Laser-induced Surface Acoustic Waves Research Articles

Remote measurement of material elastic property at elevated temperature with grating laser ultrasonic testing

ABSTRACT As a pivotal elastic property, elastic modulus plays a crucial role in characterising the mechanical performance of materials. To achieve the convenient and remote measurement of the elastic modulus of materials at elevated temperatures, a method based on diffracted grating laser-induced narrowband surface acoustic waves (SAW) is proposed. The design scheme of the diffraction grating is disclosed, and a CCD camera is deployed to confirm the generated grating pattern and determine the actual grating period. Delving into the viability of an effortless and reliable procedure for calculating elastic modulus is discussed. The elastic modulus determined by the SAW does not depend on the exact Poisson’s ratio, which can be calculated from the SAW velocity with a given, less precise value for Poisson’s ratio. Experimental results of the three metals are in line with the reference values, and the elastic modulus at elevated temperature is successfully measured remotely, thereby verifying the feasibility of the proposed method.

Photoacoustic 2D actuator via femtosecond pulsed laser action on van der Waals interfaces

Achieving optically controlled nanomachine engineering can satisfy the touch-free and non-invasive demands of optoelectronics, nanotechnology, and biology. Traditional optical manipulations are mainly based on optical and photophoresis forces, and they usually drive particles in gas or liquid environments. However, the development of an optical drive in a non-fluidic environment, such as on a strong van der Waals interface, remains difficult. Herein, we describe an efficient 2D nanosheet actuator directed by an orthogonal femtosecond laser, where 2D VSe2 and TiSe2 nanosheets deposited on sapphire substrates can overcome the interface van der Waals forces (tens and hundreds of megapascals of surface density) and move on the horizontal surfaces. We attribute the observed optical actuation to the momentum generated by the laser-induced asymmetric thermal stress and surface acoustic waves inside the nanosheets. 2D semimetals with high absorption coefficient can enrich the family of materials suitable to implement optically controlled nanomachines on flat surfaces.

Influence of top-coat thickness of thermal barrier coating on time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave

Thermal barrier coating is commonly used to insulate the hot end components of aero engine and gas turbine in order to meet extreme high temperature service conditions. The performance of the thermal barrier coating will be significantly impacted by thinning or even spalling during the service process. Given the uniqueness of thermal barrier coating in structure and composition materials, and the fact that laser-induced surface acoustic waves are extremely sensitive to material surface characteristic parameters, laser-induced ultrasonic technology is used to detect the thermal barrier coating in this paper. To characterize the size and defect of thermal barrier coating, the excitation mechanism and propagation law of surface acoustic waves are very crucial. Firstly, the surface acoustic wave propagation law is investigated using spectral analysis and wavelet transform. Then, to study the influence of top-coat thickness on laser-induced surface acoustic wave, the surface acoustic wave velocity dispersion curves of thermal barrier coating with different top-coat thicknesses are extracted using wavelet analysis. The simulative and experimental results indicate that the time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave in multilayered heterostructure are strongly affected by the top-coat thickness. By the establishment of the relationship between phase velocity of laser-induced surface acoustic wave and top-coat thickness, a theoretical basis is provided to characterize the top-coat thickness of multilayered heterostructure.

Detection of surface defects based on the optimized scanning laser source method

In this paper, the interaction between laser-induced surface acoustic wave (SAW) and the surface micro-defect with triangular cross-section is studied. It is found that the reflected wave is greatly affected by the width of the defect, which is mainly suitable for defects whose depth is equal to or larger than the wavelength of SAW. Scanning laser source (SLS) method can detect small defects whose depth is significantly smaller than the wavelength of SAW. It is found that the detection mechanism of SLS method is that the superposition of reflected wave and direct wave increases the peak-to-peak value. The SLS method based on the change of the center peak-to-peak value at the bottom of the defect proposed in this paper is more capable of detecting small damage than the traditional SLS method, and can even find the defect with a depth of only 1/28 of the center wavelength of SAW. The traditional SLS method can only locate defects. This paper proposes a defect width detection method based on SLS method. The detection efficiency of traditional SLS method is low, which limits the application of this method. This paper optimizes the scanning steps of the SLS method by the bisection method, which greatly improves the detection efficiency. Using the optimized SLS method, the position, width and depth of micro artificial defect on the surface of 7075 aluminum alloy is detected quickly and accurately.

Study on dispersion and attenuation of laser-induced surface acoustic wave by grinding surface roughness

The dispersion and attenuation of surface acoustic wave (SAW) are important indicators for evaluating the damage of machined surface layer. However, the dispersion and attenuation of SAW is also affected by surface roughness. The finite element method can simulate the rough surface without surface damage. Based on the measured surface profiles of roughness profiler, a series of grinding surfaces of C45E steel with different surface roughness Ra are simulated by finite element method, and the dispersion and attenuation of laser-induced SAW are studied. It is found that with the increase of roughness Ra, the increase of dispersion is relatively uniform, while the increase of attenuation not. The possible reason is that the dispersion is mainly affected by the average amplitude of roughness, while the occasional deep valley profile has a great influence on the attenuation. Compared with the experimental data, it is found that the velocity variation of 10–40 MHz SAW caused by Ra 0.12–0.75 μm surface roughness accounts for 8%–19% of the total velocity variation, and the proportion increases with the increase of roughness. The attenuation of 15 MHz SAW caused by surface roughness accounts for 8%–13% of the total attenuation. For a rough surface, dispersion is approximately linear with the frequency of SAW, while the attenuation coefficient increases sharply with the increase of frequency. Therefore, it is necessary to first evaluate the influence of surface roughness, especially the influence on attenuation, when the dispersion and attenuation of SAW are used to evaluate the machined surface damage. The results of this paper can be used to evaluate the influence of roughness on the dispersion and attenuation of SAW, and reduce the deviation in the evaluation of machined surface damage.

Real-time laser ultrasonic monitoring of laser-induced thermal processes

Intra- and inter-layer integrity of components fabricated with advanced manufacturing techniques, such as laser powder bed fusion, is dependent upon rapid heating, melting, and solidification processes. There is a need for new techniques to provide in situ feedback of these processes. Here a laser-based ultrasonic technique to probe thermal effects induced by a high-power continuous wave laser in titanium samples is described. Numerical simulations were performed to show that, for a spatially uniform heating beam, laser-induced surface acoustic waves are strongly influenced by surface heating conditions, are dispersive in the case of rapid heating, and that an abrupt velocity reduction happens upon the onset of surface melting. Furthermore, laser-based ultrasound experimental results which monitor the transient change of surface wave travel time associated with high power laser surface heating are provided. A pulsed laser is used to generate high frequency surface acoustic waves that propagate through the laser-heated region and are detected using a photorefractive crystal-based interferometer. Qualitative agreement is observed between theory and experiment with both showing a rapid reduction in the surface wave velocity at the onset of illumination and further decrease in surface wave velocity associated with melting. It is demonstrated that changes in the surface wave velocity can be used to track local heating and detect the onset of surface melting in real time.

Remote in-line evaluation of acousto-elastic effects during elastic–plastic transition in an aluminum plate under uniaxial tensile and dynamic fatigue loading by laser generated, optically detected surface acoustic waves

Early detection and monitoring of heavy load induced plastic deformation in the structure is crucial for timely intervention before cracking occurs and the material completely fails. Ultrasound can be used for detecting plastic deformation provided the mechanical modulus, which can be probed via changes in velocities. This work presents a measurement scheme that makes use of optically detected, laser-induced surface acoustic waves (SAWs) for remote, real-time, online monitoring of the wave velocity during the elastic–plastic transition occurring in an aluminum plate undergoing a uniaxial tensile test and a dynamic fatigue test. Monochromatic SAWs were photoacoustically generated in a wavelength-controlled way through a transient thermal grating based laser ultrasonics excitation scheme. The SAWs were detected by a home-built photorefractive interferometer. The results reveal both regions of acceleration and slowing down, indicating that the material first stiffens and then starts to form microcracks that make it behave effectively softer for propagating SAWs.

Evaluation of the surface damage of brittle materials based on the attenuation of laser-induced surface acoustic wave

In this paper, the surface damage of machined surface of brittle material is evaluated by using the attenuation of laser-induced surface acoustic wave (SAW) in propagation. Taking the typical brittle material single crystal silicon as an example, the feasibility of evaluating surface layer damage based on the attenuation of SAW is demonstrated theoretically and experimentally. Theoretical analysis shows that the micro-cracks produced in the processing of brittle materials can cause significant changes in the attenuation coefficient of SAW. After the lapped silicon wafer is etched, the surface roughness Ra is almost unchanged, while the attenuation coefficient of SAW decreases from 4.3 dB/cm to 2.0 dB/cm, which indicates that the surface layer damage of the silicon can still significantly change the attenuation coefficient after removing the influence of the surface roughness. After measuring the attenuation coefficients of SAW of silicon wafers in different processing steps, it is found that with the improvement of processing accuracy, the attenuation coefficient of SAW decreases, and the attenuation coefficient of SAW reflects the changes in surface damage. Experiments show that surface scratches can also significantly change the attenuation coefficient of SAW. Therefore, the wave attenuation coefficient of the laser-induced SAW can be used to evaluate the surface damage of the brittle material machined surface.

Laser-Induced Surface Acoustic Wave Sensing-Based Malaria Parasite Detection and Analysis

Malaria parasite- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Plasmodium falciparum</i> infection is a severe healthcare problem in tropical and subtropical regions. Early detection of such disease is vital in reducing mortality and controlling the spread in the affected areas. In this article, detecting the ring-stage malaria parasite is carried out by using the proposed photoacoustic (PA)-surface acoustic wave (SAW) sensing and detection system. By optical radiation, the PA signal is generated from the red blood cell (RBC), and then, the SAW sensor could transfer the acoustic wave into the electrical signal for the analysis. The SAW sensor is manufactured via the standard two-step lithography techniques. The proposed system consists of a tunable pulse laser device with the adjusting optical components, a PA-SAW sensor, a function generator, two analog front-end (AFE) circuits, a digital oscilloscope, a workstation for data postprocessing, and several auxiliary coaxial cables for data communications. With the acquired PA signal and its fast Fourier transform (FFT) spectrum from the various specimens, the cultured ring-stage malaria parasite in the infected whole blood could be detected at a low concentration level of 0.5% with the PA-SAW sensor. The proposed system demonstrates its excellent potential in the early diagnosis of the malaria parasite.

Study on the relationship between the damage of machined surface layer and the dispersion curve of laser-induced surface acoustic wave

Laser-induced surface acoustic wave is a new surface damage detection technology, which uses the transient thermal effect of pulse laser to excite the surface acoustic wave on the sample surface. The damage of machined surface layer will cause the velocity change of laser-induced surface acoustic wave. This phenomenon is introduced into the evaluation of the surface layer damage of silicon wafers in this paper. The three-dimensional propagation model of the single-layer structure is extended to the double-layer structure, and it is proved that the gradient change of the surface layer damage will change the trend of velocity curve. In order to characterize the change of velocity curve, the surface layer damage coefficients are introduced. The cross-sectional transmission electron microscope (TEM) image of the lapped silicon wafer shows that the surface of the silicon wafer after lapping will produce a surface damage layer. The test data of the nanoindenter shows that the modulus of the surface damage layer is lower than that of the bulk material. The modulus estimated by dispersion curve is very close to the data measured by nanoindenter, indicating that the dispersion curve can truly reflect the damage degree of silicon wafer surface layer.

Laser-induced surface acoustic waves for measurement of elastic constants based on mode conversion

The measurement of elastic constants is very important to the study of materials. Laser ultrasonic technology has been widely concerned for its unique advantages and used to measure elastic constants, but the current measurement methods have their limitations. In general, traditional measurement methods require the use of a fatigue tester or measurement of all modal waves during the experiment. It is found that cracks can transform the mode of ultrasonic wave, in other words, the surface acoustic waves can be transformed into shear waves by mode conversion. So, we proposed a new method of measuring elastic constants of materials by laser ultrasound based on mode conversion. Surface acoustic waves are induced by laser ultrasound system, the reflection and mode conversion of surface acoustic waves can be realized by cracks. Therefore, direct waves, reflected waves and mode converted waves are included in the collected surface wave signals, then the wave velocity of surface waves and shear waves are calculated by the surface acoustic waves, and the elastic constants of materials are obtained by the acoustic elasticity equation. Since the experiment and processing are all based on surface acoustic waves, multiple waveform acquisition and processing are avoided. After calculation, the error is kept below 3% compared with the standard measurement value. In order to further confirm the application range of the method, cracks of different depths are used in the experiment, it is further proved that this method has nothing to do with the crack size. The research results of this paper can extend the application range of laser ultrasound. It also provides a new approach to the characterization of materials.

Analysis of velocity calculation methods of laser-induced surface acoustic wave

The key to measuring residual stress by surface acoustic wave method is the accurate measurement of velocity. In this paper, the velocity of laser-induced broadband surface acoustic wave is studied, and three velocity calculation methods of surface acoustic wave, time domain method, phase method and wavelet method are compared. The calculation error of the time domain method under the condition of dispersion is analyzed. A recursive method for calculating phase difference is proposed to improve the efficiency of phase method. The simulated surface acoustic waves are used to compare the phase method and wavelet method under the conditions of attenuation and dispersion. Compared with the wavelet method, the phase method cannot distinguish the time when the frequency band appears, and the velocity calculation of adjacent frequency points is related, while the wavelet method is independent of each other. The wavelet method can improve the calculation accuracy of the velocity curve by interpolating the original data. After interpolation, the trend of curve is more obvious, and the fitting error is greatly reduced.

Study of laser-induced surface acoustic wave propagating on materials with machined surfaces based on wavelet analysis

The present paper is concerned with the calculation of the dispersion curves of surface acoustic waves (SAWs) propagation on materials with machined surfaces. Using a DPSS (Diode Pumped Solid State) laser as the excitation source, experiments are performed on the surfaces of silicon wafer, steel and aluminum alloy plates respectively. The theoretic analysis shows that the location of the wavelet coefficients peaks at different scales in time-frequency domain are related to the arrival times of surface acoustic waves with different frequencies. The wavelet transform using the Sym8 wavelet is applied to the time-frequency analysis of weakly-dispersive SAWs and the propagation velocities of SAWs with different frequencies are calculated by extracting relevant data. Combined with the nanoindentation experiments, the relationships between the variation trend of the SAWs' dispersion curves and the surface properties of the materials are analyzed. The repeatability of the experimental method and analysis method is also verified by multiple experiments on different materials, the results indicate that the dispersion curves of multiple groups of signals on the propagation path have good consistency.

Nondestructive determination of film thickness with laser-induced surface acoustic waves**Project supported by the National Natural Science Foundation of China (Grant No. 61571319).

The application of surface acoustic waves (SAWs) for thickness measurement is presented. By studying the impact of film thickness h on the dispersion phenomenon of surface acoustic waves, a method for thickness determination based on theoretical dispersion curve v(fh) and experimental dispersion curve v(f) is developed. The method provides a series of thickness values at different frequencies f, and the mean value is considered as the final result of the measurement. The thicknesses of six interconnect films are determined by SAWs, and the results are compared with the manufacturer’s data. The relative differences are in the range from 0.4% to 2.18%, which indicates that the surface acoustic wave technique is reliable and accurate in the nondestructive thickness determination for films. This method can be generally used for fast and direct determination of film thickness.

Determination of Mechanical Properties of Battery Films from Acoustic Measurements

Measuring the mechanical properties of lithium-ion battery films, such as thickness and elasticity, is important for predicting and improving homogeneity of the film and performance of the battery. Problems with film heterogeneity could be identified and addressed early on through accurate, non-destructive inspection of the electrode as it is being manufactured. There are several techniques currently used to measure the mechanical characteristics of films, including nanoindentation and laser-induced surface acoustic waves (SAW). However, their use for analyzing battery electrodes is fairly limited. This research investigates the use of acoustic measurements as an alternative means of non-destructive quality control that could be adapted for on-line use. Mechanical properties of thin films, including elastic modulus and thickness, have been determined using SAW [1,2]. Additionally, acoustic time-of-flight measurements have been taken on complete battery cells to find the state of charge [3]. Here we report on our efforts to measure mechanical properties with SAW. A film under tension is excited to produce a wave. This wave is measured at multiple points along the film to determine the speed of the wave. Because wave dispersion is based on properties such as elasticity and density, we can calculate mechanical properties of the film from the wave speed measurements. Preliminary results indicate that acoustic tests can demonstrate observable differences between films with different properties, and that changes in the stiffness of the film (such as what happens during the drying process for battery films) can also be detected as a difference in wave speed.

Evaluation of annealing and double ion beam irradiation by a laser-induced and laser-detected surface acoustic wave diagnostic system

The effects of annealing and double ion irradiation on nuclear structural materials were investigated using a novel, non-destructive, non-contact diagnostic method. A laser-induced and laser-detected surface acoustic wave (SAW) was adopted as a diagnostic system. The SAWs propagation velocity and the SAWs vibration velocity along the normal direction of the surface were measured to investigate mechanical properties of the substrates. Change of the shear modulus was detected in the annealed substrates. Non-linear effect on amplitude of the excited SAW was observed on the double ion irradiated materials. The potential of the SAW diagnostic system for assessing nuclear structural materials was demonstrated.

Laser-induced surface acoustic waves: An alternative method to nanoindentation for the mechanical characterization of porous nanostructured thin film electrode media

The mechanical characterization of electrode materials in thin film lithium ion batteries is currently a sparse area. However, mechanical studies could offer valuable insight since the performance and breakdown of active materials is electromechanically coupled. In this paper, a porous nanostructured V2O5 cathode thin film with demonstrated high electrochemical performance was investigated by a laser-induced surface acoustic wave technique (LiSAW) that mitigates some of the challenges associated with the popular nanoindentation technique. The intent was to explore the capability of LiSAW in measuring the elastic modulus of the nanostructured film such that a reliable methodology could be produced to mechanically characterize challenging electrode materials. LiSAW measured a modulus of 53±4GPa for the porous V2O5 film and had no problems coping with the 40nm roughness and delicate structure. On the other hand, nanoindentation produced a modulus of 50±10GPa, which is comparable to LiSAW, but with considerably higher uncertainty from roughness. For porous nanostructured electrodes, and other challenging films, that are too soft, thin, or delicate for traditional nanoindentation measurements, LiSAW is a potentially excellent alternative. LiSAW testing on many other electrode materials would be instrumental in developing a better understanding between the mechanical and electrochemical properties of thin film battery materials.

Simulation study and guidelines to generate Laser-induced Surface Acoustic Waves for human skin feature detection

Abstract Despite the seriously increasing number of people contracting skin cancer every year, limited attention has been given to the investigation of human skin tissues. To this regard, Laser-induced Surface Acoustic Wave (LSAW) technology, with its accurate, non-invasive and rapid testing characteristics, has recently shown promising results in biological and biomedical tissues. In order to improve the measurement accuracy and efficiency of detecting important features in highly opaque and soft surfaces such as human skin, this paper identifies the most important parameters of a pulse laser source, as well as provides practical guidelines to recommended proper ranges to generate Surface Acoustic Waves (SAWs) for characterization purposes. Considering that melanoma is a serious type of skin cancer, we conducted a finite element simulation-based research on the generation and propagation of surface waves in human skin containing a melanoma-like feature, determine best pulse laser parameter ranges of variation, simulation mesh size and time step, working bandwidth, and minimal size of detectable melanoma.

Laser-induced surface acoustic waves and their detection via diagnostic systems for detecting radiation damage on steel materials of nuclear devices

The development of a non-destructive, non-contact diagnostic system to detect radiation damage is very important for measuring radioactive materials. A system using surface acoustic waves (SAWs) induced and detected by lasers was developed. The propagation velocities of SAWs on stainless steel irradiated by 20keV He and Ar ions were investigated, and a tendency for the velocity to increase with an increase in ion irradiation was observed. This tendency may be due to surface modification. A non-linear effect on ion irradiation versus normal surface velocity in the vertical direction was confirmed.

Correlation Between Laser-Induced Surface Acoustic Waves and Nanoindentation on Elastic Modulus Measurement of a Nanoporous Zeolite Thin Film

Porous thin films are challenging for mechanical characterization due to their soft, thin, and delicate nature. In this study, a nanoporous zeolite thin film is characterized with both laser-induced surface acoustic wave technique (LiSAW) and nanoindentation. The LiSAW technique measured moduli of 38 GPa and 35 GPa for the non-calcined and calcined zeolite films respectively, whereas the respective nanoindentation results of 51 GPa and 47 GPa demonstrated significant substrate influence even at indentation depths within 10 % of film thickness. More accurate indentation results were achieved through an alternative indentation analysis scheme, but at the cost of significant computation time. The results show the distinctive advantage of the LiSAW technique on porous thin films while demonstrating the challenges and current adaptations of traditional nanoindentation.