Thermoelastic Regime Research Articles

Modelling and diagnostic of pulsed laser cleaning of oxidized metallic surfaces

Laser cleaning appears to be a suitable replacement for solvent based cleaning techniques in a wide range of technological domains. The short pulsed laser irradiation (few nanoseconds or shorter) in the range of 1–100 MW cm−2 has been tentatively applied in a large range of wavelengths (from UV to IR) for cleaning of various kinds of contaminated surfaces, e.g. metallic surface, ceramic surface, stone, etc. This paper will be focused on the modelling and development of diagnostics of pulsed laser-solid interactions to be applied to laser cleaning of oxidized metallic surfaces. Removal of thin oxide films (thickness below 1 μm) has been studied following a methodological approach which is based on three steps: (1) definition of a cleaning strategy, (2) analysis of the physical and chemical changes of the cleaned surfaces, and (3) development of new techniques for the cleaning diagnostics. Concerning the thin oxide films, the physical and chemical mechanisms of laser-induced removal have been modeled taking into account the role of the environment (gaseous or liquid). Momentum transfer induced by the laser irradiation of the short laser pulse was studied by acoustical methods demonstrating the thermoelastic regime for the removal of the film. Mechanical sensors, such as piezoelectric sensor or interferometric probe, have been used to quantify this mechanical regime and it was shown that they can be used to follow the laser cleaning efficiency. Examples of technological transfer will be mentioned.

Experimental evaluation of enhanced generation of ultrasonic waves using an array of laser sources

An array of ten pulsed Nd: YAG lasers was constructed in order to study the effects of generating ultrasound with an array of laser sources. The laser system permitted the spatial and temporal control of the firing of the individual lasers in the array necessary for the production of both narrow-band ultrasonic signals and phased array single pulses. The increase in sensitivity of a laser ultrasonic system associated with the generation of narrow-band and phased array acoustic waves is discussed theoretically and verified experimentally for surface and bulk wave generation. The ultrasonic signals were generated in aluminium samples of various thicknesses and with source laser power densities consistent with generation in the thermoelastic regime, thus causing no damage to the surface of the specimens. The signals were detected using a path stabilized Michelson interferometer. In the narrow-band case, the waveforms were digitally filtered in order to take advantage of the reduced spectral range of the generated acoustic energy. A significant increase in the sensitivity of the laser ultrasonic system, consistent with theoretical predictions, was observed in both the narrow-band and phased array cases.

Experimental studies of directivity patterns of laser generated ultrasound in neutral glasses

An experimental study on the directivity patterns of laser-generated longitudinal and shear ultrasound in semi-transparent neutral glasses is presented. The measured directivity patterns in the thermoelastic regime in the AB-series neutral glasses, which have various optical absorption coefficients, are in good agreement with our theory, which is briefly summarized in the Appendix.

Experimental study of directivity patterns of laser ultrasound in an unpolarized PZT ceramic in the thermoelastic regime

The experimental longitudinal directivity patterns of laser-generated ultrasound for both line and disc laser sources, in an unpolarized PZT-4 ceramic in the thermoelastic regime, are presented. The exciting source is a Q-switched Nd:YAG laser operating at the wavelength of 1.06 μm and having a pulse width of 8 ns and maximum pulse energy of 200 mJ. The acoustic pulses are detected by a 5 MHz piezoelectric ultrasonic transducer. The results show that there are noticeable differences between the directivity patterns in the non-metal PZT and those in the metal aluminium. Discussions of the observations are offered.

Green Functions for Cylindrical Elastic Waves in Aluminum: Comparison Between Theory and Experiments

A comparison between experimental and theoretical displacement fields for the tangential force problem (which is a model of the thermoelastic regime for laser generated ultrasound) at an aluminum interface is reported. The numerical predictions are based on a standard formulation of the transient generation of elastic waves along a line source. In the configuration of a purely elastic thin plate, the cylindrical elastic waves are then detected on the other side by a point-like receiver. The experimental works is carried out with a pulsed Nd:YAG laser for the generation and a laser interferometric probe for the detection of ultrasonic waves.

Transient elastic wave generation by an array of thermoelastic sources

A set of 16 thermoelastic strip sources, equivalent to a phased array of ultrasonic transducers has been implemented on a solid by irradiating its surface with a multiple beam pulsed YAG laser. Longitudinal elastic waves were focused in the sample by time delaying each laser pulse. Results of experiments performed on duraluminum in the thermoelastic regime are presented and compared with simulations based on the surface center of the expansion model. It is shown that with this technique, a high energy focusing, together with an ultrasonic beam scanning can be achieved in a wide angular range leading to a significant improvement in the signal-to-noise ratio of a laser based ultrasonic system.

The laser-generated ultrasonic phased array: Analysis and experiments

Focused ultrasonic waves have been generated in a solid by irradiating its surface with a multiple beam-pulsed YAG laser. A set of 16 rectilinear sources is used, equivalent to a phased array of ultrasonic transducers. Longitudinal waves are focused in the sample by introducing an appropriate time delay between each laser pulse. The elastic waves are detected either by a broadband optical heterodyne probe to analyze the wide ultrasonic signal spectrum (0–20 MHz), or by a narrow-band piezoelectric transducer to achieve the sectorial acoustic beam scanning of the sample. Neglecting heat diffusion in the solid and considering the source as a surface center of expansion, the impulse directivity patterns of laser-generated longitudinal acoustic waves have been computed. Experiments performed on duraluminum samples in the thermoelastic regime and steel samples in the ablation regime are presented and compared with this analysis. It is shown that a high focusing and a significant improvement of the signal sensitivity for longitudinal waves can be achieved with this technique.

Characterization of thin diamond-like carbon films by ultrasonic surface waves

The method of ultrasonic surface waves is suitable to determine the elastic modulus of thin diamond-like carbon films deposited onto silicon (111) wafers by laser-induced vacuum arc deposition (Laser-Arc) if frequencies in the range up to 100 MHz are used. Surface wave pulses were generated in the thermoelastic regime by an excimer-laser (XeCl, 308 nm). Ultrasonic signals influenced by the film structure were detected by a piezoelectric transducer. The surface wave phase velocity, which is dependent on frequency, was measured using a Fourier transformation technique. To calculate the elastic modulus of the films the inverse problem of surface wave dispersion in coated materials was solved. Carbon films with a thickness of 300 nm deposited at different substrate temperatures in the range between 60 and 500 °C were studied. Young's modulus values up to 230 GPa were obtained for substrate temperatures below 150 °C. The very small modulus for T > 150 °C suggests a drastic decrease in the sp 3-to-sp 2 bonding ratio in the amorphous diamond-like carbon structure. These results correlate with optical studies. High frequency surface waves were demonstrated to be a sensitive indicator for evaluating the structure of the carbon film as dependent on the deposition conditions.

The thermoelastic surface strip source for laser-generated ultrasound

The vertical ultrasonic surface displacement generated by a laser-induced, thermoelelastic, rectangular surface strip source is formulated. The main features of this source, ultrasonic rise times comparable to those of the generating light pulse derivatives, and a large amplitude, double-pulsed waveform, are advantageous for ultrasonic spectral- and time-domain measurements for nondestructive material evaluation. The analysis shows that, for laterally symmetric sources in the thermoelastic regime, only one tangential thermoelastic stress component contributes to the vertical displacement. Therefore, the strip source is equivalent to two, tangential line forces acting outward at the strip’s front and back edges. The leading-edge rise time of the signal is virtually independent of the lateral extent of the source, which mostly affects the trailing portions of the ultrasonic pulses. Consequently, a particularly simple expression, which compares favorably with experimental results, is obtained for short strips (subtending small angles at the observation point). In conjunction with this formulation, the thermoelastic strip source is an important tool for quantitative, laser-based, ultrasonic nondestructive material evaluation.

Laser generation of narrow-band surface waves

Tone-burst-like narrow-band surface waves are generated in the thermoelastic regime by illuminating the surface of a solid with an array of laser-generated line sources. The laser line array is formed by a system of lenses and an optical diffraction grating that provide for flexible and easy control of the line-array parameters. It is shown experimentally, consistent with theory, that the generation of narrow-band surface waves can be controlled by adjusting the line-array parameters such as the number of line sources in the array, the width of each line source, and the separation distance between them. Certain optimum generation conditions are experimentally determined whereby the amplitude of the narrow-band surface waves can be increased by a factor of N (the effective number of lines in the array) over the corresponding broadband signals that would be generated using only a single line source. A laser-interferometric system is used to detect the generated surface waves. By suitably bandpass filtering the detected signal, the greater part of the white noise in the system can be eliminated, leading to a significant improvement in the signal-to-noise ratio and hence the sensitivity of the whole laser ultrasonic system.

Thermal emission from a mechanical notch Calculations and measurements

The temperature variations due to elastic-plastic deformations are both calculated and measured, for the notch zone of a Compact Tension specimen subjected to monotonic loading. The finite element calculation adopts standard models of the plastic flow and the simplest model of the thermoplastic effect. The agreement between calculations and measurements is very good in the thermoelastic regime, and generally good in the thermoplastic regime; a global confirmation is thus obtained for the ensemble of the adopted models. Local discrepancies are discussed, and found consistent with the limitations of the model.

TEA-CO 2 laser generation of ultrasound in non-metals

An industrial TEA-CO 2 laser, operating at a wavelength of 10.6 μm, has been used to produced broadband ultrasonic pulses in polymers. The generation mechanism falls into three categories. At low power densities ≤ 10 7 W cm −2 a thermoelastic regime predominates. As the power density is increased in the range (1–5) × 10 7 W cm −2 ablation of the material surface plays an increasingly important role in the acoustic generation. Thirdly, at greater power densities, plasma breakdown just above the material surface serves as the means of generation. This paper describes the acoustic sources for these types of generation mechanism and presents theoretically calculated acoustic waveforms to match those recorded experimentally.

Experimental study of laser-generated shear waves using interferometry

The shear displacements generated by short laser pulses have been measured in aluminum semicylindrical samples, both in the thermoelastic and ablation regimes. We measured the waveforms at different angles and obtained the angular distribution pattern of the amplitudes. For the thermoelastic regime good agreement has been found between the measured and the theoretically predicted shear waveforms. In the ablation regime, the absolute values of the amplitudes are comparable to the ones of compressional waves. The shear waveforms are difficult to interpret, particularly in the case where both thermoelastic and ablative effects play a role, because the phases of the shear pulses are opposite to one another in these two regimes. To measure the in-plane displacements, and hence the shear displacement field generated by the pulsed laser, a speckle heterodyne interferometer was used.

On the precursor in laser-generated ultrasound waveforms in metals

A laser pulse, when focused on a metal sample, produces characteristic elastic waveforms, which depend on whether thermoelastic or ablative/evaporative mechanisms dominate the generation process. In the thermoelastic regime, with an unconstrained surface, the predominant axial displacement is opposite to the direction of propagation (negative), but there is typically a small transient positive displacement. This precursor is not predicted by elastic point source models, but is predicted by models including thermal diffusion. A recent formulation of pulsed photoacoustic generation is used to show how the precursor arises from interaction of the thermal and elastic modes at the illuminated surface.

Experimental Study of Laser-Generated Shear Waves Using Interferometry

Abstract The shear displacements generated by short laser pulses have been measured in aluminum semicylindrical samples, both in the thermoelastic and ablation regimes. We measured the waveforms at different angles and obtained the angular distribution pattern of the amplitudes. For the thermoelastic regime good agreement has been found between the measured and the theoretically predicted shear waveforms. In the ablation regime, the absolute values of the amplitudes are comparable to the ones of compressional waves. The shear waveforms are difficult to interpret, particularly in the case where both thermoelastic and ablative effects play a role, because the phases of the shear pulses are opposite to one another in these two regimes. To measure the in-plane displacements, and hence the shear displacement field generated by the pulsed laser, a speckle heterodyne interferometer was used.

Calculation of ultrasonic surface waves from an extended thermoelastic laser source

A method is developed to calculate ultrasonic surface waveforms generated by an extended laser source, operating in the thermoelastic regime of laser-pulse energy density. This approach integrates over a suitably weighted distribution of point surface centers of expansion, for observation to within 1 mm of the edge of the source. Power spectra as well as both horizontal and vertical displacements are presented and discussed for ultrasonic waveforms on an aluminium surface, for incident laser pulses having Gaussian lateral profiles of various sizes. Far from the source, the waveform is dominated by a dipolar Rayleigh (R) wave, whose amplitude and spectral content depend on laser spot size. Weak, monopolar pulses also occur at the intersection of bulk pressure and shear wavefronts with the surface (denoted assP andsS, respectively). Close to the source, thesP wave amplitude approaches that for theR wave, and overlaps theR wave for large source sizes. The fall-off with distance for bothsP andR waves is given. Finally, the changes in pulse shape and amplitude are calculated when anR wave from an extended thermoelastic source is reflected or transmitted by a right-angled corner of an aluminium block.

Precision laser-ultrasonic velocity measurement and elastic constant determination

A laser-ultrasonic method to measure the acoustic velocities and the elastic constants of solid materials based on the crosscorrelation of successive echoes is presented. Measurements are performed in the thermoelastic regime or slight ablation regime, which permits simultaneous measurement of longitudinal shear velocities and an unlimited number of shots at the specimen. Diffraction corrections are calculated, and dispersion effects are evaluated by frequency analysis. The precision and accuracy of the technique are estimated in terms of sampling rate, thickness of the sample, signal-to-noise ratio, frequency bandwidth and laser-beam alignment. Examples of applications of the technique are given for PZT ceramic, metal-ceramic composites and single crystal germanium.

Time- and frequency-domain characteristics of laser-generated ultrasonic surface waves

The generation of elastic waves by illumination of a free metal surface with a laser pulse was studied to establish a quantitative basis for laser ultrasonics in nondestructive evaluation of surfaces. Experiments were carried out using a Q-switched Nd:YAG laser for generation and a wideband piezoelectric sensor for detection. A theoretical model for laser generation was developed for the thermoelastic energy regime. This model integrates over point source Green’s functions, suitably spread throughout the illuminated region. Good agreement was found between experiment and theory for characteristics in both time and frequency domains, for surface waves excited in the thermoelastic regime. For a given laser pulse energy, the highest Rayleigh wave peak frequency and bandwidth occurred when the Gaussian laser-beam half-width a was reduced just enough to begin surface damage. Once such damage commenced, further spot size reduction slightly lowered the peak frequency. Quantitative agreement with theory was found for the observed dependence, on both source–receiver distance and a, of the amplitudes for both the Rayleigh wave and the surface wave traveling at the bulk pressure wave speed (sP). In the nearfield, the sP wave overlapped the Rayleigh wave and made a greater contribution to the observed waveform than in the farfield.

Interaction of IR laser radiation with liquids

The different laser radiation interaction mechanisms are briefly discussed together with the theoretical models which explain the thermoelastic regime. A general survey of experimental measurements includes optical techniques and acoustic methods.