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Schematic diagram of electromagnetic acoustic transducers: (a) an in-plane detector system; (b) an out-of-plane detector system.
Source publication
Using laser-generated Rayleigh waves in the thermoelastic regime as an ultrasonic source, a range of ultrasonic detector systems have been evaluated experimentally for their sensitivity. Systems examined were those based on electromagnetic acoustic transducers (EMATs), a non-steady state photo-emf detector, a confocal Fabry-Pérot interferometer (CF...
Contexts in source publication
Context 1
... EMAT arrangements were used for comparison in these experiments. The in-plane EMAT, figure 4(a), consisted of a stack of two sintered NdFeB magnets orientated such that the poles were perpendicular to the inspection surface. The local magnetic field penetrated the inspection surface along this axis and provided a preferential sensitivity for the device to ultrasound signals travelling along the surface. ...
Context 2
... an EMAT with out-of-plane sensitivity, the magnetic field direction is required to be parallel to the surface. The probe, figure 4(b), was therefore constructed with a soft iron 'horseshoe' magnetised by two sintered NdFeB bar magnets. The two legs of the magnet were then placed near to the surface such that the field would 'leak' sideways creating the desired local field. ...
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Resolution and signal-to-noise ratio performance improvement of ultrasound applications in measurement or imaging demands both high energy and wideband signals for excitation. Conventional signals like pulse and toneburst do not satisfy both requirements, but spread spectrum signals are capable to increase the signal energy and maintain the resolut...
Citations
... Usually, acoustical signals are detected with the polyvinylidene fluoride (PVDF)/piezoelectric transducer (PZT) [13,14] , optical beam deflection [2,15] , and a laser vibrometer [16] . However, with these techniques, the motion was recorded pointwise, and a long time was needed to finish the measurement [17,18] . ...
... This technology (not to be confused with Optical Beam Deflection [31], which is unsuitable for use on poorly reflecting surfaces) is based on the deviation sustained by a laser beam when it intercepts an ultrasonic pressure field. This 80 deviation, attributable to changes in the refractive index of the fluid by the pressure field, is detected by a position-sensitive photodetector [30]. ...
Note: The pre-print significantly differs from the published manuscript. -
The use of non-contact ultrasonic detection techniques is essential to develop an apparatus capable of inspecting
components in service, operating in adverse environmental conditions or whose surfaces are challenging
to access. Among these techniques, Gas-Coupled Laser Acoustic Detection (GCLAD) is based on measuring
the deviation that a laser beam sustains when intersecting an ultrasonic wavefront. The method has been
recently developed, being hence unestablished in the non-destructive testing (NDT) field. The objective of
the present work is to expand knowledge on the physical operating principles of the GCLAD system; this is
obtained by investigating, both theoretically and experimentally, the influence of specific working parameters
on the detected signals with the aim of maximizing the ultrasonic amplitude. These analyses are essential for
proposing effective non-destructive investigation methodologies benefitting from the employment of the GCLAD
technique. Based on the information obtained and from an NDT standpoint, test configurations are proposed
with a different relative arrangement between the GCLAD system and the structure; these configurations
provide peculiar results also based on the type of wave under investigation (surface or bulk waves). Finally,
to highlight the capabilities of the GCLAD system in inspections of mechanical pieces, signals detected
experimentally on a component (i.e., a railway axle) are presented in the presence and absence of a surface
defect.
... An additional ultrasonic detection technique is represented by the Beam Deflection Probe or Gas-Coupled Laser Acoustic Detection (GCLAD) [31,32]. This technology (not to be confused with Optical Beam Deflection [33], which is unsuitable for use on poorly reflecting surfaces) is based on the deviation sustained by a laser beam when it intercepts an ultrasonic pressure field. This deviation, attributable to changes in the refractive index of the fluid by the pressure field, is detected by a position-sensitive photodetector [32]. ...
Note: The published version of the article significantly differs fom the preprint -
The use of non-contact ultrasonic detection techniques is essential to develop an apparatus capable of inspecting components in service, operating in adverse environmental conditions or whose surfaces are challenging to access. Among these techniques, Gas-Coupled Laser Acoustic Deflection (GCLAD) is based on measuring the deviation that a laser beam sustains when intersecting an ultrasonic wavefront. The method has been recently developed, being thence unestablished in the non-destructive testing (NDT) field. The objective of the present work is to expand knowledge on the physical operating principles of the GCLAD system; this is obtained by investigating, both theoretically and experimentally, the influence of specific working parameters on the detected signals with the aim of maximizing the ultrasonic amplitude. These analyses are essential for proposing effective non-destructive investigation methodologies benefitting from the employment of the GCLAD technique. Based on the information obtained and from an NDT standpoint, test configurations are proposed with a different relative arrangement between the GCLAD system and the component; these configurations provide peculiar results also based on the type of wave under investigation (surface or bulk waves). Finally, to highlight the capabilities of the GCLAD system in inspections of mechanical organs, signals detected experimentally on a component (i.e., a railway axle) are presented in the presence and absence of a surface defect.
... To improve the reliability of the extracted BDF values, each group of experiments is carried out three times and the extracted BDF values are averaged on each specimen. To improve the signal-to-noise ratio, the averaging method is utilized [46], so the response of each time experiment is measured 512 times and averaged in the time domain. Because the amplitude and frequency of an ultrasonic response can be easily altered by material properties and operational conditions (e.g. ...
... To improve the reflectivity of theprobe laser of the nonlinear laser interferometer, the surfaces of the connecting blocks shown in figure 24 are slightly polished. To improve the signal-to-noise ratio, the averaging method is utilized [46], so the response of each time experiment is measured 64 times and averaged in the time domain. First, the obtained intact and damage response signals are simply bandpass filtered to remove noises. ...
Laser nonlinear wave modulation spectroscopy (LNWMS) has been used to detect nonlinear ultrasonic signatures caused by fatigue cracks in materials. The nonlinear feature extraction from the spectral plot plays a significant role in LNWMS. This paper uses the bispectrum to distinguish harmonically related signals which result from nonlinear interactions and to remove those which do not. A feature called bispectrum damage factor (BDF), which is defined as the ratio of the bispectrum peak difference value of the damage and intact signals to the bispectrum peak value of intact signals at frequencies F 1 , F 2 , is extracted from the bispectral plot to characterize the crack features. The basic premise of the proposed laser nonlinear wave modulation bispectrum (LNWMBS) is that this BDF value will vary regularly as the level of crack-induced nonlinearity increases. A finite element model and a noncontact laser ultrasonic system have been built for LNWMBS measurement. The simulation results show that the BDF value is sensitive to crack depth and angle, and is insensitive to crack length and detecting position. BDF values of reflected Rayleigh waves are positive and approximately linearly increase with an increase of the crack depths in the range of 0.1 λ ∼ 0.6 λ (λ is the corresponding wavelength of adopted frequency); meanwhile, the values of the transmitted waves are negative and decreasing. The BDF values are larger than the crack angle at 90° when they are less than 90°, which is beneficial for crack detection. The proposed LNWMBS technique has been successfully tested for detecting artificial sub-millimeter surface-breaking cracks in aluminum alloy plates and closed fatigue cracks of torque load simulator connection blocks.
... Проте їх застосування обмежується складністю обладнання, яке часто можна застосовувати лише в лабораторних умовах, а також в деяких випадках викликана необхідністю підготовки поверхні об'єкту контролю до вимірювань. Для вимірювання швидкості ПАХ в зразку №1 використовували дефлекційний метод реєстрації цих хвиль [12]. Метод базується на відхиленні відбитого лазерного променя від поверхні зразка в результаті його зміщення під дією акустичної хвилі. ...
... Lasers for ultrasonic generation need to satisfy a high repetition rate and an appropriate tunable and absorbable wavelength. Also, under the thermoelastic regime, a laser can generate a longitude wave [11] and Rayleigh wave [12] at the same time, which is benefit for complex-shaped parts testing. ...
In this paper, we demonstrate 1-kHz-repetition-rate laser ultrasound in carbon-fiber-reinforced plastics (CFRPs). A ZGP master oscillator and power amplifier (MOPA) system was used to generate high-repetition-rate laser radiation in the spectral range 3.2–3.4 μm. At the output wavelength of 3.4 μm, the maximum output energy of the ZGP MOPA system is 5.62 mJ with a pulse width of 24 ns, corresponding to a peak power of approximately 233.9 kW. The ultrasound was generated by the laser converted from 3.2 to 3.4 μm in the graphite/epoxy composite. The maximum ultrasonic signal amplitude in the CFRP sample was 36.2 mV in the condition of thermoelastic excitation at 3.4 μm. Ablation occurred in the CFRP sample when the energy fluence was over 90 mJ / cm 2 . Compared in different samples, laser-ultrasound generation was influenced by the wavelength of the laser.
... Although there are various techniques that are used for conversion of wide-band acoustic waves (mechanical displacement) into electronic signals, including electromagnetic [17] and capacitive transducers [18], in this review, we focus on the two widely used ultrasound detection techniques [18]: piezoelectric transducers [13,19,20] and optical interferometers and deflectors [21][22][23][24]. The non-contact nature of laser generation of ultrasound makes it attractive to develop fully non-contact (all-optical) PAI and laser ultrasonic (LU) evaluation systems employing both optical generation and optical detection of acoustic transients. ...
The current paper reviews a set of principles and applications of photoacoustic and laser ultrasonic imaging, developed in the Laser Optoacoustic Laboratories of ILIT RAS, NUST MISiS, and ILC MSU. These applications include combined photoacoustic and laser ultrasonic imaging for biological objects, and tomographic laser ultrasonic imaging of solids. Principles, algorithms, resolution of the developed methods, and related problems are discussed. The review is written in context of the current state-of-art of photoacoustic and laser ultrasonic imaging.
... The detection of the acoustic signal by piezoelectric transducers belongs to the contact method. Non-contact method includes microphone detection, eddy current and optical methods [2][3][4][5][6][7][8][9]. ...
Photoacoustic technique has emerged as a powerful tool for nondestructive evaluation and characterization. The high signal to noise ratio, sensitivity, and the least quantity of sample required make the technique suitable for wide range of applications. The intensity modulated laser beam focused on to the sample in the photoacoustic cell generates acoustic waves. Analysis of the acoustic signal enables the measurement of thermal and optical properties of the material. In the present work, the photoacoustic technique has been extended to find the thermal diffusivity and to demonstrate how the technique can be used for measuring optical reflection coefficient of a precious metal mirror—Aranmula mirror. The mirror being metal with high reflection coefficient, it finds application in optical instrumentation. The mirror is subjected to morphological characterization by atomic force microscopy and field emission scanning electron microscopy. The elemental and structural characterizations are done by energy dispersive x-ray spectroscopy, X-ray dot mapping, and X-Ray diffraction analysis. The fractal dimension of the particle distribution over the surface provides information about the surface roughness. The box-counting and power spectral fractal dimensions are found to be around 1.63 and 1.65 respectively.
... EMATs have been employed in thickness measurements and defect detection at high temperatures due to their noncontact nature, although this requires active cooling of a permanent magnet [16,17] or the use of a bulky electromagnet [18,19], limiting their use in some industrial settings. Laser-EMAT systems have also been employed successfully for high temperature operation [20][21][22], often with a water-cooled EMAT receiver, as EMATs generally provide greater efficiency in detection [23]. ...
Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 hours, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.
... The interaction of this with the dynamic magnetic field and any external static magnetic field via the Lorentz force leads to ultrasound generation [11,13]. Detection works similarly in reverse, except that the magnet becomes essential, and the efficiency is greatly improved as it is detecting moving electrons rather than using electron motion to excite the sample bulk [13,14]. As the mechanism works via electromagnetic induction the transducer does not need to be in direct contact with the sample and no couplant is required, offering the potential for fast scanning and easy use on rough, hot and rusted surfaces, and through certain coatings. ...
Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet are used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0 to 180°, and the set up has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils.
