Leaky Rayleigh Wave Research Articles

Virtual source total focusing method for surface defects using leaky Rayleigh waves

Leaky Rayleigh waves are sensitive to surface defects and beneficial for automated detection due to their non-contact characteristics. However, the amplitude of the leaky Rayleigh waves is low due to the attenuation of waveform conversion and acoustic waves propagation, which limits the imaging quality for long-distance detection. This study introduces a novel method combining leaky Rayleigh waves detection with virtual source total focusing method. The virtual source (VS) technology enhances the emission energy of leaky Rayleigh waves. Then, the total focusing method (TFM) is applied to acquire high-accuracy images of defects. To reduce noise and artifacts, a weighting function based on the coherence factor (CF) is developed to weight the TFM superimposed signals, thereby achieving high-quality image reconstruction. Experimental results show that compared with the conventional TFM method, the proposed method can effectively improve the amplitude of imaging signals while reducing system noise and imaging artifacts. The lateral accuracy of defects is improved, with the average lateral error of defect size being 0.178 mm. The signal-to-noise ratio (SNR) of ultrasound images is increased by 27.59 dB, and the array performance index (API) of ultrasound images is decreased by 33.78 %. The proposed method provides a new and effective approach for quantitatively assessing the surface defects of metal components using leaky Rayleigh waves.

Automated ultrasonic scattering energy method for evaluation of small-sized cracking damage in concrete

The aim of this study was to develop an automatic ultrasonic scattering energy method (USEM) to evaluate small-scale damage to concrete. The automated system measured Leaky Rayleigh waves from the specimens. Herein, the energy of the scattering portion of waves caused by cracks was extracted using a 2D Fourier transform. USEM has the advantage of detecting cracking damage smaller than the applied ultrasonic wavelength, which is problematic when conventional ultrasonic methods are applied to concrete. The proposed methodology was evaluated using laboratory-scale specimens and a concrete bridge exposed to freeze–thaw cycles in the field. The results of the extracted scattering energy demonstrated that the USEM is sufficiently sensitive to detect and localize subwavelength damages of 0.05 mm in concrete. Furthermore, the proposed USEM was compared with conventional ultrasonic pulse velocity with respect to the accuracy of damage detection.

Imaging method of surface defect using leaky Rayleigh wave with ultrasonic phased array

ABSTRACT Leaky Rayleigh wave testing has the advantages of non-contact, high sensitivity, and good beam directivity. However, current leaky Rayleigh wave testing mostly uses monolithic transducers and is seriously affected by acoustic wave attenuation and diffusion, resulting in poor image quality, limited detection range and low imaging efficiency. In this paper, a novel leaky Rayleigh wave testing method using an ultrasonic phased array is developed, and the total focusing method (TFM) is used to achieve high-quality imaging of defects. To improve imaging efficiency, the root-mean-square velocity (RMSV) model is utilised to rapidly acquire an approximate analytical solution for the time of flight of the acoustic wave. To obtain more accurate defect images further, the amplitude of the total focusing imaging signal is calibrated. The experimental results demonstrate that the proposed method achieves a 48% improvement in the imaging accuracy of a 1 mm hole, compared to the traditional Synthetic Aperture Focusing Technique (SAFT) imaging method. The proposed method achieves an 92% reduction in imaging time compared to the conventional TFM. The proposed method provides a new non-contact approach for high-accuracy and high-efficiency defect imaging using leaky Rayleigh wave.

Development of non-contact ultrasonic IoT system for monitoring hardening process of concrete

The objectives of this study are to develop a non-contact ultrasonic IoT (Internet of Things) system and to investigate its applicability in monitoring the hardening process of cementitious materials. The conventional measurement methods, which include contact or embedded sensors with wires, have limited applicability on complex construction sites. Therefore, in this study, we propose the non-contact ultrasonic method for identifying the properties of cementitious materials using the characteristics of the leaky Rayleigh waves and demonstrate the effectiveness of an IoT system in improving the limitations of wired systems. The apparatus developed includes an ultrasonic transmitter, sensor board and operating software. First, we demonstrated that the non-contact ultrasonic IoT system delivers commands up to a distance of 20 m at a Wi-fi strength greater than −70 dBm. Furthermore, experimental validation was performed on three different cementitious materials (cement paste, mortar, and concrete) to investigate the applicability of the system.

Thermal and mechanical variations of cementitious composites with phase change materials using a contactless ultrasonic method

Recently, research on reducing building energy by applying phase change materials (PCMs), such as n-octadecane, to cementitious materials, is being actively conducted. PCMs were modified as a type of lightweight aggregate (LWA) to prevent leakage in concrete. In this study, a fully contactless ultrasonic system was applied to monitor thermal and mechanical variations of concrete plates including PCM-LWAs subjected to temperatures up to 48 °C. Because surface preparation and direct coupling processes were not required, this method collected reliable information without thermal interference. The measurements were conducted during the melting and free cooling process of PCM-LWAs, where the methods monitored the variation of leaky Rayleigh waves for dynamic modulus changes of the solid material. The experimental results illustrated that the contactless ultrasonic system enabled a 5.5 % reduction in dynamic modulus for a concrete plate mixed with 160 kg/m3 PCM-LWAs, and showed the time lag effect of heat transfer and surface temperature reduction through heat storage performance. Further comparison studies of the dynamic modulus variation among different mix proportions of PCM-LWA concrete plates were conducted using temperature cycles. Based on the experimental study, the unique thermal and mechanical behaviors of PCM-LWA mixed concrete plates were discussed.

Shock waves generated by toroidal bubble collapse are imperative for kidney stone dusting during Holmium:YAG laser lithotripsy

Holmium:yttrium–aluminum-garnet (Ho:YAG) laser lithotripsy (LL) has been the treatment of choice for kidney stone disease for more than two decades, yet the mechanisms of action are not completely clear. Besides photothermal ablation, recent evidence suggests that cavitation bubble collapse is pivotal in kidney stone dusting when the Ho:YAG laser operates at low pulse energy (Ep) and high frequency (F). In this work, we perform a comprehensive series of experiments and model-based simulations to dissect the complex physical processes in LL. Under clinically relevant dusting settings (Ep = 0.2 J, F = 20 Hz), our results suggest that majority of the irradiated laser energy (>90 %) is dissipated by heat generation in the fluid surrounding the fiber tip and the irradiated stone surface, while only about 1 % may be consumed for photothermal ablation, and less than 0.7 % is converted into the potential energy at the maximum bubble expansion. We reveal that photothermal ablation is confined locally to the laser irradiation spot, whereas cavitation erosion is most pronounced at a fiber tip-stone surface distance about 0.5 mm where multi foci ring-like damage outside the thermal ablation zone is observed. The cavitation erosion is caused by the progressively intensified collapse of jet-induced toroidal bubble near the stone surface (<100 μm), as a result of Raleigh-Taylor and Richtmyer-Meshkov instabilities. The ensuing shock wave-stone interaction and resultant leaky Rayleigh waves on the stone surface may lead to dynamic fatigue and superficial material removal under repeated bombardments of toroidal bubble collapses during dusting procedures in LL.

Contactless estimation of soil moisture using leaky Rayleigh waves and a fully convolutional network

AbstractSoil moisture is a key factor that influences various aspects of ecosystem functioning. Measuring soil moisture without installing any objects in the soil is desirable because it allows for accurate characterizations of soil moisture while minimizing impacts on soil structure and ecology. In this study, we explored the potential of leaky Rayleigh waves as a proxy to contactlessly estimate soil moisture. We developed an ultrasonic system containing a transducer, receivers, and acoustic barrier. The specimens of sand, silt, and clay were utilized. Experiments were conducted over 4 months. We used a widely used soil‐embedded moisture sensor to compare and develop relationships between leaky Rayleigh waves and soil moisture. Our results showed that as soil moisture increased, the velocity and amplitude of leaky Rayleigh waves decreased because water molecules attracted to the soils led to their attenuation. However, their magnitudes were not considerable except for very dry soils. To overcome these limited relations to estimate soil moisture from leaky Rayleigh waves, we constructed authentic images based on the observed leaky Rayleigh waves and used them as inputs for a fully convolutional network. We found that the combination of the ultrasonic system and deep learning approach developed in this study were suitable for estimating soil moisture without soil disturbances (RMSE = 0.01 m3 m−3). This study suggests that leaky Rayleigh waves have the potential to serve as a reliable proxy for determining soil moisture without the need for physical contact.

Controlled inertial nano-cavitation above 100 MHz

The maximum size and lifetime of an acoustically nucleated cavitation bubble is inversely proportional to the driving frequency and has achieved a limit of approximately 10 MHz. Smaller cavitation bubbles that are critical to microscopic applications require shorter lifetimes that correspond to higher oscillation frequencies. Here, we demonstrate that acoustic cavitation in the 100 MHz range and beyond can be achieved through wave propagation in a solid rather than in a liquid. The cavitation bubble is nucleated at a nano-sized fracture on a glass substrate, and its expansion is driven by a leaky Rayleigh wave, while the inertial collapse is induced by a trailing shock wave. As both waves travel at different velocities, the time interval between these two events is a function of the distance to the source. In this way, we demonstrate experimentally control of the lifetime of the bubbles in a range between 6 and 80 ns, corresponding to oscillation frequencies between 13 and 166 MHz. Our results agree with finite-volume fluid–structure interaction simulations.

Attenuation and dispersion of leaky Rayleigh wave in polycrystals.

In this work, we use the characteristic equation of leaky Rayleigh waves (LRWs) and a unified approach of bulk waves proposed by Stanke and Kino [J. Acoust. Soc. Am. 75, 665-681 (1984)] to calculate the attenuation and velocity dispersion of LRWs in polycrystals. Numerical results demonstrate that the total attenuation including the leakage attenuation and scattering attenuation is proportional to frequency and independent of grain size in the Rayleigh scattering regime. Meanwhile, the variation of phase velocity in all scattering regimes remains at ∼0.7% according to the theoretical expectation; this means that the velocity dispersion of the LRWs can be ignored, consistent with the conventional viewpoint. Measurements are conducted on stainless steel at different ultrasonic frequencies (all in the Rayleigh scattering regime). The non-paraxial sound field model is used here to eliminate the diffraction loss and to obtain the total attenuation. Experimental results verify that LRWs have very little velocity dispersion. Meanwhile, experimental fitting data reveal that the modified theoretical model can be used to evaluate the total attenuation (only ∼2% discrepancies) of LRWs under the consideration of the diffraction effect. The relative errors between experimental scattering attenuation and theoretical value ranged from 11% to 18%, mainly owing to the effect of surface roughness and measurement inaccuracy.

A Novel and Non-Invasive Approach to Evaluating Soil Moisture without Soil Disturbances: Contactless Ultrasonic System.

Soil moisture has been considered a key variable in governing the terrestrial ecosystem. However, it is challenging to preserve indigenous soil characteristics using conventional soil moisture monitoring methods that require maximum soil contacts. To overcome this issue, we developed a non-destructive method of evaluating soil moisture using a contactless ultrasonic system. This system was designed to measure leaky Rayleigh waves at the air–soil joint-half space. The influences of soil moisture on leaky Rayleigh waves were explored under sand, silt, and clay in a controlled experimental design. Our results showed that there were strong relationships between the energy and amplitude of leaky Rayleigh waves and soil moisture for all three soil cases. These results can be explained by reduced soil strengths during evaporation processes for coarse soil particles as opposed to fine soil particles. To evaluate soil moisture based on the dynamic parameters and wave properties obtained from the observed leaky Rayleigh waves, we used the random forest model. The accuracy of predicted soil moisture was exceptional for test data sets under all soil types (R ≥ 0.98, RMSE ≤ 0.0089 m m). That is, our study demonstrated that the leaky Rayleigh waves had great potential to continuously assess soil moisture variations without soil disturbances.

Application of a Wireless and Contactless Ultrasonic System to Evaluate Optimal Sawcut Time for Concrete Pavements.

A recently developed contactless ultrasonic testing scheme is applied to define the optimal saw-cutting time for concrete pavement. The ultrasonic system is improved using wireless data transfer for field applications, and the signal processing and data analysis are proposed to evaluate the modulus of elasticity of early-age concrete. Numerical simulation of leaky Rayleigh wave in joint-half space including air and concrete is performed to demonstrate the proposed data analysis procedure. The hardware and algorithms developed for the ultrasonic system are experimentally validated with a comparison of saw-cutting procedures. In addition, conventional methods for the characterization of early-age concrete, including pin penetration and maturity methods, are applied. The results demonstrated that the developed wireless system presents identical results to the wired system, and the initiation time of leaky Rayleigh wave possibly represents 5% of raveling damage compared to the optimal saw cutting. Further data analysis implies that saw-cutting would be optimally performed at approximately 11.5 GPa elastic modulus of concrete obtained by the wireless and contactless ultrasonic system.

Assessment of degradation index in freeze-thaw damaged concrete using multi-channel contactless ultrasound

• Efficient data acquisition framework for contactless ultrasonic testing utilizing multi-channel MEMS array. • Introduction of Degradation Index based on leaky Rayleigh waves with increasing freeze-thaw damage in concrete. • Numerical and experimental Validations with data obtained from freeze-thaw damaged concrete. • Improvement of surface damage detection with ultrasound scanning compared to the point measurement-based methods. Concrete is among the most widely used construction materials, especially in national infrastructure such as bridges, dams, and ports. This material fulfills an important role in ensuring the durability of structures that incorporate it. With recent climate change, issues related to degradation of concrete resulting from combined deterioration, e.g., freeze-thaw damage and chloride attack, have been increasingly reported, and, accordingly, there have been many studies focusing on the assessment of concrete durability using non-destructive testing. Non-contact ultrasonic testing measures leaky Rayleigh waves propagating through concrete, where the measurement procedure is a fully non-contact manner with the help of advanced sophisticated MEMS (Microelectromechanical systems) hardware technology. In the present study, a 64-channel non-contact ultrasonic system was developed to assess freeze-thaw damage of concrete elements, and an algorithm to assess concrete damage based on the velocities of leaky ultrasonic waves, the degradation index (DI), was proposed. The proposed system and algorithm were verified through a numerical analysis and experiments with varying degrees of freeze-thaw damage. The numerical analysis results showed that the velocity of ultrasonic waves, along with the degree of degradation, decreased with an increasing simulated damage ratio. The experimental freeze-thaw test results also confirmed that the DI was more sensitive to damage from the initial freeze-thaw cycles compared to the existing evaluation indexes, such as the relative dynamic elastic modulus.

Setting time in Roller-Compacted Concrete (RCC)

The principal differences of RCC from conventional concrete pavements are workability, rheology, paste content, and compactive effort. RCC pavements obtain significant strength during compaction and, therefore, can be opened to traffic in less time than conventional concrete pavements. For both types of concrete, there are methods to evaluate workability in the fresh state, e.g., slump and Vebe test. However, no standard method is available for either concrete pavement type for evaluating setting time in-situ. Construction activities such as tining/texturing, curing membrane, and sawing of contraction joints are related to the concrete setting time. A recently developed technique for setting time of conventional mortar and concrete with fully contactless ultrasound has shown promising results with higher paste content mixtures. This recently proposed method monitors the initiation time of leaky Rayleigh waves (LR-wave) signals in time and space over the conventional concrete surface. The objective of this study is to determine the setting time in RCC pavements which has significantly lower paste volume and higher aggregate content. Preliminary results have shown that the setting time of an RCC mixture with overfilled paste can be successfully determined. Additional tests on a wider range of RCC mixture constituents and proportions, e.g., lower aggregate void and paste contents, are being conducted to evaluate the limitations of this contactless sensing method and necessary adjustments. Reliable set time determination in RCC pavements can improve QC/QA practices and indicate repeatable triggers for other construction activities like sawing contraction joints.

Early Age Monitoring of High Cement Replacement Mixtures for Pavement

Decarbonization of concrete will require a combination of alternative binding materials and higher cement replacement rate with supplementary cementitious materials. Developing tools and methods that help engineers evaluate the properties of early age concrete pavement, such as setting time and strength gain, will be necessary to adopt new concrete mixes. Two concrete pavement test sections were constructed, a high early strength concrete mix with 25% (control) and 40% replacement of cement with fly ash (HVFAC), along with monitoring of the concrete’s setting time, saw-cut timing, and strength gain. A non-contact ultrasonic device was used to estimate setting time through measuring leaky Rayleigh wave energy transmission. The laboratory setting time of the control and HVFAC mix measured with the non-contact device was 5.5 h and 15 h, respectively, and agreed with measured isothermal calorimetry results. Further calorimeter tests showed that adding an accelerating admixture or replacing part of the cement with nano-limestone decreased the HVFAC setting time up to 4.4 h. The field setting time of the control mixture with the non-contact device measured 4.2 h, which was shorter than the laboratory estimate. Based only on the experience of construction personnel, saw-cutting for these mixtures was initiated too early and caused significant joint raveling, reinforcing the importance of in situ setting time measurement. The maturity method was successfully implemented with embedded wireless temperature sensors that rapidly and easily estimated the in-place compressive strength and improved opening time determination for concrete with high cement replacement levels, which are sensitive to the volume of supplementary cementitious materials, admixtures, and ambient conditions.

Propagation of leaky Rayleigh waves along a curved fluid-solid interface.

A characteristic equation is derived for a leaky Rayleigh wave (LRW), propagating on a curved fluid-solid interface. The equations of motion for the curved solid and fluid are formulated using the constitutive equations of a homogenous isotropic curved solid and an inviscid fluid, respectively. The displacement potential functions are used to simplify the derivation. The interface conditions are used to ensure continuities of the mass, momentum, and energy across the interface. Then, with the consideration of the interface radius of the curvature, the characteristic equation for the LRW is established and solved numerically by Muller's method. One important outcome is that there is weaker directional dependence for the velocity of the LRWs on the radius of curvature in comparison with the Rayleigh waves at an air-solid interface. However, the numerical results show a strong directional dependence for the attenuation due to the LRW leakage on the complex curvatures. Moreover, a quantitative relation between the curvature and attenuation caused by the leakage for different materials is shown. The results are significant especially with respect to relevant future applications of ultrasonic testing.

Surface acoustic waves and Mach stem formation produced by lithotripter shock wave-stone interaction

Lithotripter shock wave (LSW)-stone interaction may vary significantly due to respiratory motion of the patient. In this work, we have investigated the variations of the LSW-stone interaction and associated stone fracture pattern using photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to the T-shaped fracture pattern observed in a disk-shaped stone under symmetric loading, we have observed a gradual transition of the fracture pattern to a tilted L-shape under asymmetric loading. Importantly, our model simulations reveal the generation of surface acoustic waves (SAWs)—a leaky Rayleigh wave on the anterior and a Scholte wave on the posterior surface of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral are produced by SAWs on the stone boundary under asymmetric loading, which drive the initiation and extension of surface cracks into the bulk of the stone that has been confirmed by microCT analysis.

Variations of stress field and stone fracture produced at different lateral locations in a shockwave lithotripter field

During clinical procedures, the lithotripter shock wave (LSW) that is incident on the stone and resultant stress field is often asymmetric due to the respiratory motion of the patient. The variations of the LSW-stone interaction and associated fracture pattern were investigated by photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to a T-shaped fracture pattern often observed in the posterior region of the disk-shaped stone under symmetric loading, the fracture pattern gradually transitioned to a tilted L-shape under asymmetric loading conditions. Moreover, the model simulations revealed the generation of surface acoustic waves (SAWs), i.e., a leaky Rayleigh wave on the anterior boundary and Scholte wave on the posterior boundary of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development and growth of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral were produced by SAWs on the stone boundary under asymmetric loading conditions, which drove the initiation and extension of surface cracks into the bulk of the stone that is confirmed by micro-computed tomography analysis.

Monitoring Hardening Behavior of Cementitious Materials Using Contactless Ultrasonic Method.

We propose a novel contactless ultrasonic method for monitoring the hardening behavior of cementitious materials. The goal of this method is to obtain high-quality data to compare the unique hardening process between rapid setting cement (RSC) and ordinary Portland cement (OPC) mortars without physical coupling to the surface of the specimens. To monitor the hardening behavior of cementitious materials, conventional approaches use contact or embedded-type sensors, which limit field application. Our solution is to measure leaky Rayleigh waves at the interface between air and cementitious materials, which allows for the estimation of the physical state of the medium in real time. The modulus development was back-calculated based on the increment of wave velocity using the developed sensor array and transform-based signal processing. We experimentally demonstrated that the proposed method possibly exhibits unique hardening information about flash setting, effects of a retarder, and modulus increments from RSC specimens.

PCA-WHMM-based Frequency-domain Synthetic Aperture Focusing Imaging Using Ultrasonic Leaky Rayleigh Waves

PCA-WHMM-based Frequency-domain Synthetic Aperture Focusing Imaging Using Ultrasonic Leaky Rayleigh Waves

Study on PCA-SAFT imaging using leaky Rayleigh waves

Efficient and accurate nondestructive testing of surface or sub-surface flaws is essential for metal components. This study focuses on the high-quality image reconstruction of these flaws. An imaging detection method is introduced which combines the leaky Rayleigh waves testing and the synthetic aperture focusing technique (SAFT) with an immersion pulse-echo scanning. The principal component analysis (PCA) is used to reduce the noise of the leaky Rayleigh waves, and then the imaging is obtained using the SAFT algorithm. The experimental results show that the PCA-SAFT imaging method can reduce structural noise and imaging artifacts comparing with the B-scan imaging method. The lateral resolution of defects is improved and the mean error of defects sizing can be reduced by 48.81%. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the ultrasonic image are increased by 6.95 dB and 4.20 dB respectively. The proposed method can be a new choice for effective evaluating the surface quality of key components.