Thermoelastic Fluctuations in Solids

Adiabatic thermoelastic heating can be used to monitor stress fluctuations in solids. Previous studies of the effects on the temperature fluctuations both of applied static stress and of the finite amplitude of the stress fluctuations have used approximate theory. The present rigorous thermodynamic treatment distinguishes between adiabatic second-order derivatives needed for finite amplitude and mixed derivatives needed for static applied stress. A detailed analysis is given for purely compressive stress, followed by computations for KCl, NaCl, Al, Cu, Ti, and the alloy Ti-6Al-4V. Additional terms revealed by the new analysis prove to be substantial, including the difference between the adiabatic and mixed derivatives. Revised forms are then proposed for earlier approximations. For unidirectional stress, expressions are taken from an analysis given elsewhere; and computations made for Al, Cu, Ti and Ti-6Al-4V. Corrections to earlier approximations are relatively smaller than for compressive stress, and of opposite sign because the shear component of the unidirectional stress dominates the second order effects.     

The Influence of Gravitational Field on Generalized Thermoelasticity with Two-Temperature under Three-Phase-Lag Model

The problem of the generalized thermoelastic medium for three different theories under the effect of a gravitational field is investigated. The Lord- Shulman, Green-Naghdi III, three-phase-lag theories are discussed with twotemperature. The normal mode analysis is used to obtain the analytical expressions of the displacement components, force stress, thermodynamic temperature and conductive temperature. The numerical results are given and presented graphically, when the thermal force is applied. Comparisons are made with the results predicted by three-phase-lag model, Green-Naghdi III and Lord-Shulman theories in the presence and absence of gravity as well as two temperature.     

The exothermal effect upon interaction of calcium and silicon oxides

Thermodynamic analysis is performed (TERRA software) of the lime-silica interaction for all possible lime-to-silica ratios at normal pressure. The components of the exothermal reactions are calculated. The adiabatic temperature of silica combustion in the lime is found. The heat of the chemical reaction is determined. The enthalpies of the initial rough material and the combustion materials are calculated. The evolution of the silica combusting in the lime, including formation of the stochiometric glass, wollastonite, rankinite, and pseudo-cement, is considered. The thermodynamic analysis is performed under the supposition of use of chemically pure substances with the absence of external heat and specific additions.     

Dynamic Piezoresistivity Calibration for Eddy Current Nondestructive Residual Stress Measurements

It has been recently demonstrated [M. P. Blodgett and P. B. Nagy, J. Nondestruct. Eval. 23, 107 (2004)] that eddy current conductivity measurements can be exploited for near-surface residual stress assessment in surface-treated nickel-base superalloy components. To quantitatively assess the prevailing residual stress from eddy current conductivity measurements, the piezoresistivity coefficients of the material must be first determined using known external applied stresses. These calibration measurements are usually conducted on a reference specimen of the same material using cyclic uniaxial loads between 0.1 and 10Hz, which is fast enough to produce adiabatic conditions. Therefore, the question arises whether dynamic calibration measurements can be used or not for accurately assessing the sensitivity of the eddy current method for static residual stress. It is demonstrated in this paper that such dynamic calibration measurements should be corrected for the thermoelastic effect, which is always positive, i.e., it increases the conductivity in tension, when the material cools down, and reduces it in compression, when the material heats up. For low-conductivity titanium and nickel-base engine alloys the thermoelastic corrections are relatively modest at ≈5–10%, but for high-conductivity aluminum alloys the difference between the adiabatic and isothermal properties could be as high as 50%.     

Energy and entropy of crystals,melts and glasses or what is wrong in Kauzmann's paradox?

In order to understand the thermodynamic properties of solids and melts one has to consider simultaneously their entropy and energy as a function of temperature. Therefore, the molar entropy, S, and enthalpy (energy), H, of crystals, glasses and melts of the same one‐component systems have been suitably depicted including the transformation from the melt into a solid, i. e. a glass or crystal. S and H of glasses correspond to a simple continuation of these functions from the molten state to lower temperatures. Since crystallisation occurs spontaneously such a process necessarily produces entropy causing the temperature to increase. Thus, the glassy and the crystalline state are not connected by an isothermal process, which is in contrast to the assumption in the classical nucleation and crystallisation theory as well as in the arguments causing Kauzmann's paradox. For the temperature T → 0K the enthalpy and entropy of the glass are larger by ΔH₀ and ΔS₀ as compared to the stable crystal. The calculations are illustrated using experimental data for quartz and silica glass from P. Richet, Y. Bottinga, L. Deniélou, J. P. Petitet and C. Téqui.     

Thermomechanical aspects of dynamic crack initiation

Dynamic fracture is generally addressed as an isothermal phenomenon. When specific phenomena such as shear band formation and propagation are involved, the analyses include thermomechanical conversion of strain and/or fracture energy into heat (thermoplasticity). In this case, it has been shown that very significant temperature rises can develop which cause softening of the crack-tip material. In these works, thermoelastic temperature changes at the tip of the crack are implicitly neglected. In a recent work, we have questioned this issue and shown that for a stationary crack subjected to transient loading, adiabatic thermoelastic effects were noticeable, thus causing a large temperature drop in the elastic zone surrounding the crack-tip (Rittel, 1998). In the present work, we pursue this line of investigation by presenting additional experimental results about temperature changes ahead of a dynamically loaded crack in commercial polymethylmethacrylate. We investigate mode I and mode II loading configurations. We observe, as expected, that the temperature drops for mode I loading while it rises for the mode II case. In each case, the crack initiates during the phase where the temperature changes (drop or rise). While showing that thermoelastic aspects of fracture should certainly be taken into account, the present results indicate that thermomechanical aspects in general should not be overlooked when addressing dynamic crack initiation.     

Fabrication of transparent silica glass by powder sintering

Transparent silica glasses were obtained by sintering a green compact (fabricated by slip-casting methods for high-purity silica glass powder) in diverse atmospheres. The relationships between sintering atmosphere and sintering temperature that result in transparent, sintered silica glass were shown. The results indicate that there are four forming phases for each sintering atmosphere and temperature: (1) nontransparent glass resulting from an overabundance of pores (2) crystal, such as cristobalite or β-quartz, (3) moganite, and (4) transparent glass. Optimum sintering temperature for fabricating transparent silica glass was above 1673 K in a high-vacuum (10^–4 Pa: p(O₂) = 10^–14) atmosphere. We investigate the fabrication of transparent and hydroxyl-free silica glass by a powder-sintered method. After studying the effect of sintering schedule on residual [OH^–] concentration for transparent, sintered silica glasses, we sintered a green compact prepared by silica powders with a mean particle size of 1.6 µm, first heating it to 1523 K for dehydration and then to 1873 K for densification. This typical fabricated condition resulted in a transparent, sintered silica glass with <1 ppm [OH^–] concentration.     

轴向拉力对微梁质量传感器精度和热弹性阻尼的影响

针对存在轴向拉力的矩形截面微梁谐振式质量传感器中的质量传感灵敏度、热弹性阻尼以及最小检测质量等问题进行了深入的研究。推导了质量传感器在存在轴向拉力情况下的检测灵敏度、热弹性阻尼以及最小检测质量的表达式。揭示了轴向拉力对质量传感器的工作性能的影响机理。结果表明:轴向拉力会提高质量传感灵敏度;轴向拉力会降低谐振器的热弹性阻尼;轴向拉力可以使得质量传感器捕获更微小的检测质量。     

Calorimetric techniques for the evaluation of thermal efficiencies of shape memory alloys

The latent heat and entropy changes of NiTi shape memory effect (SME) alloys have been evaluated by three different calorimetric techniques; adiabatic calorimetry, differential scanning calorimetry and a Clapeyron analysis of isothermal stress-strain data. It is found that these techniques provide consistent estimates for the enthalpy and entropy to within 20% for NiTi and noble metal SME alloys. From published thermodynamic data for SME alloys, thermal efficiencies were calculated based on an ideal SME heat engine cycle. It was found that NiTi provides the maximum thermal efficiency with the highest temperature transformation range.     

The concern of elasticity in stress-induced martensitic transformation in NiTi

It has been reported in the literature that moduli of elasticity of the austenite and martensite of near-equiatomic NiTi differ often by a factor of 3. It is expected that a phase transformation between the two phases may be induced by the application of a stress, according to thermodynamic principles. This is due to the fact that the difference in elastic energy caused by the change in modulus of elasticity serves as a driving force for the transformation, similar to the effect of the lattice distortion of the martensite on the transformation. A thermodynamic equation expressing this effect is derived. It is expected, based on the understanding of this equation, that the relationship between the critical stress and the temperature for a thermoelastic martensitic transformation is non-linear if the transformation involves a large change in modulus of elasticity. Therefore, this equation may be used for either of two purposes: to clarify the reliability of the experimentally determined moduli of elasticity of the two phases or to verify the Clausius–Clapeyron relation between the critical stress and the temperature for the transformation.     

Sol-gel glasses: some recent trends

The basic experimental steps in the preparation of sol-gel glass as developed in recent times, and their relevance have been discussed taking high purity silica glass as an example. Current developments in sol-gel derived ultra-low expansion glasses, rare earth doped laser glasses, semiconductor-doped non-linear glasses, gradient index lenses, microoptics and organic molecule-doped (mainly dyes) glasses for sensor and other applications have been discussed in brief.     

Rayleigh waves in a thermoelastic solid half space using dual-phase-lag model

The present paper deals with the study of Rayleigh waves in a thermoelastic homogeneous isotropic solid half space in the context of dual-phase-lag model. The medium is subjected to stress free, thermally insulated, boundary conditions. The equation for the phase velocity of Rayleigh waves and the analytical expressions for the amplitudes of the displacements, temperature and thermal stresses have been derived. The expressions are obtained for a wave traveling along the free surface. The results discussed numerically and illustrated graphically to show effect of the coupling parameter and phase-lags.     

Imaging of surface strain and polishing artefacts in fused silica by environmental scanning electron microscopy

The recently developed application of charge contrast imaging (CCI), available in variable pressure or environmental scanning electron microscopes (ESEM), has been found to provide images of near surface strain around micro-indentations in fused silica glass. Results suggest this strain contrast information is derived from within a few nanometres of the material surface, making CCI an invaluable tool for the study of nanometre scale surface deformation. Images of indentation strain have also been imaged using backscattered electron (BSE) imaging. The CCI technique has also been applied to the study of fused silica surfaces polished by chemically active polishing abrasives. In the samples studied, CCI provides unique images of linear defects residing below layers of chemically adhered polishing compound. Visualisation of surface strain on sub-nm rms glasses provides supporting evidence for plastic (permanent) deformation of the glass surface at the nanometre level during polishing with certain abrasives and for chemical interactions between the polishing abrasive and glass surface.     

Fabrication of transparent silica glass by powder sintering

Transparent silica glasses were obtained by sintering a green compact (fabricated by slip-casting methods for high-purity silica glass powder) in diverse atmospheres. The relationships between sintering atmosphere and sintering temperature that result in transparent, sintered silica glass were shown. The results indicate that there are four forming phases for each sintering atmosphere and temperature: (1) nontransparent glass resulting from an overabundance of pores (2) crystal, such as cristobalite or β-quartz, (3) moganite, and (4) transparent glass. Optimum sintering temperature for fabricating transparent silica glass was above 1673 K in a high-vacuum (10⁻⁴ Pa: p(O₂)=10⁻¹⁴) atmosphere. We investigate the fabrication of transparent and hydroxyl-free silica glass by a powder-sintered method. After studying the effect of sintering schedule on residual [OH⁻] concentration for transparent, sintered silica glasses, we sintered a green compact prepared by silica powders with a mean particle size of 1.6 μm, first heating it to 1523 K for dehydration and then to 1873 K for densification. This typical fabricated condition resulted in a transparent, sintered silica glass with <1 ppm [OH⁻] concentration.     

A two-temperature model for evaluation of thermoelastic damping in the vibration of a nanoscale resonators

In this work, the thermoelastic damping of a nano-scale resonator is analyzed by the generalized thermoelasticity theory based on two-temperature model (2TLS). The effect of two-temperature parameter and relaxation time in nano-scale resonator are investigated for beams under clamped conditions. Analytical expressions for deflection, temperature change, frequency shifts, and thermoelastic damping in the beam have been derived. The theories of coupled termoelasticity and generalized thermoelasticity with one relaxation time can extracted as limited and special cases of the present model. The numerical results have been presented graphically in respect of thermoelastic damping and frequency shift.     

The extended finite element method in thermoelastic fracture mechanics

The extended finite element method (XFEM) is applied to the simulation of thermally stressed, cracked solids. Both thermal and mechanical fields are enriched in the XFEM way in order to represent discontinuous temperature, heat flux, displacement, and traction across the crack surface, as well as singular heat flux and stress at the crack front. Consequently, the cracked thermomechanical problem may be solved on a mesh that is independent of the crack. Either adiabatic or isothermal condition is considered on the crack surface. In the second case, the temperature field is enriched such that it is continuous across the crack but with a discontinuous derivative and the temperature is enforced to the prescribed value by a penalty method. The stress intensity factors are extracted from the XFEM solution by an interaction integral in domain form with no crack face integration. The method is illustrated on several numerical examples (including a curvilinear crack, a propagating crack, and a three‐dimensional crack) and is compared with existing solutions. Copyright © 2007 John Wiley & Sons, Ltd.     

CuI微晶掺杂硅凝胶玻璃及其薄膜的制备与光谱性质

抄采用溶胶-凝胶工艺制成Cul微晶掺杂硅凝胶玻璃及其薄膜。通过X射线粉末衍射（XRD）和高分辨透射电镜（HRTEM）分析观察到玻璃中的晶相及其分布；由薄膜的室温透射光谱发现，随着热处理温度提高和时间延长，薄膜的特征透射谱谷向长彼方向移动（红移），并源于玻璃中的量子尺寸效应。     

Thermomechanical characterization of stress localization in glass: An experimental and numerical study

Thermoelastic stress analysis and quantitative calorimetry are full‐field noncontact techniques widely used to study the thermomechanical behaviour of materials. The first one linearly relates the sum of the principal stresses to the temperature variation, and the second one can be used to measure the mechanical dissipation. However, brittle materials such as glass are a priori bad candidates for these techniques. Indeed, their low‐temperature variations under loading lead to very noisy infrared images, and their brittle mechanical behaviour does not allow to deform them significantly. In the present paper, the thermomechanical characterization of a holed glass sample under cyclic loading is performed. A preliminary new filtering methodology has been applied to the thermal movie to remove the noise. The stress field obtained from the thermoelastic stress analysis is well correlated to the finite element model showing that this technique is adapted to study the thermoelastic response of brittle materials. Finally, the corresponding calorimetric response has been determined by using a simplified formulation of the heat diffusion equation. This permits to quantify heat sources and to carry out energy balances.     

Thermoelastic vibration analysis of Mems/Nems plate resonators with voids

In this paper, the effect of voids, relaxation times, thermomechanical coupling, surface conditions, and plate dimensions on energy dissipation induced by thermoelastic damping in microelectronics mechanical systems (MEMS)/ nanoelectronics mechanical systems (NEMS) resonators are investigated. Closed form expressions for the transverse vibrations of a homogenous isotropic, thermoelastic thin plate with voids, based on Kirchhoff theory have been derived. The exact solutions for the free vibrations of plates under clamped-simply supported (CS) and simply supported-simply supported (SS) conditions are obtained. Analytical expressions for deflection, temperature change, frequency shifts, and thermoelastic damping in the plate have been derived. Some numerical results with the help of MATLAB programming software in case of silicon nitride and magnesium like material have also been presented.     

Model of Fractional Heat Conduction in a Thermoelastic Thin Slim Strip under Thermal Shock and Temperature-Dependent Thermal Conductivity

The present paper paper, we estimate the theory of thermoelasticity a thin slim strip under the variable thermal conductivity in the fractional-order form is solved. Thermal stress theory considering the equation of heat conduction based on the time-fractional derivative of Caputo of order α is applied to obtain a solution. We assumed that the strip surface is to be free from traction and impacted by a thermal shock. The transform of Laplace (LT) and numerical inversion techniques of Laplace were considered for solving the governing basic equations. The inverse of the LT was applied in a numerical manner considering the Fourier expansion technique. The numerical results for the physical variables were calculated numerically and displayed via graphs. The parameter of fractional order effect and variation of thermal conductivity on the displacement, stress, and temperature were investigated and compared with the results of previous studies. The results indicated the strong effect of the external parameters, especially the time-fractional derivative parameter on a thermoelastic thin slim strip phenomenon.