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This article investigates the interactions of two-plane waves in weakly nonlinear elastic solids containing quadratic and cubic nonlinearity. The analytical solutions for generated combined harmonic waves are derived using the Green's function approach applied to a generated system of quasi-linear equations of motion. Wave mixing solutions are obtained and include shape functions that permit closed-form solutions for a variety of interaction geometries. An explicit example is highlighted for a spherical interaction volume assuming isotropic elastic constants. Several parameters of the generated field after mixing are analyzed including resonant and nonresonant mixing, the role of interaction angle, and the frequencies of the two incident waves. Wave mixing offers the potential for sensing localized elastic nonlinearity and the present model can be used to help design experimental configurations. [ABSTRACT FROM AUTHOR]

This work presents a peri-ultrasound theory based on ordinary state-based peridynamics for modeling elastic waves propagating in three-dimensional (3-D) plate structures and interacting with multiple cracks. A recently developed nonlinear ultrasonic technique called sideband peak count-index (or SPC-I) is adopted for monitoring one or more cracks with thickness values equal to 0 mm (crack-free), 1, 2, and 4 mm. Three separate scenarios—one crack, two cracks, and four cracks in 3-D plate structures—are investigated. These cracks can be classified as thin and thick cracks depending on the horizon size, which is mentioned in peri-ultrasound theory. Computed results for all three cases show larger SPC-I values for thin cracks than for thick cracks and the case of no cracks. This observation is in line with the previously reported results in the literature and proves that the state-based peri-ultrasound theory can capture the expected nonlinear response of elastic waves interacting with multiple cracks without changing the cracks' surface locations artificially, and this is always needed in most of the other numerical methods. The proposed state-based peri-ultrasound theory is more flexible and reliable for solving 3-D problems, and the out-of-plane wave field can be obtained for engineering analysis. [ABSTRACT FROM AUTHOR]

The ultrasonic measurement model (UMM) has been theoretically studied over the past few decades, but there is little published work on its practical application for guiding defect detection. In this work, we present a model-based ultrasonic method to detect flaws in curved components using immersion of spherically focused transducers. The UMM of focused transducers is further investigated theoretically and verified experimentally, and some key issues in the model with focused transducers are addressed, including the necessity of correctly using effective geometrical parameters and the corresponding transfer function. The UMM is employed to guide the inspection of a magnesium alloy shell component which consists of several curved surfaces. The alignment between the transducer and the shell is conducted by comparing the measured and theoretically predicted interface-echo signals, the water path is selected by simulating the effective detection region, and the system gain is set by quantitatively analysing the amplitudes of flaw signals and noises. When the inspection conditions are determined, a variable threshold based on the predicted flaw signals is introduced to distinguish noise and flaw signals. The ultrasonic C-scan images of the shell are compared with the digital radiography (DR) images, and the comparison shows that the flaws of interest can be detected in the C-scan images, which verifies the effectiveness of the UMM-based detection method. [ABSTRACT FROM AUTHOR]

The responsivity of an ultrasonic transducer is an important parameter in evaluating its effective frequency band, the electroacoustic conversion efficiency, and the measurement capability of the system. The determination of the responsivity of a traditional immersion or contact piezoelectric transducer has been well investigated. However, due to the high attenuation of waves propagating in air and the large acoustic impedance mismatch between the active piezoceramic material and the load medium, there are few reports of the calibration of an air-coupled piezoelectric transducer. In this work, we present a comparative method of measuring the responsivity of an air-coupled transducer: the air-coupled transducer is used to receive a broadband pulse signal to evaluate its frequency spectrum, and a toneburst signal with known vibration displacement is measured by the air-coupled transducer in order to calibrate the amplitude of the responsivity. The effects of transmitter responsivity, input pulse characteristics, attenuation, and diffraction are taken into account to improve the accuracy of the responsivity determination. In addition, the measurement of the amplitude of the responsivity by comparing the measured displacements avoids the complicated task of characterizing the effects of electrical equipment. The determined responsivity is checked by comparing the measured displacements using different methods at different frequencies in order to evaluate its frequency spectrum and by measuring the nonlinearity parameters of the material to evaluate its amplitude. The agreement between results obtained using different methods demonstrates that the calibrated responsivity of the air-coupled transducer is valid, and the proposed method is effective. [ABSTRACT FROM AUTHOR]

In this study, the properties of Rayleigh wave fields scattered at a surface from open vertical cracks with different shapes are investigated. The scattered Rayleigh wave is determined by both the incident Rayleigh wave and its interaction with the crack, and the wave fields are modeled using the Green's function method based on the reciprocity theorem. The reflection coefficient of the Rayleigh wave in two-dimensional coordinates is employed to simplify the area integration over the crack surface for the scattered Rayleigh wave to a line integration over the length of the crack. Numerical simulations are carried out, and the effects of the crack length and depth on the scattered Rayleigh wave fields are discussed. The results indicate that the distribution of a scattered Rayleigh wave field is mainly determined by the crack length, while the amplitude is closely related to the crack depth. Since the reflection coefficient becomes almost stable when the ratio of the crack depth to the Rayleigh wavelength is larger than 1, the variation in the scattered Rayleigh wave field is insignificant for cracks with the same length but varying depths. This work will be helpful for solving the inverse problem of evaluating crack geometry using ultrasonic Rayleigh waves and for further work on Rayleigh waves scattered by cracks with different directions of extension and angles with the surface. [ABSTRACT FROM AUTHOR]

• Crack propagation is modeled using ordinary state-based peridynamics theory. • Waves propagating and interacting with cracks are simulated using peri‑ultrasound modeling. • SPC-I technique is used to extract the nonlinear response due to crack propagation. • SPC-I is shown to be useful for analyzing interactions between propagating cracks and ultrasonic waves. This work presents a nonlocal theory based on ordinary state-based (OSB) peridynamics for numerical modeling of crack propagation under loadings and peri‑ultrasound modeling for elastic wave propagation that interacts with generated cracks in 3-D plate structures. Propagating cracks in plate structures for two types of loading – tensile and shear are considered. For both loading cases, at six different times after the start of the loading the analysis is carried out to investigate how the results vary for these six different stages of crack propagation. An ultrasonic excitation is applied at a point on the surface of the plate to generate guided waves propagating through the plate structure and interacting with generated cracks at different stages of crack formation. The out-of-plane velocity fields are recorded at the receiving material point for further analysis. A relatively new and promising nonlinear ultrasonic (NLU) technique called sideband peak count-index (or SPC-I) is adopted to extract the nonlinear response due to the generated cracks. Results show that when the crack propagation is at its early stage the SPC-I value first increases, and then decreases with time at the advanced stages of crack propagation. This is because for both types of loading (tensile or shear) the newly generated cracks are small cracks (or micro-cracks) in the beginning and then they evolve into macro-cracks under increasing loadings. The observation for SPC-I trends is consistent with other numerical modelings and experimental observations reported in the literature, and such qualitative matching gives confidence in the proposed method. The proposed method combining the peri‑ultrasound modeling and SPC-I technique presented in this paper can provide a new insight for monitoring crack propagation and give some guidance for experimental structural health monitoring (SHM) applications. [ABSTRACT FROM AUTHOR]

With respect to harmonic generation measurements in isotropic materials, the amplitude of the third-harmonic wave generally depends on both $\beta ^{2}$ (the square of the second-order nonlinear parameter $\beta $) and $\gamma $ (the third-order nonlinear parameter). Therefore, some care should be taken when measuring these parameters using the third-harmonic amplitude. The purpose of this paper is to investigate detailed theoretical and measurement techniques for the accurate determination of $\beta ^{2}$ which dominates the third-harmonic amplitude in most biological fluids and crystalline solids. The theory deals with harmonic generation in materials with cubic nonlinearity and defines the formula for measuring $\beta ^{2}$ with corrections for diffraction and attenuation. These corrections are derived from the Westervelt equation and play an important role in the measurement of nonlinear parameters. The third-harmonic amplitude that varies with the distance in water is obtained for the measurement of $\beta ^{2}$. $\beta $ is also measured from the second-harmonic amplitude for comparison. We also confirm the required input voltage to stably generate the third harmonic and discuss the effects of diffraction and attenuation correction on the $\beta ^{2}$ determination. The measured $\beta ^{2}$ in the propagation range of 0.05–0.2 m agrees well with the square of the measured $\beta $ , revealing that the third-harmonic amplitude is closely related to $\beta ^{2}$ , not $\gamma $. This paper covers comprehensive theories, experimentation, and analysis related to the measurement of third-harmonic generation in isotropic media and can be immediately applied to crystalline solids. [ABSTRACT FROM AUTHOR]

Determination of the sensitivity of a transducer is essential in evaluating its central frequency and effective bandwidth, its electroacoustic conversion capability, or the measurement ability of an ultrasonic test system. In this work, a calibration method based on self-reciprocity is proposed for the determination of transducer sensitivity, which can be applied to both planar and focused transducers. The two-port electrical network of the experimental setup is analyzed, and a simplified measurement procedure is described in which the “impedance mismatch” problem is solved, and only input and output currents are needed. An acoustic transfer function is introduced, both to reduce the effects of wave energy loss on the determination of transducer sensitivity and to help extract the effective geometrical parameters of the transducer through these measured output current signals. The effects of diffraction, attenuation, and effective geometrical parameters on sensitivity determination are discussed in this work; the experimental results show that when all these factors are taken into account, accurate sensitivities for both planar and focused transducers can be obtained at various experimental distances. [ABSTRACT FROM AUTHOR]

• An ultrasonic high throughput method is developed for SLM manufactured elements. • Both pore parameters and Young's modulus can be determined in a single experiment. • The log-normal distribution is introduced to obtain quartiles of pore parameters. • The results of other methods reveal the effectiveness of this work. Both pore parameters and elastic properties are important for the elements manufactured by the Selective Laser Melting (SLM) technology; however, their measurements are usually destructive and time-consuming. Thus, the ultrasonic longitudinal and transverse wave velocities (dual-mode velocities) method is extended here; then it is combined with the longitudinal wave attenuation to develop an ultrasonic high throughput rapid detection for SLM manufactured elements. As the focal transducer is used, a multi-Gaussian beam model is introduced to correct the diffraction attenuation. Twenty-eight parameters can be obtained by a single ultrasonic experiment, including the pore parameters and their distributions, the two-point spatial correlation function, and the Young's modulus. Ti-6Al-4V samples with different porosities manufactured by SLM technology are used to conduct the ultrasonic experiment. The experimental results agree well with the results of metallographic, tensile experiment and micro-CT results. This method provides a useful tool for nondestructively, rapidly and accurately evaluating the pore parameters and elastic properties of SLM manufactured elements. [ABSTRACT FROM AUTHOR]

Although there are methods to machine free-form surfaces, serious distortion in the concave–convex characteristic of the flattened-plane boundary, high deformation of the surface geometry, and single limitation of the surface topology are usually produced. Thus, a novel surface flattening method is proposed in this paper to retain the concave–convex characteristic and reduce the deformation, and a spiral path is generated to machine the free-form surface with various topologies. The machined surface is mapped to a planar region with a free boundary using a fusing constraint mapping method. First, the spring-mass-based stretching constraint is used to minimize the length differences of the triangular sides, which are caused by surface flattening. Subsequently, in order to flatten surfaces with an isometric deformation, we perform this operation under the constraint of hinge-based bending. Eventually, the global constraint, the minimization of the global energy, is employed to acquire a less distorted plane. Then, to generate a planar spiral path fit for machining various planes, which are concave, convex, simply-connected, or multiply connected, enrichment of the conventional Voronoi diagram, interpolation between the wave fronts, and rounding of the spiral polyline are implemented. For machining a free-form surface, by inversely mapping the planar path, a spiral tool path is planned. Experimental results are given to illustrate the effectiveness of the presented methods. [ABSTRACT FROM AUTHOR]

• Effects of cracks' geometry and orientations on nonlinear response is investigated. • State-based peri-ultrasound and SPC-I can capture cracks induced nonlinearities. • Thin cracks show higher SPC-I values than that of thick cracks. • Crack evolution process can be effectively monitored with the proposed method. • The proposed method can provide useful guidance for experimental SHM. Since it is almost impossible to carry out a comprehensive parametric investigation experimentally for internal cracks with different geometry and orientation, a good numerical modeling and simulation technique is necessary to have a clear understanding of the physics of wave propagation and its interaction with cracks. Such investigation is helpful for structural health monitoring (SHM) with ultrasonic techniques. This work presents a nonlocal peri-ultrasound theory based on ordinary state-based (OSB) peridynamics for modeling elastic wave propagation in three-dimensional (3-D) plate structures containing multiple cracks. A relatively new and promising nonlinear ultrasonic technique called S ideband P eak C ount – I ndex (or SPC-I) is adopted to extract the nonlinearity generated from the interactions between elastic waves and multiple cracks. Effects of three main parameters – the distance between the acoustic source and the crack, the crack spacing and the number of cracks are investigated using the proposed OSB peri-ultrasound theory together with the SPC-I technique. For each of these three parameters investigation, different crack thicknesses were considered − 0 mm (crack-free), 1 mm (thin crack), 2 mm (intermediate thickness) and 4 mm (thick crack); thin and thick cracks are defined after comparing the crack thickness value with the horizon size mentioned in the peri-ultrasound theory. It is found that for obtaining consistent results the acoustic source should be placed at least one wavelength away from the crack and crack spacings also play an important role on the nonlinear response. It is concluded that the nonlinear response diminishes when the cracks become too thick, and thin cracks show higher nonlinearity than that of thick cracks and no cracks. Finally, the proposed method which is combining the peri-ultrasound theory and SPC-I technique is used for monitoring cracks' evolution process. The numerical modeling results are compared with the experimental findings reported in the literature. Consistent qualitative trends in SPC-I variations predicted numerically and obtained experimentally are observed, thus it gives confidence in the proposed method. [ABSTRACT FROM AUTHOR]

Angle beam wedge transducers are widely used in nonlinear Rayleigh wave experiments as they can generate Rayleigh wave easily and produce high intensity nonlinear waves for detection. When such a transducer is used, the spurious harmonics (source nonlinearity) and wave diffraction may occur and will affect the measurement results, so it is essential to fully understand its acoustic nature. This paper experimentally investigates the nonlinear Rayleigh wave beam fields generated and received by angle beam wedge transducers, in which the theoretical predictions are based on the acoustic model developed previously for angle beam wedge transducers [S. Zhang, et al., Wave Motion, 67, 141–159, (2016)]. The source of the spurious harmonics is fully characterized by scrutinizing the nonlinear Rayleigh wave behavior in various materials with different driving voltages. Furthermore, it is shown that the attenuation coefficients for both fundamental and second harmonic Rayleigh waves can be extracted by comparing the measurements with the predictions when the experiments are conducted at many locations along the propagation path. A technique is developed to evaluate the material nonlinearity by making appropriate corrections for source nonlinearity, diffraction and attenuation. The nonlinear parameters of three aluminum alloy specimens – Al 2024, Al 6061 and Al 7075 – are measured, and the results indicate that the measurement results can be significantly improved using the proposed method. [ABSTRACT FROM AUTHOR]

In order to increase cutting width, decrease tool path length, and improve machining efficiency, the spiral enter assistant line (SEAL) is embedded into a convex pocket to plan a spiral tool path. First, the inner offset curve of the pocket boundary undergoes equidistant division to acquire the external equidistant points. Subsequently, principal component analysis (PCA) and length factors optimization are employed to obtain SEAL. Next, vector operation is performed on the external equidistant points and SEAL to calculate the corresponding internal points. After connecting the external equidistant points and the corresponding internal points, a B-spline spiral tool path is planned based on linear interpolation and B-spline curve fitting. In addition, the length factors can be adjusted to modify the distribution and length of the tool path. Theoretical analysis and machining experiments demonstrate that compared to other conventional algorithms, the spiral tool path presented in this study has obvious advantages on cutting width, tool path length, and machining efficiency. These advantages are especially pronounced when the major principal axis is much longer than the minor principal axis. [ABSTRACT FROM AUTHOR]

Surface roughness of a sample has a great effect on the calculated grain size when measurements are based on ultrasonic attenuation. Combining modified transmission and reflection coefficients at the rough interface with a Multi-Gaussian beam model of the transducer, a comprehensive correction scheme for the attenuation coefficient is developed. An approximate inverse model of the calculated attenuation, based on Weaver’s diffuse scattering theory, is established to evaluate grain size in polycrystals. The experimental results showed that for samples with varying surface roughness and matching microstructures, the fluctuation of evaluated average grain size was ±1.17 μm. For polished samples with different microstructures, the relative errors to optical microscopy were no more than ±3.61%. The presented method provides an effective nondestructive tool for evaluating the grain size in metals with rough surfaces. [ABSTRACT FROM AUTHOR]

To improve the machining efficiency and cutting force stability for milling a pocket with an island, a lofting-based spiral tool path generation algorithm is presented. First, the machining area undergoes partition based on lofting lines through the profile vertices, and isogonal lofting lines are assigned according to the sub-area angular weight. Linear interpolation is proposed to get equal-spaced points between internal and external lofting points, and these interpolation points are circumferentially connected to form a spiral polyline. Next, control vertices that follow an exponential function distribution are inserted. Using these control vertices, a B-spline curve of any order can be defined. Finally, a spiral tool path for machining a pocket with an island is obtained. The algorithm can control the machining efficiency and machining precision by adjusting shape factor, lofting line number, and B-spline order according to specific processing requirements, and it can be directly applied to a machining center with a Non-uniform rational B-spline (NURBS) interpolation function. The experimental results show that, compared to contour-parallel, the C continuous B-spline curve tool path has an obvious advantage on tool path length, machining efficiency, cutting force continuity, and workpiece surface quality. [ABSTRACT FROM AUTHOR]

The pulse-echo nonlinear acoustic wave fields generated by focused transducers are calculated numerically and verified experimentally. Simulation results show that in the reflection direction, phase differences between newly generated second harmonic and reflected waves are about π/2, regardless of the rigid and stress-free boundaries; thus, the total second harmonic wave will neither be strongly enhanced nor be cancelled. Experiments are conducted with a focused transducer working in pulse-echo mode and the measured amplitudes of nonlinear waves reflected after water-air and water-aluminum/steel interfaces agree with the simulated results. This work helps advance applications using pulse-echo nonlinear acoustics. [ABSTRACT FROM AUTHOR]

Ultrasonic wave propagation in curved components was simulated based on the Multi-Gaussian beam theory to eliminate the negative influence of the surface curvature and water path on grain size attenuation evaluation. The multi-frequency weighted attenuation evaluation model with diffraction correction was introduced into the measured attenuation spectrum to control the systematic error. The experimental results show that for 6 blocks with different curvatures and the same mean grain size, the relative error was reduced from 15.41% to 4.28% by using the presented method, and the standard deviation of the error was 15.92% of that of the traditional method. [ABSTRACT FROM AUTHOR]

A more general two-dimensional wave motion equation with consideration of attenuation and nonlinearity is proposed to describe propagating nonlinear Rayleigh waves of finite amplitude. Based on the quasilinear theory, the numerical solutions for the sound beams of fundamental and second harmonic waves are constructed with Green's function method. Compared with solutions from the parabolic approximate equation, results from the general equation have more accuracy in both the near distance of the propagation direction and the far distance of the transverse direction, as quasiplane waves are used and non-paraxial Green's functions are obtained. It is more effective to obtain the nonlinear Rayleigh sound beam distributions accurately with the proposed general equation and solutions. Brief consideration is given to the measurement of nonlinear parameter using nonlinear Rayleigh waves. [ABSTRACT FROM AUTHOR]

An evaluation model based on the multi-scale ultrasonic attenuation coefficient was developed to control both systematic error and random error. AISI 304 stainless steel was used to validate the presented model. Wavelet transformation was used to obtain the variation of ultrasonic signal over time and scale. Particle swarm optimization was utilized to correlate the coefficient with grain sizes. The model shows the attenuation of all scales increased with the grain size, and ultrasound attenuates faster on smaller scales. Compared with the ultrasonic velocity method and the traditional attenuation method, the proposed method has less systematic error and random error. [ABSTRACT FROM AUTHOR]

It is important to accurately and nondestructively evaluate the grain size of structural materials used in nuclear power plants. The current ultrasonic non-destructive methods are so dependent on the thickness measurement of a square pipe that it reduces their practicality and reliability. In this paper, a novel method using the coefficient of ultrasonic attenuation rate is developed by using the transmission and reflection coefficients. As a result, the mean grain size of the pipe can be nondestructively evaluated without measuring its thickness. Moreover, the signal preprocessing is studied to improve the stability and accuracy of evaluation results. The experimental results show that the dependence of the attenuation rate on grain sizes is much higher than that of the ultrasonic velocity. The relative error of the attenuation rate method is lower than that of the backscatter method if the thickness of the sample is less than 5 mm. When evaluating a TP304 stainless steel square pipe whose thickness is not convenient to measure, the mean grain sizes are measured 103.5 ± 2.6 μm, 96.9 ± 3.5 μm and 94.0 ± 1.7 μm by the attenuation method, the attenuation rate method and the electron backscattering diffraction method, respectively. The result verifies that the presented method works better than the attenuation method due to the fact that the error of the thickness measurement has no effect on the ultrasonic attenuation rate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A narrowband signals technique is improved to measure attenuations of solids. • The attenuations at broadband frequency ranges are measured to obtain α s - f curves. • Density is an important parameter that influences the α s - f relationship of solids. • The attenuation coefficients of nineteen frequently-used materials are provided. A well-established narrowband pulse-echo technique is employed and improved further to investigate the frequency-dependent ultrasonic wave attenuation in various solids. During attenuation coefficient measurement, the diffraction correction is introduced to minimize wave beam-spreading loss, and frequency domain signals are used to minimize both the effects of downward shift of frequency and the error in determining the amplitude of time-domain signals. The frequency-dependent attenuation curves for 19 frequently-used solid materials are obtained using this method in the frequency range 1–25 MHz. It is observed from the measurement results of these materials that the attenuation can vary linearly or nonlinearly with the signal frequency, and the potential impact factors of the attenuation-frequency relationship are discussed. The experimental results presented in this paper is expected to provide a good reference for researchers interested in developing and using frequency-dependent attenuation coefficients of materials that are currently missing in the literature. [ABSTRACT FROM AUTHOR]

The sound fields generated by ultrasonic transducers can be modeled using the Quasi-Monte Carlo (QMC) method with a high level of accuracy and efficiency from Zhang [J. Acoust. Soc. Am. 149(1), 7–15 (2021)]. In this work, this method is extended to simulate transmitted wave fields through complicated interfaces. When a wave propagates in two-layer media, the vibrating waves over the interface radiated by the transducer can be treated as the source for generating waves in the second medium, thus, a nested-form Rayleigh integral expression can be used as a model equation for the transmitted wave calculation. When the QMC method is used to solve the nested integral, pseudo-random samples for constructing the transducer and the interface are sampled separately and the transmitted wave fields are obtained using the final sample mean. Numerical examples and results are presented when the wave transmits normally or obliquely through planar or curved interfaces. The results indicate that the high level of accuracy and efficiency remains when the QMC method is used to model the transmitted wave fields. One important advantage is that wave fields can be well simulated using the QMC method when the wave transmits through a complicated interface as long as the interface can be constructed using pseudo-random samples. [ABSTRACT FROM AUTHOR]

• Surface roughness is introduced to modify a doubly scattered response model. • The theoretical confidence bounds of grain noise can be calculated accurately. • We treat the upper bound as the time-dependent threshold for C-scan imaging. • Sub-wavelength flaws can be effectively detected using time-dependent thresholds. Surface roughness degrades the performance of ultrasonic methods when detecting sub-wavelength flaws. In this work, a roughness-modified doubly-scattered response (DSR) model is developed to enhance the flaw detection method for two-phase Ti-6Al-4V with rough surfaces. Extreme value statistics are used to calculate the confidence bounds of grain noise, then the bounds are treated as a time-dependent threshold for segmenting flaws in C-scan images of two-phase Ti-6Al-4V with rough surfaces. Three Ti-6Al-4V samples with different surface roughness are designed and manufactured for validating the present method; ultrasonic C-scan results show that it can distinguish sub-wavelength flaws (about 1/3 wavelength) in two-phase Ti-6Al-4V with rough surfaces, which can be extended to industry applications. [ABSTRACT FROM AUTHOR]

Tube/Pipe (TP) 304 stainless steel has been widely used in industry, but a change in its microstructures may endanger its service safety, and it is essential to evaluate its microstructural evolution. In this work, a pulse-echo nonlinear method is proposed to characterize the microstructural evolution of the TP304 stainless steel. The detailed pulse-echo nonlinear experimental process is presented, and it is shown that the absolute nonlinear parameter can be determined when the effect of attenuation is taken into account. The microstructural evolution of TP304 stainless steel is artificially controlled by annealing treatments before it is evaluated by using nonlinear ultrasonic method and metallographic method. The results show that the grain sizes increase as the annealing time increases, which leads to the performance degradation of the TP304 steel and an increase in the nonlinear parameters, with the reason discussed considering the variation in the microstructure. The present pulse-echo nonlinear method is easier to conduct than the traditional transmission-through method and the absolute nonlinear parameter can be determined for quantitative characterization. The variation in determined nonlinear parameters provides a reference to evaluate the microstructural evolution of TP304 stainless steel. [ABSTRACT FROM AUTHOR]

The mechanical properties of 6061 aluminum alloys can be enhanced by appropriate heat treatment, but strongly weakened due to over-aging treatment. In this work, the material properties of aged 6061 aluminum alloys are nondestructively evaluated using nonlinear Rayleigh waves. Nonlinear Rayleigh wave experiments are performed on these aluminum specimens through long distances (c.a. 150 mm) to measure the velocity, attenuation and nonlinear parameter simultaneously. The wave velocity is calculated using the time-of-flight method, and the attenuation coefficient is extracted using a data fitting method. By taking into account the effects of diffraction, attenuation and source nonlinearity, the nonlinear parameter is measured. The results show that the nonlinear parameters have a positive correlation with the hardness, which is associated with the precipitate sequence due to aging treatment. It is also found that the Rayleigh wave attenuation is an effective indicator for monitoring the changes of the material microstructures. A technique is developed for evaluating material properties using multiple parameters of nonlinear Rayleigh waves, and its efficacy is verified through experiments. [ABSTRACT FROM AUTHOR]

• A laser-EMAT ultrasonic longitudinal wave resonance thickness measurement method based on surface constraint mechanism is proposed. • The influence of surface constraint mechanism, irradiation laser source parameters, and EMAT receiving coil dimensions on the accuracy and amplitude of resonance method thickness measurement are analyzed. • Achieve accurate and rapid detection of the thickness thinning of stepped aluminium plates and the thickness of 500 ℃ high-temperature metal thin-walled parts. In this paper we propose a novel ultrasonic longitudinal wave resonance method for measuring the thickness of metal walls using a laser-electromagnetic acoustic transducer (Laser-EMAT). The method is based on the surface constraint mechanism (SCM) of the material and is expected to be capable of accurately detecting local thinning of metal walls in a non-contact manner and at high temperatures. Based on finite element analysis of laser-EMAT ultrasonic resonance measurement of aluminum alloy thickness, we investigated the effects of such key factors as SCM, irradiation parameters of laser source, and the size of EMAT receiving coil on the accuracy of thickness measurement (resonance frequency position) and on the amplitude of the resonance wave. Both numerical simulations and experiments are conducted to demonstrate that the measurement accuracy of the proposed method is not affected by SCM, irradiation laser source parameters, and EMAT receiving coil size, and that accurate detection of stepped aluminum plates with thickness thinning from 3.0 mm to 0.5 mm is achieved. Furthermore, we were able to perform rapid detection of aluminum thin plate thickness at 500 °C temperature with an EMAT lift-off of 5.0 mm and achieved a relative experimental error as small as 3.40 %. The results obtained in this study showed that the proposed method performed well in non-contact measurement of metal thinning in harsh environment of high temperature. [ABSTRACT FROM AUTHOR]

The sound fields generated by ultrasonic transducers are modeled using the quasi-Monte Carlo (QMC) method, which is found to overcome the conflict between accuracy and efficiency that occurs in existing wave field calculation methods. The RI equation, which is frequently used as a model equation in ultrasonic field calculation, is used here as an exact method and for comparison purposes. In the QMC method, the judgment sampling method and Halton sequence are used for pseudo-random sampling from the sound source, and then the sound field distributions are found by solving the integral solution using the sample mean. Numerical examples and results are presented when modeling unfocused, focused, and steered and focused beam fields. The accuracy and efficiency of the QMC method are discussed by comparing the results obtained using different modeling methods. The results show that the proposed method has a high level of efficiency due to the nature of the QMC algorithm and a high level of accuracy because no approximation is required. In addition, wave fields can be modeled with the QMC method as long as sound sources can be effectively pseudo-randomly sampled, allowing the proposed method to be applied to various types of transducers. [ABSTRACT FROM AUTHOR]

Material grain size is related to metallic material properties and its elastic behaviour. Measuring and monitoring material grain size in material manufacturing and service is an important topic in measurement field. In this paper, three materials, i.e., aluminium 2014 T6, steel BS970 and copper EN1652, were chosen to represent materials with small, medium and large grain size, respectively. Various techniques of measuring material grain size were demonstrated and compared. These techniques include the measurements from material microstructure images, backscattered ultrasonic grain noise using a conventional transducer, longitudinal wave attenuation using ultrasonic arrays and shear wave attenuation using a lead zirconate titanate (PZT) plate. It is shown that the backscattered ultrasonic noise measurement and material attenuation measurement are complementary. The former is pretty good for weak scattering materials, e.g., aluminium, while the latter for materials with large grains, e.g., steel and copper. Consistent measured grain size from longitudinal and shear wave attenuations in steel and copper suggests that shear wave attenuation can be calculated from the measured longitudinal wave attenuation integrated with Stanke–Kino's model or Weaver's model, if there is a difficulty to either excite or capture shear waves in practice. The outcome of the paper expects to provide a further step towards the industrial uptake of these techniques. [ABSTRACT FROM AUTHOR]

The practical implementation of nonlinear ultrasonic technique has been limited to the through-transmission setup for measuring the second harmonic component induced by the nonlinearity or microstructural changes of test material. A more practical technique such as the pulse-echo testing has been ruled out because a traction-free reflecting boundary destructively alters the nonlinear generation process. A focusing acoustic beam or rigid boundary condition was often employed for biomedical imaging and fluid nonlinearity in the pulse-echo inspection. In this article, we further explore a more general and efficient method to improve the generation of the second harmonic component in the pulse-echo mode with traction-free surface. A dual element planar transdcer with optimal phase shift of the input signal in one element relative to another is proposed for this purpose. The validity of the phase shift concept is confirmed by comparing the enhanced generation of second harmonic amplitudes and the resulting nonlinear parameters with the rigid-boundary case equivalent to the conventional through-transmission setup. [ABSTRACT FROM AUTHOR]

A new calibration method is proposed for characterizing spherically and cylindrically focused circular piston-like transducers. The calibration procedure employs a single frequency pulse-echo configuration with a large plane reflector. Analytical solutions based-on multi-Gaussian beam model are introduced to predict the received wave distributions, and a least squares fitting method for extracting the effective geometrical focal length and radius from the measurements near the transducer focus is described. The proposed method is easy to implement and can minimize errors due to measurements, uncertainties and material attenuation. Experimental results show good agreements with the theoretical predictions using the calibrated parameters and measurements. [ABSTRACT FROM AUTHOR]

Based on the wave-mode-converted principle, an immersion-focused transducer is employed to determine the longitudinal wave and shear wave velocities. The experimental condition is then investigated to obtain the converted shear wave, which is used to analyze the relationship between the reinforcement content and the dual-mode ultrasonic velocities. In addition, the elastic modulus is calculated. Magnesium-based composite samples with different reinforcement contents are manufactured to conduct an ultrasonic experiment, wherein the dual-mode velocities vary with the change in the reinforcement content; the correlation coefficient is 99.17%. An ultrasonic dual-mode velocity model is developed to analyze the distribution of the reinforcement content. By employing the measured values obtained from the destructive method, the largest errors in the reinforcement content and elastic modulus evaluated using the proposed method are found to be −5.76% and 5.85%, respectively. The shear wave velocity determined using a normal-incidence shear-wave transducer reveals the accuracy with which the errors are measured. This method provides an effective tool to nondestructively evaluate the microstructure and elastic properties of Mg-based composites. [ABSTRACT FROM AUTHOR]

In nonlinear ultrasound testing, the relative nonlinear parameter is conveniently measured as a sensitive means of detecting and imaging overall variation of microstructures and damages. Compared to the quadratic nonlinear parameter ( β ′ ), the cubic nonlinear parameter ( γ ′ ), calculated as the third harmonic amplitude divided by the cube of the fundamental amplitude, has generally a higher value, providing better sensitivity in nonlinear parameter mapping. Since the third harmonic amplitude is about two orders of magnitude lower than the fundamental amplitude, efficient excitation and highly sensitive reception of third harmonic is very important. In this paper, we explore an odd harmonic thickness resonant transducer that meets the requirements for pulse-echo third harmonic generation (THG) measurements. We also address the problem of source nonlinearity that may be present in the measured amplitude of the third harmonic and propose a method to properly correct it. First, we measure γ ′ for a series of aluminum specimens using the through-transmission method to observe the behavior of γ ′ as a function of specimen thickness and input voltage, and examine the effects of various corrections such as attenuation, diffraction and source nonlinearity. Next, we apply the odd harmonic resonant transducer to pulse-echo THG measurements of precipitation heat-treated specimens. It is shown that such transducer is very effective in generation and detection of fundamental and third harmonics under finite amplitude toneburst excitation. The highly sensitive detectability of γ ′ are presented as a function of aging time, and the sensitivity of γ ′ is compared with that of β ′ and β ′ 2 . [ABSTRACT FROM AUTHOR]

Harmonic generation measurement is recognized as a promising tool for inspecting material state or micro-damage and is an ongoing research topic. Second harmonic generation is most frequently employed and provides the quadratic nonlinearity parameter (β) that is calculated by the measurement of fundamental and second harmonic amplitudes. The cubic nonlinearity parameter ( β 2 ), which dominates the third harmonic amplitude and is obtained by third harmonic generation, is often used as a more sensitive parameter in many applications. This paper presents a detailed procedure for determining the correct β 2 of ductile polycrystalline metal samples such as aluminum alloys when there exists source nonlinearity. The procedure includes receiver calibration, diffraction, and attenuation correction and, more importantly, source nonlinearity correction for third harmonic amplitudes. The effect of these corrections on the measurement of   β 2 is presented for aluminum specimens of various thicknesses at various input power levels. By correcting the source nonlinearity of the third harmonic and further verifying the approximate relationship between the cubic nonlinearity parameter and the square of the quadratic nonlinearity parameter ( β ∗ β ), β 2 ≈ β ∗ β , the cubic nonlinearity parameters could be accurately determined even with thinner samples and lower input voltages. [ABSTRACT FROM AUTHOR]

This work presents theoretical and numerical models for the backscattering of two-dimensional Rayleigh waves by an elastic inclusion, with the host material being isotropic and the inclusion having an arbitrary shape and crystallographic symmetry. The theoretical model is developed based on the reciprocity theorem using the far-field Green's function and the Born approximation, assuming a small acoustic impedance difference between the host and inclusion materials. The numerical finite element (FE) model is established to deliver a relatively accurate simulation of the scattering problem and to evaluate the approximations of the theoretical model. Quantitative agreement is observed between the theoretical model and the FE results for arbitrarily shaped surface/subsurface inclusions with isotropic/anisotropic properties. The agreement is excellent when the wavelength of the Rayleigh wave is larger than, or comparable to, the size of the inclusion, but it deteriorates as the wavelength gets smaller. Also, the agreement decreases with the anisotropy index for inclusions of anisotropic symmetry. The results lay the foundation for using Rayleigh waves for quantitative characterization of surface/subsurface inclusions, while also demonstrating its limitations. [ABSTRACT FROM AUTHOR]

The roughness of a flaw's surface significantly affects the scattering behavior of ultrasonic waves. It is vital to understand the impact of roughness on flaw echoes, especially when performing ultrasonic nondestructive inspection on safety-critical components. However, the current approach for creating rough flaw models fails to reconstruct complicated cracks with secondary cracks. Here, a multi-scale distortion method is developed to generate a rough flaw by using an optical microscope image of a real flaw. The finite element (FE) is then implemented to simulate the near-surface rough flaws in nickel-based bars, which are detected by an offsetting immersion transducer with mode-converted transverse waves. Numerical results show that the randomness and complexity of flaw echoes from rough flaws are exceptionally high. The gap between the maximum and minimum normalized amplitude values of flaw echoes from a rough crack with secondary cracks can reach 7.125 dB. Meanwhile, the maximum time of flight (TOF) is almost twice as large as the minimum TOF. Therefore, the present method can generate effective rough flaw models in terms of macroscopic rough geometry and microscopic rough surface. Moreover, the impact of the rough flaw surface on the flaw echoes goes beyond amplitude changes and may make flaw location challenging. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is a new manufacturing technology for building complex 3D components by progressively adding material in a layer-by-layer fashion. Despite the many apparent advantages of AM, the instable heat transfer during the melting of metallic material and the stacking manufacturing processing increases the probability of generating internal defects that may cause the AM components to deform or fail. To ensure the quality and safety of additively manufactured metallic components, nondestructive testing technologies that are fast, accurate and low-cost are needed. In this paper we review the defect formation mechanism and the nondestructive testing methods for metallic AM components, and focus on the development of ultrasonic testing in AM. We analyze the influence of the manufacturing process, material microstructure and detection parameters on ultrasonic testing and summarize the application of ultrasonic testing for offline and online detection of defects in metallic AM components. On this basis, the application and advantages of phased array ultrasonic testing, laser-generated ultrasonic testing and hybrid nondestructive testing detection are discussed. The development trends of ultrasonic testing for metallic AM are discussed and some suggestions are made. [ABSTRACT FROM AUTHOR]

A new method for determining the acoustic nonlinearity parameter using a nonlinear data fitting method is proposed. Based on the quasilinear theory of Westervelt's equation, the fundamental and second harmonic beam fields are expressed as a multi-Gaussian beam model that separates the attenuation and diffraction correction terms from the propagating plane waves. A nonlinear least squares curve-fitting method is developed to extract the nonlinearity parameter without knowing the attenuation coefficients of the material being tested. The nonlinearity parameter of water is determined using the proposed method, and the result agrees well with the literature value. The attenuation coefficients of the fundamental and the second harmonic are also extracted and discussed. [ABSTRACT FROM AUTHOR]

• A method is proposed combining nonlinear SPC based techniques and pulse-echo method. • The proposed method has a high sensitivity to detect fatigue damage. • The method maintains the easier operability as traditional linear methods. • The length and width of cracks are more sensitive to low and high frequencies, respectively. The nondestructive ultrasonic detection and evaluation of fatigue cracks is critical to ensure the safe operation of equipment. Although linear ultrasonic methods are easy to implement in practice, their parameters are insensitive to micro-scale cracks that are smaller than the wavelength. Conventional nonlinear techniques have better sensitivity but require a relatively complicated configuration and demanding experimental setups. In this work, a simple approach for the inspection of fatigue damage is presented to overcome the above shortcomings by combining nonlinear sideband peak count (SPC) techniques with the pulse-echo experimental method. Related nonlinear wave theories are briefly reviewed, and experiments are performed in stainless steel specimens with varying crack levels to verify the effectiveness of the proposed method. The linear and nonlinear parameters are extracted from the measurements for comparison. The results demonstrate that, under the same pulse-echo mode with low excitation amplitude, both the SPC-I (sideband peak count-index) and SPI (sideband peak intensity) variations are more sensitive to the quantitative detection of fatigue cracks than the conventional linear and nonlinear ultrasonic based parameters, especially in the initial stages of cracking. Experiments using pitch-catch configuration and an additional pulse-echo experiment using a different input frequency are carried out to verify the reliability and superiority of the proposed method. This work provides a convenient and efficient method for robust measurements of early damage of materials in practice. [ABSTRACT FROM AUTHOR]

To detect flaws in strongly attenuating materials, we propose an ultrasonic phased array imaging method that combines Filtered-Delay Multiply And Sum (F-DMAS) nonlinear beamforming algorithm with pseudo-color technique. Based on the spatial coherence of signals, three filters with different frequency ranges are selected to reconstruct F-DMAS images from the full matrix capture (FMC) data. Then, three channels of F-DMAS images are combined with the RGB color model to obtain the pseudo-color image. Finally, an automatic flaw extraction algorithm is developed using the techniques of color image segmentation, data fusion, and edge detection. Experiments are performed on samples of polymer bonded explosives with side drilled holes. The results show that the proposed method has significant advantages over the linear beamformed Total Focusing Method (TFM) in terms of signal-to-noise ratio, detection ability, and visual presentation of flaws. • F-DMAS algorithm is introduced to improve the SNR and lateral resolution of image. • FRR zero-phase filter reduces filter-induced phase distortion of F-DMAS image. • Pseudo-color is combined with F-DMAS to get more intuitive flaw image. • Color image segmentation improves the robustness of automatic flaw extraction. • Proposed method has advantages for flaw detection in highly attenuative materials. [ABSTRACT FROM AUTHOR]

• A FFT method using comb transducers is proposed to determine the velocity; • The Rayleigh wave velocity can be calculated using the central frequency; • The experiments are easy to do as the distance and time are not required; • The proposed technique has high levels of precision and accuracy; • It is applicable for materials with different velocities using a same transducer. A convenient, accurate and precise method is proposed to determine velocities of ultrasonic Rayleigh waves in different materials by extracting central frequencies of signals, which are measured by a comb transducer and converted to the frequency domain using the fast Fourier transformation (FFT). The velocities can be calculated as c r = fl , where f is the central frequency of the wave signal and l is the teeth spacing or period of the comb transducer. The experimental measurements are easy to do, as long as the Rayleigh wave reflected from the standard reflectors are measured using one comb transducer, without knowing the wave propagation distances and times. Results show that the proposed technique has a high level of precision, as the central frequencies are very stable. The same comb transducer is used to measure the Rayleigh wave velocities in different materials where the velocities vary from 2100 m/s to 3400 m/s. Comparison of the experimental results with those measured using the time-of-flight method showed a high level of accuracy - all relative errors were found to be less than 1%. [ABSTRACT FROM AUTHOR]

Multi-layered structures play a critical role in aerospace, automotive and electrical industries, so the non-destructive evaluation and maintenance of such structures must be accurate and rapid. The application of conventional ultrasonic phased array imaging methods is limited due to computational complexity, image quality and image size. In this paper, we proposed a cost-effective, high-quality imaging method that uses full-matrix migration total focus method (FM-TFM) and produces large-area scanned image with the aid of normalized cross-correlation (NCC) template matching technique. The FM-TFM method eliminated the need for iterative refraction point calculations by using full matrix migration extrapolation, and the optimization of NCC was also investigated in the study. The verification was divided into two parts, fixed transducer experiment and manual scanned experiment, to demonstrate the advantages of FM-TFM with improved SNR (22 % or greater) and one order of magnitude reduction in computational time when compared to ray tracing total focus method (RT-TFM) and NCC matching accuracy of multilayer specimen, respectively. The proposed method can help eliminate the need for costly scanning equipment and shows great potential for extension to other TFM-based approaches. • The proposed algorithm facilitates high-quality phased array imaging in less time. • Large-area scanned imaging is realized without encoders. • Filtering enhances the matched robustness. • Reducing the imaging distortion improves the matched accuracy. [ABSTRACT FROM AUTHOR]

This paper presents theoretical and numerical models for the backscattering of 2D Rayleigh waves in single-phase, untextured polycrystalline materials with statistically equiaxed grains. The theoretical model, based on our prior inclusion-induced Rayleigh wave scattering model and the independent scattering approximation, considers single scattering of Rayleigh-to-Rayleigh (R–R) waves. The numerical finite element model is established to accurately simulate the scattering problem and evaluate the theoretical model. Good quantitative agreement is observed between the theoretical model and the finite element results, especially for weakly scattering materials. The agreement decreases with the increase of the anisotropy index, owing to the reduced applicability of the Born approximation. However, the agreement remains generally good when weak multiple scattering is involved. In addition, the R–R backscattering behaviour of 2D Rayleigh waves is similar to the longitudinal-to-longitudinal and transverse-to-transverse backscattering of bulk waves, with the former exhibiting stronger scattering. These findings establish a foundation for using Rayleigh waves in the quantitative characterisation of polycrystalline materials. • A theory for backscattering of Rayleigh waves in polycrystals has been developed. • Theory is constructed assuming single scattering with Born and IS approximations. • Backscattering of Rayleigh waves is similar to that of bulk waves. • FE model is developed to simulate backscattering of Rayleigh waves. • FE model verifies that theory can well describe Rayleigh waves backscattering. [ABSTRACT FROM AUTHOR]

Diffuse scattering of ultrasound by the microstructure of polycrystal specimens can be used to evaluate grain size and grain elongation. The existing diffuse scattering models mostly dealt with circular transducers whose symmetrical sound field is insensitive to the asymmetric elongated grain. The sound field of a rectangular transducer provides a new perspective for acquiring additional information. First, the existing single scattering response (SSR) and double scattering response (DSR) models are modified for a rectangular transducer, where the sound field of a rectangular transducer is equivalent to that of an elliptical transducer in the far-field. Therefore, an equivalent single Gaussian beam model is derived using amplitude-equivalent and beamwidth-equivalent coefficients. Then, the spatial correlation function of elongated grains is transformed into the wavenumber domain, giving rise to the SSR and DSR of a rectangular transducer that reveals the interaction effect of an asymmetric sound field and elongated grains on ultrasonic backscattering. The experimental results show that the sizes of elongated grains in a cold-rolled aluminum are evaluated as 1086 ± 8, 90 ± 4, and 10 ± 1 μm in the x, y, and z directions, where the exact values are 1184.2 ± 11.9, 80.7 ± 5.2, and 8.3 ± 0.5 μm according to metallographic measurements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A reliable method is proposed to measure frequency-dependent shear wave attenuation. • Experimental and theoretical predicted results are compared and discussed. • The proposed method and the attenuation results provide good references for future studies. A pulse-echo oblique incidence technique for determining the frequency-dependent attenuation of ultrasonic shear waves in metallic materials is considered and improved. To measure the attenuation coefficient, the wave signals are measured when the waveform-converted shear waves propagate over different distances. A diffraction correction is introduced to minimize the beam-spreading loss of the wave, and narrowband signals are used to minimize the effects of a downward shift in frequency. The attenuation coefficient is measured at a certain frequency, and experiments are performed over a broad bandwidth range of frequencies; a curve fitting method is then applied to these attenuation values to ensure an accurate estimate of the frequency-dependent shear wave attenuation. Experiments are performed to evaluate the effectiveness of the proposed method, and the frequency-dependent shear attenuation curves for some commonly used metallic materials are obtained. One of the most significant findings from the results is that the shear wave attenuation follows a power law dependence on frequency with exponents between 2 and 3, which lie between the theoretical values of the exponents from the classical stochastic and Rayleigh scattering theories. The effects of grain size on shear wave attenuation are investigated, and a quantitative comparison is drawn between the experimentally observed attenuation and that predicted by the classical theory. The proposed measurement method and the experimental results presented here are expected to provide good references for researchers interested in frequency-dependent shear wave attenuation behavior. [ABSTRACT FROM AUTHOR]

• Surface defect visualization is attained using leaky Rayleigh waves. • PCA-SAFT imaging method is proposed to improve resolution and reduce noise. • The SNR and CNR of the PCA-SAFT image are both increased. • The proposed method is verified on both isotropic and anisotropic specimens. 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. [ABSTRACT FROM AUTHOR]

A planar stress-free boundary is known to destructively alter the nonlinear wave generation process including the second harmonic component. This is the reason why the harmonic generation measurements in the study of nonlinear fluids and solids have been limited to the through-transmission setup. Recently, we have found through acoustic modeling and simulation that a dual element transducer composed of the annular ring emitter and the inner disk receiver improves the second harmonic generation in the pulse-echo mode with the stress-free boundary. This paper utilizes a dual element transducer for nonlinearity parameter (β) measurements of solid specimens in the pulse-echo testing configuration. The theoretical derivation provides the received beam fields and the formula for β determination with all necessary corrections incorporating diffraction, attenuation and boundary reflection. The dual element transducer is applied to pulse-echo β measurements of aluminum alloy samples, and the results are found to be in good agreement with the through-transmission method. [ABSTRACT FROM AUTHOR]