Laser-generated Surface Waves Research Articles

Velocimetry of GHz elastic surface waves in quartz and fused silica based on full-field imaging of pump–probe reflectometry

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump–probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6.0nsto11.5ns, image stacks of wave field propagation were created.Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2% to 5%.These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Laser-generated surface waves for quantitative detection of inclined surface cracks based on finite element analysis

ABSTRACT In this paper, transmitted surface waves are used to quantitatively detect the length and angle of inclined surface cracks based on laser ultrasonic technique. The interaction between laser-generated surface waves and inclined surface cracks is analysed, and the propagation path and mode conversion process of the transmitted surface waves are considered. Then, the propagation paths of some peaks and troughs of the transmitted surface waves are obtained. Based on the obtained path and modal conversion information, a quantitative detection method for the length and angle of inclined surface cracks is proposed. The finite element method is used to verify the propagation paths and proposed method, and the results show that the arrival time of the theoretical path is consistent with that of the simulation. The relative measurement errors of the surface crack length and angle are less than 15% and 10%, respectively, which verifies the rationality and correctness of the proposed method.

Quantitative evaluation of subsurface cracks with laser-generated surface wave based on back propagation neural network

Quantitative evaluation of subsurface cracks with laser-generated surface wave based on back propagation neural network

Defocus generation method for measuring the thickness of surface-hardened layers by a laser ultrasonic technique.

We developed a noncontact inspection method for measuring the thickness of surface-hardened layers using a laser ultrasonic technique in which a laser beam for generating ultrasonic surface waves was defocused on a specimen. Our method was based on the dependence of the ultrasonic surface-wave velocity on the thickness of the hardened layers. In the measurement, it is essential to generate a surface wave with a wavelength comparable to the hardened-layer thickness. It is known that the wavelength of the laser-generated surface wave depends on the beam diameter of the generation laser. We applied this control method of the wavelength of the surface wave (i.e., the defocus generation method) for measuring the hardened-layer thickness with higher accuracy and found that the results were in good agreement with the calculation.

Measuring the Depth of Subsurface Defects in Additive Manufacturing Components by Laser-Generated Ultrasound

A new method to measure the depth of subsurface defects in additive manufacturing components is proposed based on the velocity dispersion analysis of Lamb waves by the wavelet-transform of laser ultrasound. Firstly, the mode-conversion from laser-generated surface waves to Lamb waves caused by subsurface defects at different depths is studied systematically. Secondly, an additive manufactured 316L stainless steel sample with six subsurface defects has been fabricated to validate the efficiency of the proposed method. The measured result of the defect depth is very close to the real designed value, with a fitting coefficient of 0.98. The defect depth range for high accuracy measurement is suggested to be lower than 0.8 mm, which is enough to meet the inspection of layer thickness during additive manufacturing. The result indicates that the proposed method based on laser-generated ultrasound (LGU) velocity dispersion analysis is robust and reliable for defect depth measurement and meaningful to improve the processing quality and processing efficiency of additive/subtractive hybrid manufacturing.

Numerical simulation of laser-generated ultrasonic waves for detection surface defect on a cylinder pipe

The non-destructive detection of cylindrical pipe defects is still open to research. In this letter, we report a study of the detection method for the surface defect in the cylindrical pipe by using the laser ultrasonic wave. In this study, the propagation characteristics of laser-generated ultrasonic waves in a cylindrical pipe have been analyzed by the finite element method. The simulation results indicated that the laser-generated surface wave and shear wave converted to surface wave can be used to detect the surface defect in the cylindrical pipe. The maximum negative peak of reflected surface waves due to surface defects increases as the depth of surface defects increases. The position of surface defects through B-scan images can be detected. The proposed method may provide a useful way to detect of the surface defect in the cylindrical pipe.

Imaging and Sizing of Surface Defects Using Synthetic Aperture Focusing of Laser-generated Surface Waves

The synthetic aperture focusing technique (SAFT) can be easily adapted to laser ultrasonic testing and can improve the signal-to-noise ratio (SNR) of laser ultrasonic images, to enhance defect detectability. In this study, the imaging and sizing of surface defects were carried out when SAFT was adopted for laser-generated surface waves. Six different surface breaking notches were made on an aluminum plate and were visualized using a laser ultrasonic scanning system consisting of a pulsed laser for ultrasonic generation and a laser interferometer for ultrasonic reception. B-scan images and SAFT-processed images for the six different notches were compared, which highlighted the advantages of the SAFT post-processing compared to B-scan imaging. A methodology to size the defects in SAFT-processed images was also proposed. The methodology utilizes the linear relationship between the suitable threshold decibel value, which determines the exact defect length with the use of an amplitude-drop method, along with the SNR of the defect echo in the SAFT-processed images. The experimental results demonstrate that the estimated defect lengths are similar to actual defect lengths, which showing that the proposed methodology can be effectively used for sizing of surface defects in SAFT-processed images.

Acoustical breakdown of materials by focusing of laser-generated Rayleigh surface waves

Focusing of high-amplitude surface acoustic waves leading to material damage is visualized in an all-optical experiment. The optical setup includes a lens and an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern at the surface of a gold-coated glass substrate. Optical excitation induces a surface acoustic wave (SAW) that propagates in the plane of the sample and converges toward the center. The evolution of the SAW profile is monitored using interferometry with a femtosecond probe pulse at variable time delays. The quantitative analysis of the full-field images provides direct information about the surface displacement profiles, which are compared to calculations. The high stress at the focal point leads to the removal of the gold coating and, at higher excitation energies, to damage of the glass substrate. The results open the prospect for testing material strength on the microscale using laser-generated SAWs.

Measurement of weld penetration depths in thin structures using transmission coefficients of laser-generated Lamb waves and neural network

The Laser/EMAT ultrasonic (LEU) technique has shown the capability to measure weld penetration depths in thick structures based on ray-tracing of laser-generated bulk and surface waves. The ray-tracing method is not applicable to laser-generated Lamb waves when the LEU technique is used to measure weld penetration depths in thin structures. In this work, transmission coefficients of Lamb waves present in the LEU signals are investigated against varying weld penetration depths. An artificial neural network is developed to use transmission coefficients of sensitive Lamb waves and LEU signal energy to predict weld penetration depths accurately. The developed method is very attractive because it allows a quick inspection of weld penetration depths in thin structures.

Noncontact Evaluation of Acoustic Nonlinearity of a Laser-Generated Surface Wave in a Plastically Deformed Aluminum Alloy

A noncontact method to evaluate the acoustic nonlinearity of surface waves in a plastically deformed aluminum alloy is proposed. Line-arrayed laser beams modulated with slit masks were used for the generation of narrowband surface waves. A laser-ultrasonic detector using a two-wave mixing (TWM) approach was also employed to detect the surface waves. The specimens were deformed by a stroke-controlled tensile tester so as to generate various degrees of tensile deformation. The experimental results showed that the acoustic nonlinearity of the laser-generated surface waves increased according to the level of tensile deformation. This tendency was in good agreement with our previous results obtained using a contact piezoelectric (PZT)-transducer as the receiver. These results imply that our noncontact technique is suitable for the evaluation of acoustic nonlinearity and can be applied to practical damage assessment.

Research on the oscillation effect of near-surface metal defect based on laser-generated acoustic surface wave

According to the thermoelastic equations and heat conduction equations of laser-generated acoustic surface wave (SAW), the SAW propagation images are obtained by taking finite element solutions to the equations. When the SAW propagates through the aluminum plate with the near-surface defects at different depths, producing an oscillating effect between the surface acoustic wave and the near-surface defects, and then there is a gradual decrease after the gradual decrease of the oscillation waves as getting through the near-surface defect. When the SAW propagates through the aluminum plate with the near-surface defect at different depth, there is a certain variation of the center frequency of the oscillation waves. Simulation results show that, when the depth of near-surface defect changes from 0.1 to 0.5 mm, the center frequency of the oscillation signal generated by the oscillation effect changes from 0.4 to 0.76 MHz, and the center frequency of the oscillation signal is seriously attenuated with increasing width of the near-surface defect, thus providing a theoretical basis for quantitative detection of the near-surface defects.

Influence of slit width on harmonic generation in ultrasonic surface waves excited by masking a laser beam with a line arrayed slit

Masking an expanded laser beam with a line-arrayed slit is an effective and easy way to produce line arrayed illumination sources in order to excite a laser-generated surface wave within a narrowband of frequencies. The objective of this study is to identify the slit width dependency of the magnitude of harmonic waves generated by this technique. The frequency response of the generated surface wave is presented first under the assumption that the spatial energy profile of each line beam is a simple one-dimensional square pulse and the temporal profile is a delta function. Then the magnitude ratio A2/A1 (A1: magnitude of the fundamental wave, A2: magnitude of the second-order harmonic wave) is obtained as a function of the duty ratio to show that it has a minimum of zero at a specific duty ratio; that is, the initial generation of the second-order harmonic wave can be fully suppressed at this duty ratio. This is comparable with the conventional Gaussian line beam model, in which the initial generation of the second-order harmonic wave is inevitable. Next, we carried out a two-dimensional thermo-elastic analysis using FEM in order to model a practical situation, where thermal conduction and a Gaussian temperature profile are factors. Naturally, the frequency response obtained from this numerical analysis does not perfectly match that of the one-dimensional analysis; however, we prove that the magnitude ratio A2/A1 has a minimum at a specific duty ratio. Additionally, the dependency of A2/A1 on the duty ratio obtained from the numerical analysis showed very good agreement with the experimental results.

利用激光干涉法测量激光超声下非镜面钽膜的表面波速度

利用激光干涉法测量激光超声下非镜面钽膜的表面波速度

Acoustic Nonlinearity of a Laser-Generated Surface Wave in a Plastically Deformed Aluminum Alloy

The acoustic nonlinearity of surface waves is studied to evaluate plastic deformation in aluminum alloys. A narrow-band surface wave is successfully generated by a pulsed Nd-YAG laser system consisting of a beam expander and a slit mask. Various degrees of tensile deformation are induced by interrupting the tensile tests so as to obtain aluminum specimens with different degrees of plastic damage. The normalized acoustic nonlinearity increases as a function of tensile strain. The experimental results show that the acoustic nonlinearity of a laser-generated surface wave has a good correlation with the level of tensile deformation and has a potential to evaluate microdamage induced by dislocation microplasticity.

Al6061 합금의 소성변형에 따른 음향비선형 특성의 완전 비접촉식 평가

This study proposes a fully non-contact measurement method to assess acoustic nonlinearity of narrowband surface waves generated by a line-arrayed laser beam by using a laser-ultrasonic detector in the way of two-wave mixing (TWM) method. This method was applied to figure out a relationship between plastic deformation and nonlinearity characteristics of a plastically deformed aluminum specimens. The experimental results showed that the acoustic nonlinearity of the laser-generated surface wave increased proportionally to the level of tensile deformation. This tendency is in good agreement with the result of measurement by contact method with PZT-transducer.

레이저 여기 표면파의 음향비선형성을 이용한 Al6061 합금의 소성변형 평가

The objective of this study is to assess plastic deformation in aluminium alloy by acoustic nonlinearity of laser-generated surface waves. A line-arrayed laser beam made by high-power pulsed laser and mask slits is utilized to generate the narrowband surface wave and the frequency characteristics of laser-generated surface waves are controlled by varying the slit opening width and slit interval of mask slits. Various degrees of tensile deformation were induced by interrupting the tensile tests so as to obtain aluminum specimens with different degrees of plastic deformation. The experimental results show that the acoustic nonlinear parameter of a laser-generated surface wave increased with the level of tensile deformation and it has a good correlation with the results of micro-Vickers hardness test and electron backscatter diffraction (EBSD) test. Consequently, acoustic nonlinearity of laser-generated surface wave could be potential to characterize plastic deformation of aluminum alloy.

Bending Fatigue Evaluation of Al6061 Alloy by Laser-Generated Surface Wave

The bending fatigue of Al6061 alloy has been evaluated by the acoustic nonlinearity of laser-generated surface wave. The surface wave is very attractive for field applications since it enables to pulse and receive signals at the same surface of materials, and has strong acoustic nonlinear effects on the surface. A relative acoustic nonlinear parameter was measured successfully on the surface of fatigue-damaged aluminum 6061 alloy. The results show that the acoustic nonlinear parameter increased with fatigue damage accumulation in relation to dislocation evolution. Consequently, this study suggests that the acoustic nonlinearity technique of a laser-generated surface wave can be potentially used to characterize surface damage resulting from bending fatigue.

Acoustic Nonlinearity of Narrowband Laser-generated Surface Waves in the Bending Fatigue of Al6061 Alloy

Acoustic Nonlinearity of Narrowband Laser-generated Surface Waves in the Bending Fatigue of Al6061 Alloy

Laser Generation of Surface Waves on Cylinder with a Slow Coating

An analytical model of acoustic field excited by a pulsed-laser line source on a coated cylinder is presented. Surface wave dispersive behaviours for a cylinder with a slow coating are analysed and compared with that of a bare cylinder. Based on this analysis, the laser-generated transient response of the perturbed Rayleigh wave and the higher modes of steel cylinder with a zinc coating are calculated from the model using residue theory and FFT technique. The theoretical result from the superposed waveform of the perturbed Rayleigh wave and higher modes agree well with the waveform obtained in experiment. The results show that the model and numerical method provide a useful technique for quantitatively characterizing coating parameters of coated cylinder by the laser generated surface waves.

Laser-generated nonlinear surface wave pulses in silicon crystals

The absorption-layer method for inducing pressure shocks is employed to generate finite-amplitude, broadband surface wave pulses in crystalline silicon. Spectral evolution equations are used to compute the wave form distortion from the first to the second measurement location, and the results are shown to be in quantitative agreement with the measured data. The measurements also confirm that a nonlinearity matrix which describes the coupling of harmonics provides a useful tool for characterizing wave form distortion. In the (001) plane, the measurements show that the longitudinal velocity wave forms develop rarefaction shocks along $[100]$ and compression shocks along $26\ifmmode^\circ\else\textdegree\fi{}$ from $[100].$ In the (110) plane, compression shocks are observed in the longitudinal velocity wave forms in the direction $37\ifmmode^\circ\else\textdegree\fi{}$ from $[100],$ whereas rarefaction shocks are seen along $[11\ifmmode\bar\else\textasciimacron\fi{}0].$ The results in the (001) and (110) planes are consistent with sign changes in the nonlinearity matrix elements. In the (111) plane, the measured wave form distortion is consistent with the phase changes associated with the computed complex-valued matrix elements. In particular, the characteristics of propagation in the $[112\ifmmode\bar\else\textasciimacron\fi{}]$ and $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2]$ directions are shown to differ. This specific case is proved to follow from a more general result based on the symmetry properties of surface acoustic waves in this plane. In all the planes, it is demonstrated that, unlike bulk waves, the peak acoustic amplitude of surface waves can increase as they propagate, thereby allowing large stresses to be generated at surfaces. Finally, the power flux and total power of the pulses are shown to be substantially higher than in previous reports.