Laser-generated Rayleigh Waves Research Articles

Quantitative detection of lateral subsurface cracks based on laser-generated Rayleigh waves in the frequency domain

Quantitative detection of lateral subsurface cracks based on laser-generated Rayleigh waves in the frequency domain

Surface defect detection from additive manufacturing components at elevated temperatures using laser-generated Rayleigh waves

Online monitoring is a critical issue for additive manufacturing components layer by layer to ensure the integrity of the structure. However, the rough surface and high temperature make it difficult to use conventional non-destructive testing methods to carry out. In this work, the laser ultrasonic technique, as a non-contact nondestructive testing method, is applied to detect defects in a 316L stainless steel specimen manufactured using selective laser melting at elevated temperatures. The unreported phenomenon of phase evolution of Rayleigh waves for different excitation locations and temperatures is observed and systematically explored using the finite element method and experiments.Furthermore, an improved variational mode decomposition strategy based on particle swarm optimization is proposed to enhance the signal-to-noise ratio. A fitness function based on permutation entropy and mutual information is developed to determine the optimized parameters in variational mode decomposition. The experimental results indicate that, for the different excitation locations, the amplitude of the Rayleigh wave generated by laser irradiation on the defect is larger than on the small area around the defect, and the defect can be effectively detected using B-scans at elevated temperatures according to the abnormal evolution of Rayleigh waves; for the fixed excitation locations, the amplitude and velocity of Rayleigh waves gradually decreases at 100–500 °C, which will provide a scientific basis for online monitoring and evaluation of additive manufacturing components.

A novel 3D evaluation method for surface defects using broadband laser-generated Rayleigh waves with wavenumber analysis

To address the issues of large imaging errors for small defects and the difficulty in depth evaluation using local wavenumber estimation for surface defect imaging, a novel three-dimensional (3D) evaluation method for surface defects using broadband laser-generated Rayleigh waves with wavenumber analysis is proposed. A finite element model is established to investigate the interaction between the Rayleigh wave and the surface defect and reveal the wavenumber change mechanism of the non-dispersive Rayleigh wave in the case of defects. It is discovered that when the Rayleigh wave encounters the surface defect, various mode-converted scattered waves are generated, resulting in the appearance of new components with wavenumbers lower than that of the incident Rayleigh wave in the wavenumber domain. Additionally, the maximum amplitude of the Rayleigh wave in the B-scan image increases as the defect depth increases. Based on the simulation analysis, a 3D evaluation method for surface defects is proposed. Firstly, the scattered Rayleigh wave caused by the defect is extracted using frequency-wavenumber analysis. Secondly, a space-frequency-wavenumber analysis is used to determine the local wavenumber of the scattered Rayleigh wave for defect imaging. Finally, the defect depth is estimated by analyzing the maximum amplitude of the Rayleigh wave. A surface defect detection experiment is conducted to verify the effectiveness of the proposed method, and the experimental results demonstrate that the proposed method can suppress noise interference and accomplish high-precision imaging of small surface defects compared to the traditional method. Moreover, the method can establish a linear mapping relationship between the defect depth and the maximum amplitude of the Rayleigh wave for depth evaluation. The research results can provide a potential application for the 3D evaluation of surface defects.

Surface defect identification based on broadband laser-generated Rayleigh wave with wavenumber filtering

As a non-contact, couplant-free and nondestructive technique, the laser ultrasonic technology has great potential for detecting surface defects. In this paper, we propose a surface defect identification method based on broadband laser-generated Rayleigh wave with wavenumber filtering in an area scanning mode. Firstly, a particular lowpass filter is constructed in frequency-wavenumber domain to extract the scattered wave generated by the defects, and then a surface defect image can be reconstructed based on the broadband scattered wave wavenumbers. Secondly, a threshold denoising method is employed to enhance the signal-to-noise ratio and improve the image sharpness. Thirdly, the experimental validation is carried out, in which a laser ultrasonic detection system is used to detect the surface defects of aluminum alloy specimens and identify their different parameters. The experimental results verify that the proposed method can identify the location, size and orientation of surface defects effectively, meanwhile, its imaging effect shows significantly superior to that of the conventional reflected wave energy and standing wave energy methods. Furthermore, the correlation between maximum local scattered wave energy and the depth of defects is found, which can characterize the severity of the surface defects.

Quantitative detection of surface defect using laser-generated Rayleigh wave with broadband local wavenumber estimation

Laser ultrasonic technology has been widely used in surface defect detection attribute to its non-contact, non-destructive and high spatial resolution characteristics. This paper proposes a surface defect quantitative detection method using laser-generated Rayleigh wave with broadband local wavenumber estimation. In this method, considering the broadband characteristics of laser-generated Rayleigh wave, the broadband local wavenumber estimation is presented to achieve the defect imaging accurately, and then the defect geometric parameters are estimated based on image segmentation. A surface defect detection experiment using the laser ultrasonic detection system is conducted to verify the effectiveness of the proposed method. The experimental results show that the proposed method has superior imaging effect for vertical and inclined defects than the standing wave energy method or reflected wave energy method. Besides, the geometric parameters such as length, width, and inclination angle of a surface defect can be accurately identified by the proposed method, the errors of vertical defects are 1.6% in length and 4.0% in width respectively, as well as the maximum and minimum error of inclined defects are 5.0% and 1.28% in inclination angle respectively. The research results provide a potential application for the fast and non-destructive surface defect detection of metal structures.

Frequency-dependent scattering of wideband laser-generated Rayleigh waves for vertical surface crack characterization

Wideband laser-generated Rayleigh waves have been extensively exploited for surface crack characterization, most of which, nonetheless, are of a nature of either numerical simulation or experimental observation. Earlier, an elastodynamic reciprocity theorem-based theoretical model was proposed by the authors Xu L et al 2022 Ultrasonics 118 106578, Xu L et al 2021 J. Sound Vib. 509 116226, aimed at scrutinizing the interaction of narrowband Rayleigh-Lamb waves with a surface or subsurface crack. In this study, the model is extended to a wideband scenario to analytically explore the interaction of a wideband laser-generated Rayleigh wave with a surface crack, as well as the resultant crack-scattered Rayleigh wavefield. First, under the narrowband scenario, a dimensionless parameter is formulated based on the closed-form solution to the magnitude of the narrowband scattered Rayleigh wavefield, revealing that the scattering effect of a surface crack on the Rayleigh wave is frequency-dependent and there exists a characteristic frequency, at which the crack-scattered Rayleigh wavefield manifests the strongest intensity. Similarly, under the wideband scenario, such dependence can be calibrated by a spectral damage indicator (SDI), which facilitates the evaluation of the severity of the surface crack using the wideband laser-generated Rayleigh wave. Proof-of-concept simulation is performed to verify the model. Quantitative agreement between the analytical and numerical results validates the accuracy of the proposed model and SDI. Experiment is also conducted to demonstrate the effectiveness of the frequency-dependent scattering of wideband laser-generated Rayleigh waves for vertical surface crack characterization.

A novel laser-based duffing oscillator system to identify weak ultrasonic guided wave signals related to rail defects

Laser-generated Rayleigh wave is a promising candidate for noncontact and remote inspection of railway as it can inspect in line-to-line fashion. Attribute to the recent advancement of optical design, wavelength of the laser generated-Rayleigh wave can be controlled at desired frequency band. By adjusting the wavelength of the Rayleigh wave, detection of defect occurred at different depths in rail become possible. Internal and subsurface defects that occur beneath the surface of railhead usually only reflect very small portion of wave energy hence accurate determination of their presences becomes challenging. Besides, such reflected-defect signal is usually contaminated with strong random noise which further hinder identification of weak reflected-defect signal. This study presents a novel method for detection of surface, subsurface and internal defects in railhead using laser-generated Rayleigh wave and chaotic oscillator. The use of laser-generated Rayleigh wave provides a fast mean to assess the health condition of railhead without the need to physically mounting sensor on the specimen. The use of chaotic oscillator can accurately reveal wave signal that is related to defect occurred in railhead. The effectiveness of the proposed method has been verified using numerical simulation model. Three rail specimens were machined with internal, subsurface and surface-breaking defects, respectively. Experimental results revealed that the proposed method can accurately and reliably reveal the presence of reflected-defect signal under strong random noise conditions. The present study also shows that the proposed system is promising in revealing hazardous defects occurred on rai

Finite element analysis of laser-generated Rayleigh wave for sizing subsurface crack in frequency domain

In order to realize the quantitative detection of subsurface cracks, finite element method (FEM) was used to study the interaction of laser-generated Rayleigh wave and subsurface crack with different buried depths and lengths in this paper. The transmitted and reflected Rayleigh waves of subsurface cracks were analyzed in frequency domain, it is found that the center frequency of transmission wave decreases with the increase of crack length when the buried depth of crack remains unchanged, and increases with the increase of crack depth when the crack length remains unchanged. The center frequency of reflected Rayleigh wave is independent of crack length, and has a quadratic function relation with the buried depth of subsurface crack. The relationships, between the ratio of buried depth to incident Rayleigh wavelength and the ratio of reflected Rayleigh wave frequency to incident Rayleigh wave frequency, and the ratio of crack length to incident Rayleigh wavelength and the ratio of transmitted Rayleigh wave frequency to incident Rayleigh wave frequency, were obtained respectively based on the frequency analysis. The relationships were verified by the simulation signals, it is found that the maximum relative error of the buried depth of the subsurface cracks determined by the proposed relationship is 5.939%, and the maximum relative error of the length of the subsurface cracks is 12.149%. The research results show that the proposed method is promising in quantitative characterization of the buried depth and length of subsurface cracks in metallic materials.

Non-contact detection of railhead defects and their classification by using convolutional neural network

Railhead defects must be detected and classified intelligently in order for railway transportation systems to operate safely. Rail defect identification and categorization can be automated by using machine learning models to process rail image data (acquired using cameras). However, such an automated method has significant drawbacks: it cannot detect subsurface defects, picture data requires a high-end GPU with a long computational time, and machine learning model training can be influenced by image quality, which is dependent on light intensity and shooting altitude. Rayleigh waves are a potential candidate for rail inspection because they can detect both surface and subsurface defects and travel long distances on curved surfaces (like a rail) at high speed. This article looks into the possibility of combining fully non-contact laser ultrasonic technology (LUT) and a deep learning approach for intelligent detection and classification of railhead surface and subsurface defects. The fully non-contact LUT was used to actuate and capture laser-generated Rayleigh wave signals on railhead specimens in order to create a database of A-scan signals from healthy, surface, subsurface, and edge defect railheads. The classification capabilities of a support vector machine (SVM), a fully connected deep neural network (DNN), and a convolutional neural network (CNN) were examined after they were applied to the preprocessed signals without extracting any statistical/signal processing-based characteristics. The comparative analysis demonstrates that CNN is robust in classifying railhead defects. As a result, when combined with CNN, the laser ultrasonic technology may ensure automatic defection and classification of railhead surface and subsurface flaws.

Frequency-Dependent Scattering of Laser-Generated Rayleigh Waves for Surface Crack Characterization

Frequency-Dependent Scattering of Laser-Generated Rayleigh Waves for Surface Crack Characterization

Remote characterization of surface slots by enhanced laser-generated ultrasonic Rayleigh waves

Characterization of surface features is essential in many industrial applications, especially for features with large depths, high aspect ratios or under extreme conditions. This work presents a non-contact method to characterize surface slots with large lengths using ultrasonic Rayleigh waves generated by a pulsed laser. A delay-and-sum superposition technique is applied to enhance the signal to noise ratio of transmitted Rayleigh waves. The length of the slot can be calculated from the time-of-flight information of Rayleigh waves without any prior knowledge of its orientation, width or aspect ratio. Both numerical simulations and experiments are conducted to demonstrate the proposed method, showing excellent performance. Furthermore, mode conversion has been studied to understand its impact on the reconstruction accuracy. Given the non-contact feature of the laser ultrasonic technique, the proposed method provides a simple and feasible avenue for the rapid characterisation of normal and angled surface features with high aspect ratio in extreme environments.

Numerical simulation of laser-generated Rayleigh wave pulses propagation in the machined surface with residual stress using finite-difference method

Laser-excited broadband Rayleigh wave propagation behaviors can be used for characterizing material surface characteristics non-destructively. The present paper is concerned with the Rayleigh wave propagation in the machined surface with a damaged layer with residual stress. Biot’s theory of small deformations affected by initial stress forms the basis for this study. Taking ground silicon wafer as an example, we adopted a staggered finite-difference (FD) scheme for numerical simulation. By comparing the dispersion images with the theoretical dispersion curves of Rayleigh wave, the accuracy of the FD algorithm is verified. Then, some two-layer models without or with residual compressive stress are utilized to further analyze the dispersion energy characteristics of Rayleigh waves in the machined surface. The results indicate that damage layer thickness and damage degree determine the main Rayleigh wave high-order mode and the energy of the high-order mode, respectively. In addition, residual compressive stress can reduce the phase velocity of the Rayleigh wave to a small extent. Meanwhile it can significantly increase the total energy of the Rayleigh wave which excited by the same excitation source. This paper will provide a valuable reference for laser ultrasonic evaluation of residual stress on the machined surface.

Non-Contact Inspection of Railhead via Laser-Generated Rayleigh Waves and an Enhanced Matching Pursuit to Assist Detection of Surface and Subsurface Defects.

Laser ultrasonic technology can provide a non-contact, reliable and efficient inspection of train rails. However, the laser-generated signals measured at the railhead are usually contaminated with a high level of noise and unwanted wave components that complicate the identification of defect echoes in the signal. This study explores the possibility of combining laser ultrasonic technology (LUT) and an enhanced matching pursuit (MP) to achieve a fully non-contact inspection of the rail track. A completely non-contact laser-based inspection system was used to generate and sense Rayleigh waves to detect artificial surface horizontal, surface edge, subsurface horizontal and subsurface vertical defects created at railheads of different dimensions. MP was enhanced by developing two novel dictionaries, which include a finite element method (FEM) simulation dictionary and an experimental dictionary. The enhanced MP was used to analyze the experimentally obtained laser-generated Rayleigh wave signals. The results show that the enhanced MP is highly effective in detecting defects by suppressing noise, and, further, it could also overcome the deficiency in the low repeatability of the laser-generated signals. The comparative analysis of MP with both the FEM simulation and experimental dictionaries shows that the enhanced MP with the FEM simulation dictionary is highly efficient in both noise removal and defect detection from the experimental signals captured by a laser-generated ultrasonic inspection system. The major novelty contributed by this research work is the enhanced MP method with the developments of, first, an FEM simulation dictionary and, second, an experimental dictionary that is especially suited for Rayleigh wave signals. Third, the enhanced MP dictionaries are created to process the Rayleigh wave signals generated by laser excitation and received using a 3D laser scanner. Fourth, we introduce a pioneer application of such laser-generated Rayleigh waves for inspecting surface and subsurface detects occurring in train rails.

Subsurface defect detection using phase evolution of line laser-generated Rayleigh waves

An unreported phenomenon of phase evolution of Rayleigh ultrasonic waves with subsurface defects is observed and systematically explored for detection of subsurface defects using non-contact line laser ultrasonic technique and numerical simulation. The mechanism of phase evolution of Rayleigh wave signals is explained by the interference of the reflected and direct Rayleigh wave, explored by finite element analysis. Both experiments and simulation show distinct peak evolution of the Rayleigh wave signals with the width and depth of subsurface defect. A dimensionless parameter (|Neg|/Pos), defined by the ratio of absolute negative peak to positive peak of Rayleigh wave, is proposed to evaluate the phase evolution of Rayleigh wave with defect width and depth, which is further used to quantify the subsurface defects. The phase evolution of Rayleigh waves can act as a robust and sensitive feature to detect subsurface defects using laser-generated ultrasound, which has promising applications in life prediction and health monitoring of various engineering structures.

Surface circular-arc defects interacted by laser-generated Rayleigh wave

In this work, the finite element method (FEM) is used to investigate the propagation of laser-generated Rayleigh wave along the material surface at the quarter-arc transition surface under the thermoelastic regime, and to establish the relationship between the circular-arc radius and the time domain characteristics of reflected and transmitted Rayleigh waves. The simulation shows that the amplitude of the reflected Rayleigh wave decreases whereas the amplitude of the transmitted Rayleigh wave increases as the radius increases, which is significantly different from the well-studied interaction of Rayleigh waves with the perpendicular transition surface. By introducing the circular-arc defects which are easily formed in some engineering components during the material surface quenching, we find that the depth gauging of the surface circular-arc defects is more accurate in comparison to the surface rectangular defects based on the arrival time of the transmitted Rayleigh wave. This is further verified by the corresponding experimental results. These foundings are of practical values for detecting the depth of the arc defect quantitatively by the laser ultrasonic technique.

Detection of Internal Holes in Additive Manufactured Ti-6Al-4V Part Using Laser Ultrasonic Testing

For a non-contact, non-destructive quality evaluation, laser ultrasonic testing (LUT) has received increasing attention in complex manufacturing processes, such as additive manufacturing (AM). This work assessed the LUT method for the inspection of internal hole defects in additive manufactured Ti-6Al-4V part. A Q-switched pulsed laser was utilized to generate ultrasound waves on the top surface of a Ti-6Al-4V alloy part, and a laser Doppler vibrometer (LDV) was utilized to detect the ultrasound waves. Sub-millimeter (0.8 mm diameter) internal hole defect was successfully detected by using the established LUT system in pulse-echo mode. The method achieved a relatively high resolution, suggesting significant application prospects in the non-destructive evaluation of AM part. The relationship between the diameter of the hole defects and the amplitude of the laser-generated Rayleigh waves was studied. X-ray computed tomography (XCT) was conducted to validate the results obtained from the LUT system.

Numerical study on the mechanism of laser-generated Rayleigh waves’ interaction with a surface notch at high temperature

Quantitative characterization of surface notches in metallic materials has long been a hot research topic. In order to quantitatively characterize a surface notch, the finite element method is used to study the interaction of Rayleigh waves with different types of surface defects, and we then compare the difference between their displacement signals. The simulation results show that the oscillation signal following the Rayleigh pulse resulting from the mode-convened shear wave can quantitatively characterize the surface notch at different temperatures, where the notch width and depth can be characterized by the arrival time and the temporal cycles of the oscillation signal, respectively. Moreover, as the temperature increases, the arrival time of the oscillation signal will be delayed, but the temporal cycles will remain unchanged. This research provides a theoretical basis for the quantitative detection of surface defects at high temperature.

Research on laser-generated Rayleigh waves with angled surface crack by finite element method

This paper uses the finite element method to study the interaction between laser-generated ultrasonic wave and the angled surface crack. Laser-generated ultrasonic wave propagation images at different times has been analyzed in detail, when laser-generated R-wave arrived at the angled surface crack, the maximum amplitude of laser-generated R-wave increases, the maximum amplitude of R-wave increases with the crack orientation angle θ decreases from 60° to 15°, the amplitude of the corresponding center frequency spectrum increases with θ decreases. The numerical result shows that the method can effective detect different angled surface crack.

Numerical simulation of the interaction of laser-generated Rayleigh waves with subsurface cracks

In this article, the interaction of a laser-generated Rayleigh wave with a subsurface crack has been simulated in detail. The finite element method (FEM) was used to model a subsurface hollow with a rectangular shape, which was used to represent the subsurface crack. By studying the contributions of all of the reflected and mode-converted signals from the subsurface crack, the arrival time of those signals and the mechanism underlying their occurrence could be fully explained. The first negative peak of the reflected waves originated from the top corner of the subsurface crack. The arrival time of this peak first decreased and then increased as the depth of the subsurface crack increased, which was due to the mode conversion when the diffracted bulk mode interacted with the free surface. However, when the depth of the crack remained the same and the height of the subsurface crack increased, the arrival time of the first negative peak of the reflected waves remained the same but the arrival time of the following reflected peaks maintained a linear relationship with the height of the crack. Furthermore, the sources and mechanism of the reflected waves were studied. It was found that the interval time of the peaks was dependent on the height of the crack, and a method for measuring the height of subsurface cracks has been proposed in this paper. The results of the simulation enable the quantitative measurement of the size of subsurface cracks and the height measuring method has the potential to measure the height of subsurface defects and structures.

Laser-generated Rayleigh wave for width gauging of subsurface lateral rectangular defects

In this study, the widths of subsurface lateral rectangular defects were successfully gauged using laser-generated Rayleigh waves. Finite element analysis was used to simulate the interaction between Rayleigh waves and subsurface defects. The results of the simulation show that diffracted bulk modes resulting from the interaction between Rayleigh waves and subsurface defects may interact with the free surface. Modes are converted into Rayleigh waves that propagate along the surface. The arrival time of reflected mode-converted Rayleigh waves remains constant, even when the height of subsurface defects increases. However, as the depth increases, the arrival time of reflected mode-converted Rayleigh waves changes due to mode conversion caused by diffracted bulk modes interacting with the free surface. The influence of mode conversion was quantitatively analyzed and taken into account in the width measurement method for subsurface lateral defects. Experimental results are consistent with simulation results and show that Rayleigh waves induced by a pulsed line laser are sensitive to subsurface defects. Finally, the proposed method may be potentially useful for width measurements of other types of subsurface structures.