Showing papers on "Thermoelastic damping published in 1968"

Critique on Theories Predicting Thermoelastic Properties of Fibrous Composites

Abstract: A critique on the theories predicting the thermoelastic properties of unidirectional fibrous composites is presented. The method of approach in these theories varies from netting analysis to complex statistical methods. This critique provides a concise description and evaluation of those theories which are instructive or illustrate an interesting approach in the prediction of the thermoelastic proper ties. It is intended to contribute to better understanding and judicious application of these theories, to familiarize the researcher with the state of the art and to provide him with a basis for future effort. Thermoelastic properties predicted by the various theories are arranged in check-list form for quick reference. Typical results are included for comparison purposes and current trends are dis cussed. References and publications which provide working equa tions and/or result comparison of these theories are cited.

Thermoelastic Stress Production in Solids

Abstract: Theoretical calculations of the consequences of the thermoelastic mechanism for production of stress in solids by pulsed energy input have been performed, and the effect of variations in the acoustic impedance of a transparent backup material and variations in the pulse duration as compared to the acoustic transit time across the penetration depth of the incident energy have been investigated. Experimental verification of the theoretical calculations has been done using a Q‐switched ruby laser as an energy source with samples of absorbing glass as targets. It is found that the experimental results give good agreement in the shapes of the stress pulses produced, and fair agreement in the absolute amplitude of the stress, with no adjustable parameters being required. The calculations and experiments demonstrate the validity of the thermoelastic stress‐production mechanism in solids when no phase transformation occurs.

Generation of thermoelastic stress waves by impulsive electromagnetic radiation.

Abstract: The penetration and absorption of electromagnetic radiation through a thin layer adjacent to the surface of an elastic half-space provides a sudden heat source distribution through the layer, and, in consequence, stress waves are generated. A solution for uniaxial motion is given in the limit situation when the pulse duration approaches zero in comparison with the wave travel time over the absorption depth, together with the neglect of thermal diffusion on this time scale. The radiation absorption is assumed to decay exponentially with depth. These simplifying features lead to stress wave profiles which clearly illustrate the effective nature of the wave propagation, so that the present solution provides a useful complement to previously obtained solutions of a more general nature. In the case when the surface is stress-free, following initial build-up (here instantaneous) of compressive stress within the absorption layer, a tensile wave propagates outward from the layer. Numerical data are presented to show that the peak tensile stress can attain significant levels within distances of a few absorption depths.

Controlling wall thickness in the blowing - of hollow bodies

Abstract: Hollow-body blowing is effected by a two-part blowing device with the aid of compressed air and the thermoplastic moulding material solidifies through contact with the cold wall The method is partic useful in the production of bottles and provides for various fields of the preformed article to be reheated at different temps, in the thermoelastic region Pref the preformed item is cylindrical in shape and the three fields along the axis of the cylinder are heated to different temps

Attenuation of Longitudinal Ultrasonic Waves in Insulators at Room Temperature

Abstract: It is possible to distinguish between two intrinsic loss mechanisms for longitudinal ultrasonic waves propagating in dielectric single crystals, viz., the thermoelastic loss mechanism and the Akhiezer mechanism. In the normal derivation of the theoretical expressions of the attenuation coefficients for these mechanisms, the former involves the thermal conductivity and the thermal expansion coefficients, while the latter involves the thermal conductivity and the Gruneisen numbers of the thermal phonon modes. These Gruneisen numbers can only be deduced from a knowledge of the third‐order elastic constants. We have found experimentally that, for the 15 materials for which sufficient data are available, the attenuation from the Akhiezer mechanism exceeds that from the thermoelastic mechanism by a factor of about 40. This result is shown to be a consequence of the distribution of the Gruneisen numbers of the thermal phonon modes about the mean value. It is suggested that our conclusions can be used to predict the total ultrasonic attenuation in other materials from a knowledge of a few macroscopic properties, even if the third‐order elastic constants are unknown. Such predictions are made for GaAs, InSb, and diamond. The third‐order elastic constants of GaAs, InSb, and LiF have also been used to compute the ultrasonic attenuation in these materials, and to compute the volume Gruneisen constants. The latter are found to be in good agreement with experimental values except for InSb.

Transient Thermoelastic Problem for a Transversely Anisotropic Hollow Cylinder with Temperature-Dependent Properties

Abstract: This paper is concerned with an axisymmetric transient thermoelastic problem for a transversely anisotropic hollow cylinder, the boundary of internal surface being subjected to a sudden temperature rise. The problem is treated on assumption that the thermal and elastic properties of the cylinder are dependent on temperature. Basic equation is solved with the aid of perturbation method. The particular boundary value problems are carried out, and the corresponding displacements and stresses are clarified. For numerical examples, calculations are performed for the cases of anisotropic cylinders made of grades ATJ and ZTA graphites respectively. In these treatments, the variation of conductivity and thermal expansion is taken to be linear according to the temperature rise, but that of the Young's moduli is taken to be quadratic. The solution for an isotropic cylinder is also derived as a particular case of the present problem.

Reduction of noise by the use of damping materials

Abstract: level being thus reduced. Free bending and strain waves can transport sound energy via extended sheet-metal structures to points far away from the sources, causing sound radiation into the air. In this case too, the application of vibration damping materials will be beneficial because in this way the structure-borne sound propagation will be partially suppressed by the attenuation of the free-bending waves. Furthermore, the damping of the bending vibrations can essentially improve the sound insulation of light-weight structures, so-called sandwich walls, consisting of outer metal plates and an air cushion between, filled with sound-absorbing material. The damping of the metal plates diminishes the influence of resonances and coincidences which otherwise would lead to a diminution of the sound reduction of the sandwich wall (Stiuber I956, I965). Finally, it has been shown that the fatigue life of metal panels, for example near the outer walls of jets which are excited to flexural vibrations by the random pressure of the air-borne sound field of high intensity, is increased as a result of the damping of the panels (Kurtze & Westphal I965). There has been a systematic development of vibration damping materials for nearly 20 years. Formerly, the production of these materials was a matter of trial-and-error methods (Beranek I960). It was a long road to the high-quality products which are in use today. The measuring technique had to be developed with which the viscoelastic properties, especially the damping of the damping materials and of the combined systems consisting of metal plates or sheet metal and damping viscoelastic layers, could be investigated. Furthermore, the theory of the bending vibrations and the viscoelastic properties, especially the damping of the combined systems, had to be worked out to enable us to understand the dependence of those properties on the viscoelastic properties of the layers and their thicknesses. In the first stage, two-layer systems consisting of metal panels with a damping layer on one side were considered (Lienard I951, 1957; Oberst & Frankenfeld 1952; Oberst, Becker & Frankenfeld I954). From the results of the theory the conditions which the damping materials had to fulfil to provide high damping of the combined systems could be established. It soon became obvious that the highest damping values could be obtained with amorphous thermoplastic polymers. Next, the possibilities were studied as to how vibration damping materials with optimum damping efficiency could be

Piezoelektrisches und elastiches Verhalten von HIO3 und DIO3

Abstract: The piezoelectric, elastic, and thermoelastic constants of HIO3 and DIO3 have been found to agree within the limits of experimental error, as could be shown by earlier investigations on other non-ferroelectric compounds. A high anisotropy in the deviations from the Cauchy relations reveals strong bonds with preferred orientations.

Stresses around Circular Ring Inclusion Due to Uniform Thermal Gradient

Abstract: The thermoelastic problems for the case of an infinite elastic plate containing a circular hole or a circular inclusion subjected to a uniform temperature gradient have been analyzed by some investigators, so for the next step a problem of a plate containing a ring which has different elastic properties is analyzed in this paper. This subject is worked out in considering a case of a pipe running through a concrete medium, or of a plate containing a circular hole as reinforced by a ring. Numerical results are given in the form of curves for typical cases.

Damping of thermoelastic structures

Abstract: Report ascertains the effects of thermoelastic damping on the propagation of longitudinal waves in cylindrical rods Review of results of wave propagation in unbounded elastic solids and in elastic cylinders precedes consideration of thermal modification of elastic properties

Laser Interferometric Determination of the Dynamic Response of Solids

Abstract: Measurements of the dynamic response of a solid induced by charged particle interaction are presented. The measurements were made using a laser interferometric technique to determine the free surface motion of silicon and germanium exposed to a pulsed electron beam. The results show that the free surface displacement and velocity can be determined in detail. The measured displacement and velocity histories are directly related to the energy spectrum of the incident electron beam and proportional to the electron fluence. The measurements are compared with the motion predicted by a thermoelastic calculation using known material properties. The difference between the predicted and measured response is discussed with respect to the coupling between the absorbed energy and the acoustic phonons.

Local stress measurement using the thermoelastic effect

Abstract: A technique for measuring local stresses in metallic specimens is proposed and tested. The technique depends on the experimental measurement of temperature changes in stressed members due to adiabatic elastic deformation. At a free boundary in a body under plane stress, these temperature changes are directly related to the value of the tangential principal stress. The technique is suited for measurement of stress-concentration effects, since the temperature changes can be measured with thermocouples featuring extremely small junctions. A simple stress-concentration geometry, the finitewidth strip with a central circular hole, is chosen as a model system for this study. Heat transfer in this geometry due to the temperature gradients produced by elastic deformation is analyzed. It is shown that the ratio of the temperature change at a reference section to the change at the locale of the stress concentration can be used to determine the stress-concentration factor, allowing for heat-transfer effects. An experimental measurement system capable of obtaining reproducible results with the thermal-measurement technique is described, and experimental results are given for the model geometry which agree favorably with theoretical predictions. Application of the technique to other problems is discussed.

Calculation of the thermal stresses in a solidifying crust

Abstract: The equation for the thermoelastic displacement potential in the quasi-steady approximation is refined by taking the body force potential into account. The equation is solved for bodies of simple shape. The optimal law of solidification ensuring minimum temperature stresses in the crust is determined for an infinite slab.

Flexural vibration of beams with external viscous damping

Abstract: The primary goal of this investigation was to deter­ mine the effects of concentrated external viscous damping proportional to transverse velocity on the free and forced vibration of beams. The model used to represent this problem was the elementary Bernoulli-Euler differential equation for flexural vibration, which was modified by add­ ing terms representing the type of damping under considera­ tion. Starting from this modified equation, general solu­ tions were derived in terms of both exact and approximate representations of the normal modes of free damped vibra­ tion. Galerkin's method was used to construct the approxi­ mate solution, which may be applied to a wide variety of beam configurations for which an exact solution is either impossible or prohibitively difficult. Both general solu­ tions are presented in a form applicable to any beam configuration' with homogeneous boundary conditions, either distributed or concentrated damping, and arbitrary forcing input and initial conditions. Numerical results for a specific example were cal­ culated from both solutions. The example chosen was a uniform simply supported beam with a dashpot attached at viii ix the center of its span. Normal mode functions and charac­ teristic frequencies were calculated for a range of dashpot strength by both methods; comparison showed very good agreement between exact and approximate values. Forced vibration due to sinusoidal motion of the beam supports was considered, and the steady state strain energy and power dissipated by damping were computed using the approximate solution. Results for the example showed that the influence of concentrated damping may be interpreted in terms of its effects on the normal modes of vibration. These effects were found to differ both qualitatively and quantitatively for the various modes. The relationship between the normal modes of damped vibration and their more familiar undamped counter­ parts was examined, and a physically meaningful descrip­ tion of the nature of these modes was offered. Comments on the utility of the normal mode solutions and suggestions for further investigation are included.