i.e. the displacement field over one element depend on the nodal displacements of the element nodes and all nodes of immediately neighbouring elements. However, the use of subdivision surfaces ensures that all the local displacement fields thus constructed combine conformingly to define one single limit surface. Numerical tests, including the Belytschko et al. [10] obstacle course of benchmark problems, demonstrate the high accuracy and optimal convergence of the method.

Structural basis for the fracture toughness of the shell of the conch Strombus gigas

Abstract: Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few per cent of the composite material, the fracture toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude The judicious placement of the organic matrix, relative to the mineral phase, and the hierarchical structural architecture extending over several distinct length scales both play crucial roles in the mechanical response of natural composites to external loads Here we use transmission electron microscopy studies and beam bending experiments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two energy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shell's tougher middle layers at higher loads Both mechanisms are intimately associated with the so-called crossed lamellar microarchitecture of the shell, which provides for 'channel' cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material Despite a high mineral content of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a 'ceramic plywood' and can guide the biomimetic design of tough, lightweight structures

Oscillations of polymeric microbubbles: Effect of the encapsulating shell

Abstract: A model for the oscillation of gas bubbles encapsulated in a thin shell has been developed. The model depends on viscous and elastic properties of the shell, described by thickness, shear modulus, and shear viscosity. This theory was used to describe an experimental ultrasound contrast agent from Nycomed, composed of air bubbles encapsulated in a polymer shell. Theoretical calculations were compared with measurements of acoustic attenuation at amplitudes where bubble oscillations are linear. A good fit between measured and calculated results was obtained. The results were used to estimate the viscoelastic properties of the shell material. The shell shear modulus was estimated to between 10.6 and 12.9 MPa, the shell viscosity was estimated to between 0.39 and 0.49 Pas. The shell thickness was 5% of the particle radius. These results imply that the particles are around 20 times more rigid than free air bubbles, and that the oscillations are heavily damped, corresponding to Q-values around 1. We conclude that the shell strongly alters the acoustic behavior of the bubbles: The stiffness and viscosity of the particles are mainly determined by the encapsulating shell, not by the air inside.

Residual quenching stresses in glass-coated amorphous ferromagnetic microwires

Abstract: The calculation of the residual stress tensor components in glass-coated amorphous ferromagnetic microwire is carried out on the basis of the theory of viscoelasticity. The approach takes into account the relaxation of the stresses both in a metallic core and a glass shell of the wire within a certain temperature interval near the point of the wire's vitrification. The distribution of the residual stresses is investigated as function of mechanical characteristics of metallic core and glass shell at different ratios of the metallic core radius to the total wire radius. The magnetic behaviour of a glass-coated amorphous microwire with small negative magnetostriction is analysed and is shown to be consistent with the experimental data.

Shell corrections of superheavy nuclei in self-consistent calculations

Abstract: Shell corrections to the nuclear binding energy as a measure of shell effects in superheavy nuclei are studied within the self-consistent Skyrme-Hartree-Fock and relativistic mean-field theories. As a result of the presence of a low-lying proton continuum resulting in a free particle gas, special attention is paid to the treatment of the single-particle level density. To cure the pathological behavior of the shell correction around the particle threshold, a method based on the Green's function approach has been adopted. It is demonstrated that for the vast majority of Skyrme interactions commonly employed in nuclear structure calculations, the strongest shell stabilization appears for Z=124 and 126, and for N=184. On the other hand, in the relativistic approaches the strongest spherical shell effect appears systematically for Z=120 and N=172. This difference probably has its roots in the spin-orbit potential. We have also shown that, in contrast to shell corrections which are fairly independent of the force, macroscopic energies extracted from self-consistent calculations strongly depend on the actual force parametrization used. That is, the A and Z dependence of the mass surface when extrapolating to unknown superheavy nuclei is prone to significant theoretical uncertainties. (c) 2000 The American Physical Society.

Rotation-free triangular plate and shell elements

Abstract: SUMMARY The paper describes how the nite element method and the nite volume method can be successfully combined to derive two new families of thin plate and shell triangles with translational degrees of freedom as the only nodal variables. The simplest elements of the two families based on combining a linear interpolation of displacements with cell centred and cell vertex nite volume schemes are presented in detail. Examples of the good performance of the new rotation-free plate and shell triangles are given. Copyright ? 2000 John Wiley & Sons, Ltd.

A Shell/3D Modeling Technique for the Analysis of Delaminated Composite Laminates

Abstract: A shell/3D modeling technique was developed for which a local solid finite element model is used only in the immediate vicinity of the delamination front. The goal was to combine the accuracy of the full three-dimensional solution with the computational efficiency of a shell finite element model. Multi-point constraints provided a kinematically compatible interface between the local 3D model and the global structural model which has been meshed with shell finite elements. Double Cantilever Beam, End Notched Flexure, and Single Leg Bending specimens were analyzed first using full 3D finite element models to obtain reference solutions. Mixed mode strain energy release rate distributions were computed using the virtual crack closure technique. The analyses were repeated using the shell/3D technique to study the feasibility for pure mode I, mode II and mixed mode I/II cases. Specimens with a unidirectional layup and with a multidirectional layup were simulated. For a local 3D model, extending to a minimum of about three specimen thicknesses on either side of the delamination front, the results were in good agreement with mixed mode strain energy release rates obtained from computations where the entire specimen had been modeled with solid elements. For large built-up composite structures the shell/3D modeling technique offers a great potential for reducing the model size, since only a relatively small section in the vicinity of the delamination front needs to be modeled with solid elements.

Shape coexistence and the N = 28 shell closure far from stability

Abstract: The masses of 31 neutron-rich nuclei in the range A = 29-47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N = 28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in 43S.

Air mask with seal

Abstract: A nasal mask includes a flexible shell and a seal. The flexible shell has an inlet and includes a malleable element disposed about a periphery of the shell for maintaining the shell in a user selectable configuration. The seal is disposed along a perimeter of the shell so as to form a chamber within the shell when the mask is donned by a user.

Solar collection system

Abstract: The solar tracking mechanism is utilized in connection with a solar energy collection system. The collection system includes a light reflective shell shaped to focus solar radiation on a radiation absorbing segment of a tube which carries a heat transfer fluid. The shell is pivotally mounted on a support frame. An actuator mounted between the support frame and the shell rotatably the shell. A solar sensor is mounted deep within a sighting tube which is fixed to the shell such that a line of sight through the sighting tube is at least parallel to the optical axis of the shell. The solar sensor generates a sensor signal which is used as a control input to an actuator control system. End limit switches generate a limit stop signals when the shell reaches maximum angular positions. The actuator control system generates fluid flows to the actuator based the solar sensor signal and the limit stop signals. The method of tracking the sun includes providing a solar cell array, activating the solar collection system when solar radiation illuminating the array exceeds a predetermined threshold, providing a solar sensor shielded from the solar radiation except for direct, aligned radiation, pivotally rotating the shell westward based upon the solar sensor signal, stopping the shell at a maximum angular positions, and rotating the shell westward if the shell does not reach the maximum westward angular orientation during a predetermined daylight time period. The solar energy collection system may be further configured to include a bisected shell, which are hinged together. The shell halves can be collapsed onto each other thereby protecting the light reflective surface and the radiation absorbing segment of the tube carrying heat transfer fluid.

Bi-stable composite shells

Abstract: This paper is concerned with a new type of deployable structures that can be rolled up like a tape measure but, unlike a tape measure, are stable in two different configurations. The key to the behaviour of these structures is a particular type of composite construction based on an anti-symmetric lay-up of glass fibres in a polypropylene matrix. The paper presents a finite element analysis using the ABAQUS package of the process of rolling up a bi-stable cylindrical shell. The results of this simulation provide considerable new insights into the structural mechanics of bi-stable shells, as well as predicting the stress distribution, curvature variation, etc. in the rolledup shell.

Theory of Anisotropic Thin-Walled Beams

Abstract: Asymptotically correct, linear theory is presented for thin-walled prismatic beams made of generally anisotropic materials. Consistent use of small parameters that are intrinsic to the problem permits a natural description of all thin-walled beams within a common framework, regardless of whether cross-sectional geometry is open, closed, or strip-like, Four classical one-dimensional variables associated with extension, twist, and bend-ing in two orthogonal directions are employed. Analytical formulas are obtained for the resulting 4 × 4 cross-sectional stiffness matrix (which, in general, is fully populated and includes all elastic couplings) as well as for the strain field. Prior to this work no analytical theories for beams with closed cross sections were able to consistently include shell bending strain measures. Corrections stemming from those measures are shown to be important for certain cases. Contrary to widespread belief, it is demonstrated that for such classical theories, a cross section is not rigid in its own plane. Vlasov's correction is shown to be unimportant for closed sections, while for open cross sections asymptotically correct formulas for this effect are provided. The latter result is an extension to a general contour of a result for 1-beams previously published by the authors.

Manipulating core-shell reactivities for processing nanoparticle sizes and shapes

Abstract: This paper describes new findings of an investigation of thermally-activated core–shell reactivities of nanoparticles in solutions for processing size, shape and surface properties. Gold nanoparticles of ≈2 nm core sizes with thiolate monolayer encapsulation were chosen as a model system for the manipulation of reaction parameters, including annealing effect, core composition and shell structure. It is revealed that, upon an evolution of particle sizes, annealing treatment of the solution in the presence of encapsulating thiols can lead to the formation of highly monodispersed nanoparticles. New insights into shell desorption, core coalescence and shell re-encapsulation have been provided by dependencies of the evolution temperature on the capping thiolate chain length and the core alloy composition. Transmission electron microscopic, FTIR and UV-Visible techniques were used to characterize the morphological and chemical properties. The implication of the results for the development of abilities in chemical processing core–shell nanoparticles is discussed.

Arrangement for portable pumping unit

Abstract: An enclosure for a medical pumping device generally comprises a front shell, a back shell and a polymeric substrate interposed therewith. The polymeric substrate comprises at least one gasket seat corresponding to the perimetric edge of each shell. In use, various components are securely placed within the plurality of component compartments within the polymeric substrate and thereafter encased within the enclosure between the front shell and back shell. According to the preferred embodiment of the present invention, the polymeric substrate also serves to form bumpers about the edges of the enclosure when the shells and substrate are assembled together.

Optical properties of a photonic crystal of hollow spherical shells

Abstract: We report on optical transmission studies of three-dimensionally ordered photonic crystals of close-packed spherical shells. These samples are fabricated using a double-template method, which allows for extensive control over shell thickness. The transmission spectra exhibit an optical stop band, whose spectral position and width depend on the thickness of the shell and on the overlap between adjacent spheres, in a manner consistent with numerical simulations. These parameters can be controlled over a wide range, thus permitting systematic studies of the optical properties, and providing a valuable method for engineering the characteristics of the optical stop band in colloidal photonic media.

Nonlinear vibrations and multiple resonances of fluid-filled, circular shells, part 1: Equations of motion and numerical results

Abstract: The response-frequency relationship in the vicinity of a resonant frequency, the occurrence of travelling wave response and the presence of internal resonances are investigated for simply supported, circular cylindrical shells. Donnell's nonlinear shallow-shell theory is used. The boundary conditions on radial displacement and the continuity of circumferential displacement are exactly satisfied. The problem is reduced to a system of four ordinary differential equations by means of the Galerkin method. The radial deflection of the shell is expanded by using a basis of four linear modes. The effect of internal fluid is also investigated. The equations of motion are studied by using a code based on the Collocation Method. The present model is validated by comparison of some results with others available. A water-filled shell presenting the phenomenon of 1:1:1:2 internal resonances is investigated for the first time; it shows intricate and interesting dynamics.

Piezoelectric induced bending and twisting of laminated composite shallow shells

Abstract: The piezoelectric induced bending and twisting deformation of laminated shallow shells has been presented in this report. A simple and efficient method has been developed for the static analysis of shallow shells under mechanical, thermal, and piezoelectric fields. The governing equations are based on the linear theory of piezoelectricity and the classical thin shallow shell assumptions. Approximate solutions have been obtained using the conventional Ritz analysis for different boundary conditions. Comparisons of degenerate cases with published analytical and experimental results show good agreement. Several cases have been reported here, investigating the effects of material stiffness coupling, shell shallowness, and actuator geometric parameters on the bending and twisting deformation of piezoelectric laminated structures. It is shown that the desired shape could be better controlled by a proper selection of the lay-up configuration and geometric characteristics of the structure such as the thickness, size, and location of the piezoelectric actuators.

The mathematical shell model underlying general shell elements

Abstract: SUMMARY We show that although no actual mathematical shell model is explicitly used in ‘general shell element’ formulations, we can identify an implicit shell model underlying these nite element procedures. This ‘underlying model’ compares well with classical shell models since it displays the same asymptotic behaviours|when the thickness of the shell becomes very small|as, for example, the Naghdi model. Moreover, we substantiate the connection between general shell element procedures and this underlying model by mathematically proving a convergence result from the nite element solution to the solution of the model. Copyright ? 2000 John Wiley & Sons, Ltd.

Snap-on plug coaxial connector

Abstract: A coaxial connector assembly includes a plug connector 2 and a mating jack connector 102. The plug connector 2 is a snap-on or quick connect and quick disconnect style connector. The plug connector 2 includes a shell 10 that can be terminated to a coaxial cable outer shield or braid 114. The shell 10 has deflectable spring fingers 12 formed at its mating end with a radially extending lip 16. A collar 30 surrounds the shell 10. The collar 30 is formed by a two housing components 32 that are preferably identical and can be mated together in surrounding relationship to the shield. Spring beams 34, 34 A or elastomeric members 34 B engage the peripheral lip to urge the collar 30 and the shell 10 toward a neutral position, even though the collar 30 and the shell 10 are relatively axially shiftable to facilitate mating and unmating.