Generation Of Thermoelastic Waves Research Articles

Thermoelastic wave generation and its longitudinal wave propagation measurement by a microscopic optical interferometer

Laser ultrasonics is a noncontact measurement method that uses a laser-induced elastic wave source in combination with an optical surface displacement-tracking system. This study compared the performances of two optical interferometers with different characteristics when applied to measurement of pulsed thermoelastic waves. The surface displacement-tracking system was designed to measure the center of the microscopic view. A pulsed laser beam irradiated a black ink layer to generate the thermoelastic waves. The out-of-plane displacement on the axially opposite side was then measured using either a Michelson interferometer or a Sagnac interferometer. The objective lens of the system was of a type commonly used in biological observations. The Michelson interferometer estimated a maximum displacement of 0.43 nm and a maximum sound pressure of 24.7 kPa. The signal-to-noise ratios from 16 averages were 14.9 dB (Michelson interferometer) and 19.2 dB (Sagnac interferometer). Furthermore, this paper compares the performance of the numerically estimated Sagnac interferometer outputs calculated from the measured Michelson interferometer outputs with the experimentally obtained Sagnac interferometer outputs. The numerically estimated Sagnac interferometer’s output was shown to be identical to the experimentally acquired output. The Michelson interferometer requires a higher average operating frequency (i.e., it needs a longer data acquisition time), although this interferometer does offer superior displacement output linearity. This property enables calculation of the sound pressure from the displacement amplitude. These findings indicated that combination of the measurement points of the Sagnac interferometer with those of the sparsely distributed Michelson interferometer reduced the measurement time when compared with a single use of the Michelson interferometer while also maintaining the data acquisition quality.

Поліпшення якості оптоакустичної візуалізації: співставлення фізичного та числового експерименту

Optoacoustic imaging is based on the generation of thermoelastic waves by heating an object in an optically inhomogeneous medium with a short laser pulse. The generated ultrasonic waves contain information about the distribution of structures with predominant optical absorption. Detection of acoustic perturbations on the surface of the object and the application of the backprojection algorithm are used to create a picture of the absorbed energy inside the environment. Conventional reconstruction methods lead to artifacts due to the peculiarities of the recovery algorithm. This study proposes an iterative procedure to reduce these artifacts. The algorithm minimizes the error between the measured signals and the signals calculated from the recovered image. The paper compares the results of processing optoacoustic signals implemented in numerical experiments with the results of physical experiments. It is shown that the quality of the recovered images improves even with a small number of iterations.

High-speed angular-scan pulse-echo ultrasonic propagation imager for in situ non-destructive evaluation

This study examines a non-contact laser scanning-based ultrasound system, called an angular scan pulse-echo ultrasonic propagation imager (A-PE-UPI), that uses coincided laser beams for ultrasonic sensing and generation. A laser Doppler vibrometer is used for sensing, while a diode pumped solid state (DPSS) Q-switched laser is used for generation of thermoelastic waves. A high-speed raster scanning of up to 10-kHz is achieved using a galvano-motorized mirror scanner that allows for coincided sensing and for the generation beam to perform two-dimensional scanning without causing any harm to the surface under inspection. This process allows for the visualization of longitudinal wave propagation through-the-thickness. A pulse-echo ultrasonic wave propagation imaging algorithm (PE-UWPI) is used for on-the-fly damage visualization of the structure. The presented system is very effective for high-speed, localized, non-contact, and non-destructive inspection of aerospace structures. The system is tested on an aluminum honeycomb sandwich with disbonds and a carbon fiber-reinforced plastic (CFRP) honeycomb sandwich with a layer overlap. Inspection is performed at a 10-kHz scanning speed that takes 16 seconds to scan a 100 X 100 mm2 area with a scan interval of 0.25 mm. Finally, a comparison is presented between angular-scanning and a linear-scanning-based pulse-echo UPI system. The results show that the proposed system can successfully visualize defects in the inspected specimens.

Remote defect visualization of standard composite coupons using a mobile pulse-echo ultrasonic propagation imager

A new laser-ultrasonic-based defect monitoring system, a mobile pulse-echo ultrasonic propagation imager, has been developed to evaluate damage to composite and metallic structures. The system integrates a Q-switched Nd:YAG laser for the generation of thermoelastic waves, a laser Doppler vibrometry sensor for sensing ultrasound and a two-axis translation stage for raster scanning of the combined generation and sensing laser beams. The system allows scanning of both ultrasound generation and sensing laser beams simultaneously. Thus, a full-field pulse-echo ultrasound, as large as the scan area, can be generated that visualizes the longitudinal wave propagation through the thickness. The full-field pulse-echo ultrasonic wave propagation imaging algorithm is used for real-time structural damage evaluation. Four F-16 Lockheed Martin aircraft reference standard composite specimens with different shapes, sizes, thicknesses, compositions, and aspect ratio of damages were tested: the stepped graphite-to-aluminum, the graphite-to-graphite, and the steel-to-graphite composite panel with flat bottom holes and the graphite-to-aluminum core sandwich structure with core missing. Moreover, the effects of laser pulse energy on damage visualization using the system has been presented and tested on the 4-mm-thick aluminum plate with 2-mm-thick wall-thinning. The proposed system successfully detected and visualized all the artificially introduced defects in the standard coupons.

Mathematical Modeling of the Concentrated Energy Flow Effect on Metallic Materials

Numerous processes take place in materials under the action of concentrated energy flows. The most important ones include heating together with the temperature misdistribution throughout the depth, probable vaporization on the surface layer, melting to a definite depth, and hydrodynamic flotation; generation of thermo-elastic waves; dissolution of heterogeneous matrix particles; and formation of nanolayers. The heat-based model is presented in an enthalpy statement involving changes in the boundary conditions, which makes it possible to consider melting and vaporization on the material surface. As a result, a linear dependence of penetration depth vs. energy density has been derived. The model of thermo-elastic wave generation is based on the system of equations on the uncoupled one-dimensional problem of dynamic thermo-elasticity for a layer with the finite thickness. This problem was solved analytically by the symbolic method. It has been revealed for the first time that the generated stress pulse comprises tension and compression zones, which are caused by increases and decreases in temperature on the boundary. The dissolution of alloying elements is modeled on the example of a titanium-carbon system in the process of electron beam action. The mathematical model is proposed to describe it, and a procedure is suggested to solve the problem of carbon distribution in titanium carbide and liquid titanium-carbide solution in terms of the state diagram and temperature changes caused by phase transitions. Carbon concentration vs. spatial values were calculated for various points of time at diverse initial temperatures of the cell. The dependence of carbon particle dissolution on initial temperature and radius of the particle were derived. A hydrodynamic model based on the evolution of Kelvin-Helmholtz instability in shear viscous flows has been proposed to specify the formation of nanostructures in materials subjected to the action of concentrated energy flows. It has been pointed out for the first time that, for certain parameters of the problem, that there are two micro- and nanoscale peaks in the relation of the decrement to the wavelength of the interface disturbance.

Three dimensional evaluation of aluminum plates with wall-thinning by full-field pulse-echo laser ultrasound

We propose a full-field pulse-echo laser ultrasonic wave propagation imager (FF-PE-UPI) for the evaluation of structural defects. The FF-PE-UPI consists of a Q-switched laser for the generation of thermoelastic waves, a laser Doppler vibrometer (LDV) for sensing, and a two-axis translation stage for raster scanning of the combined generation and sensing laser beams. A-scan, B-scan, and C-scan data representations are used for the evaluation of structural defects. Three specimens were tested: a 4-mm aluminum plate with an area of 50% thickness reduction, a 6-mm aluminum plate with an area of 25% thickness reduction, and an 8-mm aluminum plate with engraved letters. The damages on the tested specimens were successfully visualized.

The Possible Explanation for Secondary Microseisms Seasonal and Annual Variations

Annual drift is typical for microseisms. We propose a model based on thermoelastic wave generation that explains the highest microseisms during winter using higher stress level at the same time. If we remove the average influence of the background stress from the microseisms, we obtain the residual microseisms, which show the semiannual periods with maxima in March and October. The histogram of anomalous microseisms has the same form as the variations in Length of the Day (LOD). This phenomenon is recognized as a secondary order mechanism after the annual drift. The synoptic situations and earthquakes were recognized as imminent triggers of anomalous microseisms. This synoptic situation is consistent with the uplift of the northern part of Europe after ice cap melting.

ONE-DIMENSIONAL MODEL OF OPTICALLY INDUCED THERMOELASTIC WAVES

A one-dimensional model of the generation of thermoelastic waves by light beam absorption in solids is described and solved. The exact thermal and mechanical boundary conditions are used. An analytical asymptotic solution of the problem is obtained. The relation between the maximum displacement of the second surface of the sample and the duration of the incident light pulse is determined.

Photothermal Generation of Thermoelastic Waves in Composite Media

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

Thermoelastic wave generation by resistive heating in silicon

We report the piezoelectric detection and thermoelastic signals resulting from resistive heating within a diffused resistor and commercial packaged transistor. cw and pulse experiments at 5 V and 300 kHz, as well as 5-V 10-MHz mixing experiments, result in transducer outputs in the 25 μV–1 mV range. Theoretical values are in good agreement with experimental findings.