Showing papers on "Thermoelastic damping published in 1986"

Thermoelastic response of fluid-saturated porous rock

Abstract: A linear theory for fluid-saturated, porous, thermoelastic media is developed. The theory allows for compressibility and thermal expansion of both the fluid and solid constituents. A general solution scheme is presented, in which a diffusion equation with a temperature-dependent source term governs a combination of the mean total stress and the fluid pore pressure. In certain special cases, this reduces to a diffusion equation for the pressure alone. In addition, when convective heat transfer and thermoelastic coupling can be neglected, the temperature field can be determined independently, and the source term in the pressure equation is known. Drained and undrained limits are identified, in which fluid flow plays no role in the deformation. In the drained case, the medium behaves as a simple thermoelastic body with the properties of the porous skeleton with no fluid present. In the undrained limit, the fluid is trapped in the pores, and the material responds as a thermoelastic body with effective compressibility and thermal expansivity determined in part by the fluid properties. The theory is further specialized to one-dimensional deformation, and several illustrative problems are solved. In particular, the heating of a half space is explored for constant temperature and constant flux boundary conditions on the thermal field, and for drained (zero pressure) and impermeable (zero flux) conditions on the fluid pressure field. The behavior of these solutions depends critically upon the ratio of the fluid and thermal diffusivities, with very large and very small values of this parameter corresponding to drained and undrained responses, respectively.

Laser generation of acoustic waves in liquids and gases

Abstract: The laser generation of sound in liquids and gases is reviewed. The sound‐generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser‐pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10− 8 cm− 1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11‐μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser‐PAS system are presented, including first results on continuous i n s i t u air pollution monitoring.

A theory of thermoelastic materials with voids

Abstract: A linear theory of thermoelastic materials with voids is considered. First, some general theorems (uniqueness, reciprocal and variational theorems) are established. Then, the acceleration waves and some problems of equilibrium are studied.

Fundamental solution of the generalized thermoelastic problem for short times

Abstract: The solution of the problem of determining stress and temperature distributions with a continuous source of heat in an infinite elastic body governed by the equations of generalized thermoelasticity are obtained by using the Laplace transform technique. Inverse transforms are obtained in an approximate manner for small values of time. Numerical computations for some particular cases are carried out and the results are compared with the corresponding solutions of the coupled case and the uncoupled classical case.

Longitudinal wave propagation in a generalized thermoelastic cylinder

Abstract: In this paper the longitudinal wave propagation in a circular infinite cylinder is studied. The infinite circular cylinder is assumed to be made of a generalized thermoelastic material. The dispersion relation is obtained for the case in which the temperature is kept constant on the surface of the cylinder. Because of the complexity of the dispersion relation, the numerical solutions are given. For various values of parameters appearing in the field equations, some dispersion, attenuation, and phase velocity diagrams are presented

Thermoelastic strain in a half-space covered by unconsolidated material

Abstract: An algorithm to predict crustal thermoelastic strain from observed local atmospheric temperature is given and applied to a 24-month crustal strain record of one test strainmeter site located near Bouquet Reservoir in southern California. We use a crustal model that consists of an elastically decoupled layer overlying a uniform elastic half-space, and a thermal source that is given by a stationary temperature wave whose wavelength is related to local topography and/or lateral material heterogeneity. The decoupled layer delays, attenuates, and low-pass filters the source temperature field. The thermoelastic strain in the underlying half-space, resulting from the temperature variations at the base of the decoupled layer, is calculated using the Berger (1975) solution for thermoelastic strain in a uniform half-space. Applying our model to the test data, we obtain a good fit between predicted and observed strains if we filter the surface thermal signal through a 63-cm-thick decoupled layer. Much of the remaining strain variations clearly correspond to other environmental sources (reservoir loading and rainfall). Our analysis suggests that the horizontal thermoelastic strain is inversely proportional to the wavelength of local topography and/or lateral material heterogeneity. Thus, the horizontal thermoelastic strain will be greater in areas of local topography and/or lateral material heterogeneity and smaller in more homogeneous and flat areas. An upper layer of loose material, natural or artificial, acts as a thermoelastic strain insulator. Burial of strainmeters in places where such a layer exists can reduce the thermoelastic strain noise considerably even for shallow strainmeter emplacements.

A coupled thermoelastic model for saturated–unsaturated porous media

Abstract: The determination of the heat and mass flow in porous media is a problem of great importance in many engineering and technical processes. Typical problems include the accumulation of moisture under...

Electron microscope observation of the early stages of thermoelastic martensitic transformation in a TiNiCu alloy

Abstract: In situ observations by electron microscopy were made on nucleation and growth of a thermoelastic martensite in a Ti39.5Ni10.0Cu(at.%) alloy. It has been found that a stress field near defect aggregates plays an important role in the martensite nucleation. The habit plane of the martensite plates in the early stages of formation was determined to be the (334) plane of the parent cubic lattice by trace analysis. The evperimentally determined habit plane was in good agreement with the theoretically predicted one. The atomic configuration along the habit plane was considered on the basis of the evperimentally determined lattice deformation. It has been found that the macroscopic (334) habit plane is composed of (112) steps.

Thermal Expansion of Elastic-Plastic Composite Materials.

Abstract: : Exact relationships are derived between instantaneous overall thermal stress or strain vectors and instantaneous overall mechanical stifness or compliance, for two binary composite systems in which one of the phases may deform plastically. Also the local instantaneous thermal strain and stress concentration factors are related in an exact way to the corresponding mechanical concentration factors. The results depend on instantaneous thermoelastic constants and volume fractions of the phases. They are found for fibrous composites with two distinct elastically isotropic or transversely isotropic phases, and for any binary composite with elastically isotropic phases. The results indicate that in the plastic range the thermal and mechanical loading effects are coupled even if the phase properties of not depend on changes in temperature. The derivation is based on a novel decomposition procedure which indicates that spatially uniform elastic strain fields can be created in certain heterogeneous media by superposition of uniform phase eigen-strains with local strains caused by piecewise uniform strss fields which are in equilibrium with prescribed surface tractions. The method is extended to discretized microstructures, and also to analysis of moisture absorption and phase transformation effects on overall response and on local fields in the two composite materials. (Author)

Photothermal Generation of Thermoelastic Waves in Composite Media

Abstract: L'accent est mis sur la comprehension des effets des parametres du materiau et dynamiques, sur le processus de generation thermoelastique. On decrit le couplage thermoelastique des modes de propagation et de non-propagation

Material and load optimization of thermoelastic solids. part i: sensitivity analysis

Abstract: The first variation of a general integral functional, representing either an objective function or a constraint, is evaluated by the adjoint variable method for the sensitivity analysis in a material and load optimization problem for a thermoelastic solid body. Primary and adjoint problems are defined in order to facilitate the derivation in terms of only design function variations.

Large thermoelastic deflections and stability of simply supported polygonal panels

Abstract: A Berger-type approximation for large deflections of thermally loaded initially curved elastic plates is derived. In case of simply supported boundaries and arbitrary polygonal planforms, dimensionless solution is found in terms of Dirichlet's Helmholtz-eigenvalues of the given domain. Stability of this equilibrium solution is exemplarily discussed, showing bifurcation and snapping behaviour in the sense of a cusp catastrophe. Additionally, possibility of mode-jumping is indicated.

Thermoplastic Instability for the Transient Contact Problem of Two Sliding Half-Planes

Abstract: We study the time dependent problem of a nonconducting half-plane sliding on the surface of a conductor with heat generation at the interface due to friction. The conducting half-plane is slightly rounded to give a Hertzian initial pressure distribution. Relationships are established for temperature and thermoelastic displacements due to a heat input of cosine type through the surface, and then these are used to obtain the solution in the form of a double Fourier integral. Numerical results show that, if the ratio of the initial size of the area of contact to that in the steady state is less than some critical value, the area of contact and the pressure distribution change smoothly toward the steady state solution. Otherwise the area of contact goes through bifurcation. The bifurcation accelerates the process. Numerical results are compared with previous approximate solutions.

Diffraction of elastic waves by defects in plates: Calculated arrival strengths for point force and thermoelastic sources of ultrasound

Abstract: This paper applies geometrical ray theory to the calculation of the surface displacements generated by point force and thermoelastic sources of ultrasound in plates containing planar defects. The calculation includes direct wave arrivals, waves undergoing back-wall reflection with or without mode conversion, and waves diffracted by the crack-tip. Ultrasonic B-scan data are also simulated so that comparison can be made with experimental data. It is shown that the thermoelastic source, which can be generated by a pulsed laser, is particularly well suited to defect detection by the ultrasonic time-of-flight technique.

Two-dimensional problem of a moving heated punch in generalized thermoelasticity

Abstract: The two-dimensional equations of generalized thermoelasticity are solved for the case of a heated punch moving across the surface of a semi-infinite thermoelastic half-space subject to appropriate boundary conditions. The exponential Fourier transform with respect to one space variable in a coordinate system moving with the punch is applied. The resulting equations are solved and numerical results are given. The results are compared with those obtained by Roberts for the coupled thermoelastic case.

Thermoelastic transient response of an infinitely long annular cylinder

Abstract: This paper analyzes, the transient response of one-dimensional axisymmetric quasi-static coupled thermoelastic problems. Using the Laplace transform with respect to time, the general solutions of the governing equations are obtained in the transform domain. Also presented are the numerically transient distributions of stress and temperature increments in the real domain for the case of an infinitely long annular cylinder. The inversion to the real domain is obtained by using a Fourier series technique and matrix operations simultaneously, therefore no thermoelastic potentials are introduced in the solution process. It is shown that the coupling effect behaves as a clear lag in both the stress and the temperature distributions.

On a new thermodynamic description of viscoelastic materials

Abstract: A new thermodynamic approach, based on extended irreversible thermodynamics, is proposed to describe viscoelastic materials. The state space is widened by including the stress tensor and the heat flux vector among the set of independent state variables. These extra variables are satisfying evolution equations taking the form of first order time differential equations. The present formalism encompasses the classical models of Maxwell, Kelvin-Voigt, Poynting-Thomson as well as the thermoelastic body and the Newtonian fluid. This is achieved without appealing to an artificial decomposition of the stress and strain tensors into elastic and inelastic parts.

Material and load optimization of thermoelastic solids. part ii: numerical results

Abstract: A material and load optimization problem in thermoelasticity has been investigated for an infinite solid slab The objective function of optimization involves linearly combined physical objectives, and there are behavioral constraints on heat flux and stress distributions Sensitivity analysis results of a companion paper are used to derive analytical expressions for the primary and adjoint variables and also for the sensitivity coefficient of ‘functional’ Nonlinear programming methods of constrained minimization are then used to calculate optimal design parameters

Thermoelastic Coupling to Lamb Waves

Abstract: We consider the thermoelastic generation of Lamb waves produced by incidence of a light beam on a solid plate. In addition to the variables relevant for thermoelastic SAW generation -- frequency and the optical, thermal, and acoustical properties of the materials used -- one has the plate thickness and mode type (symmetric, anti-symmetric). The characteristics of the problem depend on the ratios of plate thickness to thermal wavenumber and acoustic wavelength, and on the symmetry of optical beam incidence. Initial results indicate that the thermoelastic coupling efficiency for plate modes can be appreciably higher than for SAWs. Furthermore, if the plate is a suitable semiconductor structure with an applied electric field, one can increase the local heating over that produced by optical absorption alone, as shown by surface displacement measurements made in the photo-thermal-displacement microscope. These features may be useful in novel sensors that employ Lamb waves.

Thermoelastic influences in journal bearing lubrication

Abstract: A theoretical investigation is made to study the way in which thermal distortion of bearing components modifies the characteristics of journal bearings. The thermoelastic treatment developed is two-dimensional and incorporates an existing thermohydrodynamic analysis. It is applied to circular and partial arc bearings for a range of parametric groups governing the bearing operation. The results show that for a fixed journal position, the effect of thermal distortion is to reduce the minimum film thickness, increase the load capacity, increase the peak temperatures and pressures, and also to enhance considerably the stability of the bearing. The effects are more marked for larger oil-lubricated bearings and higher speeds of operation and it is suggested that discrepancies observed between experimental results and existing theory could be largely explained by this phenomenon.