Bubble-based acoustic radiation force elasticity imaging

Acoustic radiation force is applied to bubbles generated by laser-induced optical breakdown (LIOB) to study viscoelastic properties of the surrounding medium. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within a viscoelastic medium. Tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts, and crosscorrelation processing is performed offline to monitor bubble position. Maximum bubble displacements are inversely related to the Young's moduli for different gel phantoms, with a maximum bubble displacement of over 200 microm in a gel phantom with a Young's modulus of 1.7 kPa. Bubble displacements scale with the applied acoustic radiation force and displacements can be normalized to correct for differences in bubble size. Exponential time constants for bubble displacement curves are independent of bubble radius and follow a decreasing trend with the Young's modulus of the surrounding medium. These results demonstrate the potential for bubble-based acoustic radiation force methods to measure tissue viscoelastic properties.     

The reference temperature dependence of Young’s modulus of two-temperature thermoelastic damping of gold nano-beam

This work is concerning with the study of the thermoelastic damping of a nanobeam resonator in the context of the two-temperature generalized thermoelasticity theory. An explicit formula of thermoelastic damping has been derived when Young’s modulus is a function of the reference temperature. Influences of the beam height and Young’s modulus have been studied with some comparisons between the Biot model and the Lord–Shulman model (L–S) for one- and two-temperature types. Numerical results show that the values of the thermal relaxation parameter and the two-temperature parameter have a strong influence on thermoelastic damping at nanoscales.     

Thermoelastic Characterization of Changing Phase Distribution in Hardened Steel by Laser Ultrasonics

The increase of hardness of steel during a heat treatment intended to give components more performance is a result of a drastic change in grain size and microstructure, which in turn can be analyzed via changes in acoustic wave scattering. The degree of scattering is related to the grain size, alloy phases, elastic anisotropy, and phonon spectra, which are connected with the structural heterogeneity. In this study, an axially oriented hardening profile in a steel rod was induced by a Jominy test. All-optical photoacoustic excitation and detection schemes were used to establish the relation between the hardness, the elastic modulus, the elastic scattering, and the thermal diffusivity on a series of eight samples cut out from the gradient part of the rod. For each sample, the scattering of the photoacoustically excited traveling surface acoustic waves detected in a heterodyne diffraction and beam deflection setup was extracted from their damping behavior at different wavelengths and frequencies. Also, the thermal diffusivity was determined by fitting the slow time evolution of the laser-induced photo-thermoelastic displacement signal, and was found to be decreasing with decreasing grain size and increasing hardness.     

Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.     

In situ measurement on ultraviolet dielectric components by a pulsed top-hat beam thermal lens

A simple and sensitive mode-mismatched thermal lens (TL) technique with a pulsed top-hat beam excitation and a near-field detection scheme is developed to measure in situ the thermoelastic and the thermooptical responses of ultraviolet (UV) dielectric coatings as well as bulk materials under excimer laser (193- or 248-nm) irradiations. Owing to its high sensitivity, the TL technique can be used for measurements at fluences far below the laser-induced damage threshold (LIDT). We report on the measurement of both linear and nonlinear absorption of the UV dielectric coatings and bulk materials as well as the investigation of time-resolved predamage phenomena, such as laser conditioning of highly reflective dielectric coatings and irradiation-induced changes of a coating's various properties. The pulsed TL technique is also a convenient technique for accurate measurement of the LIDT of dielectric coatings and for distinguishing different damage mechanisms: thermal-stress-induced damage or melting-induced damage.     

Hydrodynamic aspects of the channel formation by a CO<Subscript>2</Subscript> laser beam penetrating deep into a liquid

It is shown that, taking into account the hydrodynamic phenomena accompanying the development and maintenance of a deep laser-induced vapor-gas channel in a liquid, it is possible to estimate the order of magnitude of the channel depth and growth rate, as well as the characteristic frequencies of the channel wall instabilities and acoustic perturbations. The estimates agree with the experimental data.     

Attenuation in sand: an exploratory study on the small-strain behavior and the influence of moisture condensation

The loss mechanisms responsible for the observed attenuation in soils are often unclear and controversial. This is particularly the case with the small-strain damping D _min in air-dry sands. Ultimately, physical explanations must accommodate the observed effects of confinement, strain level, frequency, and load repetition. Three hypotheses are explored herein: measurement bias, thermoelastic relaxation, and adsorbed layers. Micro and macro-scale experimentation using photoelasticity, thermal infrared imaging, atomic force microscopy and resonant column testing are complemented with conceptual analyses. Results show that Mindlin-contact friction cannot explain the observed response of the small-strain damping ratio D _min and thermoelastic loss is suggested. While thermoelastic relaxation is inherently frequency dependent, the superposition of multiple internal scales in soils can justify the observed low dependency on frequency. Moisture condensation leads to adsorbed water layers on grain surfaces, which has a small but observable effect on shear modulus and a significant influence on damping ratio. Participating loss mechanisms at small-strains may involve distortion and motion of adsorbed layers and hydration force hysteresis. Hysteretic capillary breakage at contacting asperities gains relevance when the strain exceeds the elastic threshold strain; this strain coincides with the strain range when frictional losses begin to dominate. Finally, the damping ratio in air-dry sands is very small, and causality-based attenuation–dispersion relations predict modulus dispersion about 1% per log cycle, therefore the medium can be considered non-dispersive for practical purposes.     

复合材料的椭圆夹杂模型线性温变问题的界面热应力分析

利用处理平面多连通域热弹性问题的一种有效方法,获得了椭圆夹杂模型线性温变问题的热弹性场解答,并讨论了夹杂和基体材料的热膨胀系数、热传导系数以及剪切模量对界面热应力的影响规律,所获得的结论为增强复合材料的设计与应用提供了有价值的参考依据.     

Thermoelastic expansion vs. piezoelectricity for high-frequency, 2-D arrays

Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated increased efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. In this paper we show that the radiated power from a black PDMS film is comparable to a 20 MHz piezoelectric two-dimensional (2-D) array element. Furthermore, we predict that a thermoelastic array element can produce similar acoustic power levels compared to ideal piezoelectric 2-D array elements at frequencies in the 100 MHz regime. We believe these results show that thermoelastic generation of ultrasound is a promising alternative to piezoelectricity for high-frequency, 2-D arrays.     

Size-dependent generalized thermoelasticity model for Timoshenko microbeams

A size-dependent, explicit formulation for coupled thermoelasticity addressing a Timoshenko microbeam is derived in this study. This novel model combines modified couple stresses and non-Fourier heat conduction to capture size effects in the microscale. To this purpose, a length-scale parameter as square root of the ratio of curvature modulus to shear modulus and a thermal relaxation time as the phase lag of heat flux vector are considered for predicting the thermomechanical behavior in a microscale device accurately. Governing equations and boundary conditions of motion are obtained simultaneously through variational formulation based on Hamilton’s principle. As for case study, the model is utilized for simply supported microbeams subjected to a constant impulsive force per unit length. A comparison of the results with those obtained by the classical elasticity and Fourier heat conduction theories is carried out. Findings indicate that simultaneous considering the length-scale parameter and thermal relaxation time has strong influence on the thermoelastic behavior of microbeams. In dynamic thermoelastic analysis of the microbeam, while the non-Fourier heat conduction model is employed, the modified couple stress theory predicts larger deflection compared with the classical theory.     

Thermomechanical behaviour of metals in cyclic loading

Temperature variation induced by repeated mechanical cyclic loading on AISI 1045 mild steel was studied.The experimental results of cyclic loading at low stress levels elucidate the coupling phenomena of thermal/mechanical behaviour which causes cooling and/or heating corresponding to the stressed state. The governing factors are thermoelastic effect and viscous dissipation. The thermoelastic effect causes the specimen temperature to go down and/or up which corresponds to the loading and/or unloading in cycling, where the viscous dissipation effect causes heat to generate inside the sample which steadily heats the specimen. As a result, a trend of increasing specimen mean temperature with periodical local fluctuation on temperature history can be observed. The heating rate, due to viscous dissipation, is increased with increasing strain rate. Cyclic loading at high stress levels results in large amounts of heat generation where thermoplasticity predominates. An abrupt temperature rise in the first few cycles, followed by a slow-down in later cycling, is to be seen. The phenomena and results were discussed. In addition, the effect of heat transfer between the specimen and its surroundings should be considered for both cases if the time is sufficiently long or the temperature gradient evolved is of significance.     

Singular surfaces in linear thermoelasticity

In this paper we give a detailed account, within the framework of the linear theory of thermoelasticity, of the propagation of surfaces of discontinuity in a homogeneous, isotropic elastic solid which is able to conduct heat. The methods used in the investigation are, in large measure, due to T. Y. Thomas. The early sections of the paper contain a derivation of the principal results of Thomas's theory which enables us to determine, from a consideration of the appropriate Cauchy initial-value problem, the characteristic surfaces of the linear thermoelastic equations. The wavefronts associated with these characteristics are found to propagate with one of the constant speeds

, ₀, E_T, v_T being respectively the density, the isothermal Young's modulus and the isothermal Poisson's ratio of the material in its reference state.

A discontinuity surface of order r in the displacement and temperature fields is referred to as a weak thermoelastic wave if r2 and a strong thermoelastic wave if r=0 or 1. Concerning the properties of these waves our main conclusions are as follows. Weak thermoelastic waves and strong waves of order 1 are characteristic and may be described as dilatational or rotational according as their speed of propagation is v_T or v_S. Dilatational strong waves of order 1 are shock waves and rotational waves of this type are propagating vortex sheets. For all thermoelastic waves of order 1 the strength (defined in a natural way) is completely determined by its distribution on an initial configuration of the wavefront. Irrespective of the shape of this initial configuration, the strength of a dilatational wave decays rapidly as the wave propagates on account of thermoelastic dissipation. For rotational waves, however, the variation of strength during propagation depends solely upon the geometrical form of the initial wavefront. A strong thermoelastic wave of order 0 is an absolute singular surface in the temperature field, discontinuities of displacement being excluded from consideration. A wave of this type may be characteristic, in which case its speed of propagation is v_S; or it may be non-characteristic, in which case it is a dilatational shock wave. In neither case is the strength of the wave completely determined by its distribution on an initial wavefront, a situation which leads us to argue that thermoelastie waves of order 0 cannot in practice be created.

In the final section of the paper the properties of singular surfaces in classical elastokinetics are discussed in the light of the foregoing analysis of discontinuous thermoelastic waves.     

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors

The sensitivity of acoustic-wave microsensors coated with a viscoelastic film to mass changes and film modulus (changes) is examined. The study analyzes the acoustic load at the interface between the acoustic device and the coating. The acoustic load carries information about surface mass and film modulus; its determination has no restrictions in film thickness. Two regimes of film behavior can be distinguished: the gravimetric regime, where the sensor response is mainly mass sensitive, and the nongravimetric regime, where viscoelasticity gains influence on the sensor response. We develop a method, which allows the assignment of the sensor signal to a gravimetric or a nongravimetric response. The critical value can be determined from oscillator measurements. The related limits for the coating thickness are not the same for the coating procedure and mass accumulation during chemical sensing. As an example, we present results from a 10 MHz quartz crystal resonator.     

Evaluation of damage in composites by using thermoelastic stress analysis: A promising technique to assess the stiffness degradation

The stiffness degradation represents one of the most interesting damage phenomena used for describing the fatigue behaviour of composites. A critical aspect of modelling the damage is represented by the simulation of the whole behaviour of the composite and by the assessment of the actual stiffness for the models validation. In this work, the stiffness degradation of quasi‐isotropic carbon fibre reinforced polymer (CFRP) obtained by automated fibre placement has been assessed by means of thermoelastic stress analysis. The amplitude of temperature signal at the mechanical frequency (thermoelastic signal) was considered as an indicator of material degradation and compared with the data provided by an extensometer. The correlation between thermoelastic and mechanical data allowed to build a new experimental model for evaluating and predicting material stiffness degradation by just using thermoelastic data. The proposed approach seems to be very promising for stiffness degradation assessment of real and complex mechanical components subjected to actual loading conditions.     

Nanoindentation technique for measuring residual stress field around a laser-induced crack in fused silica

A nanometre scale indentation technique using microprobe indentations to measure residual stresses at selected positions near u.v.-laser-induced cracks in fused silica is presented. The approach is based on the observation that the nanoindentations' penetration depths are affected by the residual stress field emanating from the laser-induced crack. A simple theoretical model based on the change of the nanoindentation penetration depth as well as the change in Young's modulus and hardness of the material is derived. The results show good agreement with the inclusion model [15] suggesting that the residual stress field around a laser-induced crack in fused silica is of shear nature. An exploratory test made on an unstressed sample (free of a laser-induced crack), yielding values for Young's modulus and hardness in accordance with handbook values, shows the high accuracy of this nanoindentation diagnostic.     

Fracture behaviors for a functionally graded superconducting cylinder with a cylindrical crack

In this study, a double exponential model is proposed to investigate the cylindrical crack problem for a functionally graded superconducting cylinder. The stress intensity factors (SIFs) are analytically obtained by transforming the corresponding crack problem into dual integral equations. The effects of applied magnetic field, model parameters, and crack configuration on the SIFs are analyzed. Some important phenomena are observed. Among others, both decreasing the graded index of Young's modulus and increasing the introduced nondimensional exponent parameter in the critical current model can inhibit crack propagation. This study should be useful for the application of superconducting devices.     

Experimental investigation of acoustic properties of titanium alloys in the temperature range of 20–1000°C

Titanium and titanium-based alloys, which are widely used in various sectors of the national economy, require deep and versatile investigations of their physico-mechanical properties in a wide temperature range. Numerous abnormal physical phenomena are observed in titanium-based alloys at high temperatures, especially in the region of polymorphic transformation. In particular, in addition to significant structural variations, which influence the strength properties of the final products, near such transformations the titanium alloys (especially with a fine structure) have a tendency to superplastic deformation, which is widely applied in modern technology. Among the physical characteristics, which provide extensive information about the structural and physico-mechanical properties of titanium alloys, are the temperature expansion and acoustic properties (in particular, the speed of ultrasound, information about the temperature dependence of which is unavailable for the majority of engineering materials), which allow the Young modulus for these materials to be calculated.     

Status of the Electrostatic Levitation Furnace (ELF) in the ISS-KIBO

The electrostatic levitation method is a containerless processing technique that utilizes Coulomb force between a charged sample and the surrounding electrodes. The Japan Aerospace Exploration Agency (JAXA) has been developing this technique for more than 20 years. In 2016, JAXA completed the flight model assembly, and the Electrostatic Levitation Furnace (ELF) for the International Space Station (ISS) was launched to the ISS. The ELF is mainly intended to handle oxide melts that are difficult to levitate on the ground based electrostatic levitator due to gravity and due to insufficient charging. ISS-ELF can measure the thermophysical properties (density, surface tension and viscosity) of high temperature melts above 2000 ^°C. The thermophysical properties data of materials at high temperature is useful for the study of liquid states and improvement of numerical simulation by modeling the manufacturing processes using the liquid state. Moreover, the interfacial energy of immiscible melts will be measured by creating a core-shell droplet configuration which otherwise cannot be obtained on the ground due to sedimentation. This paper briefly describes the ELF facility and presents the results of a functional checkout that includes the density measurement of molten alumina.     

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。     

Ultrasonic metamaterials with negative modulus

The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.