Showing papers on "Thermoelastic damping published in 1976"

Solid Friction Damping of Mechanical Vibrations

Abstract: A theory of solid friction damping of mechanical vibrations is presented that is based on a solid friction mathematical model previously proposed by the author. A summary and improved description of the general analytic features of the solid friction model are given as necessary background for the theory. The Coulomb friction damped oscillator is analyzed to establish an approach to the treatment of a simple friction damped oscillator. The approach then is generalized to treat a more general model of friction where the author's model is used to describe friction force primarily as a function of displacement. The solid friction damped oscillator studied is a wire pendulum where solid friction enters via inelastic flexing of the wire at the support. Theoretical results are generalized to be applicable to other types of oscillators and other sources of solid friction. An expression for the decay rate of the oscillation amplitude envelope of an unforced oscillator is derived. The decay rate and an equivalent linear damping ratio are determined for several values of an exponent parameter in the solid friction model.

A continuum theory of deformable ferrimagnetic bodies. II. Thermodynamics, constitutive theory

Abstract: In order to close the system of differential field equations developed in Paper I, this article proposes a rational development of the relevant macroscopic thermodynamics and of a constitutive theory. In particular, by following Coleman’s thermodynamics, exact nonlinear constitutive equations for thermoelastic antiferromagnetic insulators are formulated. According to the deductive scheme adopted in Paper I, the important case of elastically isotropic antiferromagnets with a magnetic easy axis, and possibly endowed with the property of weak ferromagnetism, is developed in detail by using approximations. In order to supplement the description of thermodynamically recoverable processes and in accordance with the Onsager–Casimir theory of irreversible processes, the constitutive equations governing phenomena such as viscosity, electric and heat conduction, and spin relaxation, the latter either for strong or weak damping, are obtained. Regarding the latter effect, it is shown, thanks to the formalism adopted ...

Thermoelastic instability in lubricated sliding between solid surfaces

Abstract: IN the past few years, it has been recognised that, in certain conditions, thermoelastic instabilities occur in dry sliding contact between two solids1–4. We here report observations of the same phenomenon in the presence of a liquid lubricant which supports analytical predictions6 of such an effect. Thermoelastic instability is caused by frictional heating and thermal deformation of the surfaces in contact, and alters the original contact configuration. Even if the surfaces are perfectly flat at the outset, instability will lead to a departure from flatness, ultimately resulting in patches of high pressure in the lubricant, with surrounding regions of low pressure and relative parting of the surfaces. This instability can lead to a malfunction of, for example, mechanical face seals.

Damper for sound conducted in solids

Abstract: A damper for sound conducted or propagating in solids is constructed to ecially damp oscillations or vibrations of the bending type particularly in plates, housings, girders, rails, rods, etc. The damper is connected to the structural member to be damped in such a manner that the damping force is taken up by the structure member itself. For this purpose a damping element having a high rigidity against bending is secured longitudinally to a structural member to be damped. The structural member and the element form the damping unit. Due to the high flexural rigidity, the damping element behaves like a solid platform or a fixed point, such that vibrations of the structural member cause a relative motion between the structural member and the damping element. This relative motion provides excellent damping characteristics which may even be improved by placing a viscous damping material between the structural member and the element.

Mathematical model and boundary conditions in stress analysis relating to steam turbine rotors, under transient operating conditions

Abstract: An appropriate mathematical model for calculation of thermoelastic stress in steam turbine rotors, under transient operating conditions, is developed in the framework of so called uncoupled quasi-static theory. The numerical analysis has been carried out by means of the finite element code ANGUS, the accuracy of which has been evaluated by comparing it with an analytical solution given by one of the authors in Reference 2. It has been shown that, with an appropriate choice of thermal and elastic boundary conditions, it is possible to carry out stress transient analysis considering only a segment of the hottest zone of high or intermediate pressure body of rotors. The paper includes a comparison study of the influence of two types of thermal boundary conditions in the labyrinth glands.

Ritz Solutions for Thermoelastic Cylinders

Abstract: The use of Rayleigh-Ritz and modified Rayleigh-Ritz techniques for quasistatic thermal stress analysis of nonhomogeneous anistropic elastic bodies is examined. General expressions for the thermoelastic energies are written, and reduction of the general formulation to the problem of a nonhomogeneous orthotropic circular cylinder under a radial temperature distribution is illustrated. Approximate solutions for the deformations and stresses are compared with exact thermoelastic solutions in special cases of homogeneous cylinders, and with finite element solutions in cases of nonhomogeneous cylinders.

Conditions for zero thermal stresses in anisotropic multiply connected thermoelastic bodies

Abstract: The conditions for zero thermal stresses in multiply connected anisotropic thermoelastic homogeneous materials are determined from a consideration of the compatibility requirements. For bodies of rectilinear anisotropy the conditions are the same as those for isotropic bodies; namely, if the body is free of surface tractions and body forces, a temperature field which is linear with respect to rectangular cartesian coordinate system results in zero stresses. A uniform temperature rise produces no stresses in a cylindrically orthotropic, multiply connected body providing the symmetry axis lies outside the body, but causes stresses when the axis intersects or is surrounded by the body. Any nonzero temperature field induces stress in a spherically orthotropic body. The deformation fields associated with each of these temperature fields are obtained by integration of the thermal strains.

On the thermoelastic analysis of solar cell arrays and related material properties

Abstract: Accurate prediction of failure of solar cell arrays requires accuracy in the computation of thermally induced stresses. This was accomplished by using the finite element technique. Improved procedures for stress calculation were introduced together with failure criteria capable of describing a wide range of ductile and brittle material behavior. The stress distribution and associated failure mechanisms in the N-interconnect junction of two solar cell designs were then studied. In such stress and failure analysis, it is essential to know the thermomechanical properties of the materials involved. Measurements were made of properties of materials suitable for the design of lightweight arrays: microsheet-0211 glass material for the solar cell filter, and Kapton-H, Kapton F, Teflon, Tedlar, and Mica Ply PG-402 for lightweight substrates. The temperature-dependence of the thermal coefficient of expansion for these materials was determined together with other properties such as the elastic moduli, Poisson's ratio, and the stress-strain behavior up to failure.

Thermoelasticity in Open Systems

Abstract: The thermoelastic properties of polymer networks are usually obtained by studying the effect of temperature on the magnitude of the stress of samples subjected to uniaxial elongation. It is assumed that polymer networks are in the region of small elongation characterized by reversible deformation, and that the results obtained represent equilibrium values. The method makes it possible to characterize the network by a series of thermodynamic parameters, of which the ratio fu/f is used most frequently. This ratio, fu/f, expresses the ratio between the energetic component fu and the total elastic force f. The values of fu/f are technically important parameters which may serve as a measure of the “ideality” of an elastomer. For an ideal elastic polymer, i.e., the polymer the elasticity of which is entirely entropic in origin, it can be written fu/f = 0. The significance of thermoelasticity was markedly increased when the phenome-nological parameter fu/f was shown to be interpretable on the molecular ...

Thermal stresses in a spherical pressure vessel having temperature-dependent, transversely isotropic, elastic properties

Abstract: Rayleigh-Ritz and modified Rayleigh-Ritz procedures are used to construct approximate solutions for the response of a thick-walled sphere to uniform pressure loads and an arbitrary radial temperature distribution. The thermoelastic properties of the sphere are assumed to be transversely isotropic and nonhomogeneous; variations in the elastic stiffness and thermal expansion coefficients are taken to be an arbitrary function of the radial coordinate and temperature. Numerical examples are presented which illustrate the effect of the temperature-dependence upon the thermal stress field. A comparison of the approximate solutions with a finite element analysis indicates that Ritz methods offer a simple, efficient, and relatively accurate approach to the problem.

Two-dimensional thermal elasticity problem for a body weakened by a system of thermally insulated cracks

Abstract: The thermally elastic state of a body in two dimensions with cracks has been investigated in a number of articles (see the survey in [1]). However, in the majority of cases problems have been investigated in which temperature stresses in a body are weakened by a single crack. The existing solutions of problems on the interaction between thermally insulated cracks in an elastic body have been confined to simple cases either with collinear [2, 3] or with arched cracks [4, 5]. Below the two-dimensional thermoelastic problem for an infinite body with arbitrarily positioned straight-lined thermally insulated cracks is studied by reducing it to a system of singular integral equations. An approximate solution is found for large distances between cracks. An exact solution is obtained in the case of a periodic system of collinear cracks.

Convection and thermoelastic effects in narrow vertical fracture spaces

Abstract: Issued as Program management report 1-5, Semi-annual technical letter report no. 1-2, and Final technical report, Project no. G-35-623

Temperature Compensated Piezoelectric Materials

Abstract: : In order to search for new temperature compensated materials for surface acoustic wave (SAW) devices with low ultrasonic attenuation and high electro-mechanical coupling, the following experimental and theoretical investigation were carried out Extensive crystal growth investigations were carried out for Pb2KNb5O15, Li2SiO3, Ba2Si2TiO8, Ba2Ge2TiO8, and Bi2MoO5, using Czochralski or flux-pulling methods Progress in reducing the cracking problem was made by improved temperature control during growth and subsequent cooling, and by reducing variations in the diameter of the boule during growth Except for LiSiO3, millimeter size or larger specimen of fair to good quality have been obtained The ferroelectric transition temperature and the lattice parameters have been studied for the system xK2O + yPbO + zNb2O5 in the vicinity of the stoichiometric composition corresponding to Pb2KNb5O15, and the possible implications for the composition dependence of the elastic, thermoelastic and piezoelectric constants and for attaining large electromechanical coupling factor with temperature compensated cuts are discussed The two piezoelectric constants of AlPO4 are presently being measured by the x-ray method

Thermal stresses as a laser heating damage mechanism

Abstract: : The relative significance of thermal stresses as a laser heating damage mechanism is assessed by comparison with the damage mechanisms of melting and thermal degradation of structural strength properties. The limiting cases of one-dimensional axial and one-dimensional radial heat flux in a thin target plate whose plane is normal to the axis of a stationary, axially symmetric heat source, are investigated. A one-dimensional radial heat conduction numerical model of the linear thermoelastic stress field including the effects of melting and structural failure is developed. Residual tensile strength and damage size are presented as functions of the laser beam and target plate parameters.