Laser Ultrasonic Waves Research Articles

Low-Power Field-Deployable Interdigital Transducer-Based Scanning Laser Doppler Vibrometer for Wall-Thinning Detection in Plates

Lamb waves have become a focal point in ultrasonic testing owing to their potential for long-range and inaccessible detection. However, accurately estimating the flaws in plates using Lamb waves remains challenging because of scattering, mode conversion, and dispersion effects. Recent advances in laser ultrasonic wave techniques have introduced innovative visualization methods that exploit the dispersion effect of Lamb waves to visualize defects via, for example, acoustic wavenumber spectroscopy. In this study, we developed an interdigital transducer (IDT)-based scanning laser Doppler vibrometer (SLDV) system without a power amplifier using a low-power IDT fabricated from lead magnesium niobate–lead zirconate titanate single crystals. To validate the proposed low-power IDT-based SLDV, four different defective plates were measured for defects. A comparison between a conventional IDT-based SLDV, a dry-coupled IDT-based SLDV, and the proposed method demonstrated that the latter is highly reliable for measuring thin plate defects.

Evaluation of 3D printed 316L stainless steel microstructure and mechanical property by laser ultrasonics

ABSTRACT In this paper, the laser ultrasonic non-destructive testing technology was used to detect the microstructure and mechanical properties of selective laser melting (SLM) 316 L stainless steel after post-treatment. The microstructure and mechanical properties of additively manufactured 316 L stainless steel were measured by Scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD), tensile testing, etc. The laser ultrasonic longitudinal wave signal characteristic values were coupled with the microstructure and mechanical properties of SLM 316 L stainless steel. The results show that: The yield strength has a good correlation with wave speed and attenuation. With the increase of holding temperature and holding time, the grain size increases, the low angle grain boundary decreases, the yield strength gradually decreases from 409 MPa to 359 MPa, the wave velocity increases from 5579 m/s to 5700 m/s, and the frequency domain attenuation coefficient increases from 0.13 dB/mm to 0.16 dB/mm, the central frequency domain is about 10 MHz. The ultimate tensile strength is related to the frequency domain attenuation. The ultimate tensile strength decreases with the increase of the frequency domain attenuation coefficient. It can provide a reference for the evaluation of mechanical properties by laser ultrasonic testing.

Efficiency analysis of laser ultrasonic longitudinal wave detection based on the constraint effect

Abstract Laser ultrasound is a non-destructive inspection technique, but its application in internal defect detection and state characterization is limited by its weak body wave displacement induced by thermo-elastic mechanism. In this paper, the influence of the constraint layer on the directivity of laser induced ultrasonic force source and the excitation efficiency of longitudinal wave is analyzed by finite element modeling with transparent glass as the constraint layer. With the optimization of the parameters of the confinement layer, the longitudinal waves with energy comparable to that of the free surface ablation mechanism can be excited under the thermoelastic mechanism, which greatly improves the efficiency of the longitudinal wave of laser ultrasonic. Applying the above research results to the detection of internal defects, the imaging results of the internal defects are obvious and the location defects are accurate.

Longitudinal Wave Defect Detection Technology Based on Ablation Mechanism

For laser ultrasound in the thermoelastic mechanism, excitation of ultrasonic body wave signal is weak, it is not easy to realize the detection of deep defects inside the workpiece. While the ablation mechanism produces a high and practical ultrasonic signal-to-noise ratio, this paper is based on the generation mechanism of laser ablation excitation of ultrasonic waves, the establishment of laser ultrasound in the ablation mechanism in the aluminum plate excitation and propagation of ultrasonic numerical model, through the solution, obtained the ultrasonic acoustic field map, discussed the ablation mechanism of the laser ultrasonic body wave acoustic field directionality. Additionally, the preliminary verification of the validity of the model is presented. Then, in order to explore the application potential of high signal-to-noise ratio longitudinal waves in defect detection, defects of different depths are preset in the model for simulation calculations, and waveform and acoustic field analyses are performed on the simulation results to study the ultrasonic propagation paths inside the member and the interaction with the defects, and the transverse position and depth of the internal defects are judged by using B-scan imaging. Finally, experimental validation is carried out. The experimental results are highly consistent with the simulation model, and the defect experiments can qualitatively determine the location of internal defects and verify the practicality and accuracy of the model.

Internal Hole Defect Detection Based on Laser Ultrasonic Shear Wave

Internal Hole Defect Detection Based on Laser Ultrasonic Shear Wave

Image quality improvement in multi-mode laser ultrasound imaging using reconstructed synthetic aperture focusing technique

The synthetic aperture focusing technique (SAFT) is used as a laser ultrasonic imaging algorithm for non-destructive internal defect testing, and multi-mode time domain SAFT (MMT-SAFT) imaging is developed to enhance the imaging capability of defect detection. However MMT-SAFT algorithm has the problem of artifacts caused by the laser ultrasound (LU) modes waves. Thus to solve this problem, an automatic imaging technique based on MMT-SAFT called Reconstructed synthetic aperture focusing technique (Re-SAFT) is proposed. This technique consists of two steps: a preliminary MMT-SAFT is firstly performed to locate the defect position tentatively and futher separate LU modes wave signals using the defect-reflected signal; then a secondary reconstructed MMT-SAFT based on the priori defect location information is achieved for artifacts removal. In addition, directivity patterns of LU modes wave have been used in Re-SAFT to achieve more accurate defect imaging in the second step. Results indicate that by using Re-SAFT, the typical artifact caused by laser ultrasonic waves can be removed and the imaging quality can be significantly improved from 19.67 to 33.85 dB. Further, the consumed time of Re-SAFT is economized by optimizing the imaging pixel size without compromising the image quality. Therefore, Re-SAFT has potential applications in industrial laser ultrasonic testing.

Efficiency enhancement and application of laser ultrasonic longitudinal wave based on glass constraint

Due to the weak longitudinal signals generated by laser ultrasound in the thermoelastic mechanism, the characteristic echoes are weak when evaluating the interior of solids, thus limiting its application to internal defect detection. A glass confinement layer is introduced to enhance the longitudinal excitation effectiveness. Specially, a thermoelastic model of laser ultrasound with a glass confinement is established to explain the mechanism of the enhancement of the longitudinal wave effectiveness, and the effect of the glass width on the longitudinal wave generated by the base ultrasound is investigated. The effect of the glass confinement layer on the enhancement of the effectiveness of the internal defects detection is studied. The simulation and experimental results show that the longitudinal waves with high signal-to-noise ratio induced from thermoelastic effect are excited similar to the ablation mechanism, which greatly improves the excitation efficiency of the longitudinal waves. The deep detection defects and the accurate localization of depth information are realized with an error of no more than 1.2%.

Noncontact measurement of bolt axial force during tightening processes using scattered laser ultrasonic waves

This paper presents a new methodology for noncontact measurement of the axial force of bolts in their tightening processes using laser-generated ultrasound waves. This method employs ultrasound waves scattered in a bolt shaft to detect axial force changes, while most conventional ultrasonic methods use ultrasound waves propagating linearly along the bolt axis. The ultrasound waves in this study are generated by laser irradiation on the top surface of a bolt. Subsequently, they propagate deeply into the shaft and return towards the top of the bolt through complicated paths due to the multiple scattering in the bolt shaft. Finally, they are detected at the top surface using another laser and a speckle knife edge detector. With an examination based on the finite element analysis and verification experiments, it has been shown that the waveform of the scattered ultrasound shifts in time linearly with increasing the axial force. The time shifts were estimated using the cross-correlation analysis between the measured waveforms and the reference waveform with no axial force. This result demonstrates the feasibility of estimating the change in axial force during tightening processes once the relationship between the time shifts and axial force is obtained for the specific type of bolt to be used in products. Furthermore, the proposed technique does not require machining to flatten a bolt's head and the end, while conventional ultrasonic methods need the flattening procedures, enabling fast, cost-effective axial force measurement in mass production manufacturing processes.

Quantitative detection of rail head internal hole defects based on laser ultrasonic bulk wave and optimized variational mode decomposition algorithm

This paper proposes a quantitative rail head internal defect detection method combining mode conversion and time-of-flight based on the theory of laser ultrasonic bulk wave scattering and diffraction. The interaction between laser ultrasonic bulk wave and rail internal hole defects is modeled using finite element simulation, and the modal conversion of bulk wave encountered defects is investigated. The feasibility of localization and the quantitative method are both confirmed. Simultaneously, an Optimized Variational Modal Decomposition (OP-VMD) method is proposed to solve the problem of low SNR of bulkwave signals and to facilitate multi-mode wave discrimination. This method can extract the defect diffraction Shear Wave (S) signal from complex ultrasonic signals. Finally, experiments are performed on the defects of artificial internal holes in rail heads with varying burial depths and sizes. The experimental results show that the method can not only detect but also quantify the defects of internal holes in rail heads. The location relative errors are within 3%, and the quantitative relative errors are within 7%.

Noncontact measurement of bolt axial force in tightening processes using scattered laser ultrasonic waves

This paper presents a new methodology for noncontact measurement of the axial force of bolts in their tightening processes using laser-generated ultrasound waves. This method employs ultrasound waves scattered in a bolt shaft to detect axial force changes, while most conventional ultrasonic methods use ultrasound waves propagating linearly along the bolt axis. The ultrasound waves in this study are generated by laser irradiation on the top surface of a bolt. Subsequently, they propagate deeply into the shaft and return towards the top of the bolt through complicated paths due to the multiple scattering in the shaft. Finally, they are detected at the top surface using another laser and a speckle knife edge detector. With an examination based on the acoustoelastic theory and finite element analysis, we demonstrate that the waveform of the scattered ultrasound shifts linearly with axial force by the cross-correlation method. The proposed technique does not require machining to flatten a bolt's head and the end, while conventional ultrasonic methods need flattening procedures. The proposed technique enables fast, cost-effective axial force measurement in mass production manufacturing processes.

A laser-induced dual ultrasonic wave method for noncontact load monitoring of pillar porcelain insulators

In this paper, a noncontact load monitoring method based on laser-induced synchronous ultrasonic surface wave and air wave is presented to improve the accuracy of load measurement on pillar porcelain insulator. In order to eliminate the measurement error caused by insulator deformation, a correction algorithm for surface wave velocity calculation is established by introducing the air wave propagation time. An experimental setup of the load measurement system for pillar porcelain insulators based on laser-induced dual ultrasonic waves was assembled. A load calibration experiment and load detection experiment were carried out under various bending and torsional loads. The results showed that the proposed method is effective to solve the problem of the surface wave propagation time changing abruptly, greatly increasing the accuracy of load measurement. In the monitoring experiments, the relative error between the calculated load value and the actual applied load value was small, the average measurement error of the bending load was 16.20%, and the average measurement error of the torsional load was 11.38%. This proved that the measurement of the insulator load value using a laser ultrasonic surface wave is more precise than the traditional methods, making it more suitable for engineering inspection.

Fast and high-resolution laser-ultrasonic imaging for visualizing subsurface defects in additive manufacturing components

Additive manufacturing (AM) is an emerging technique for efficient fabrication of individually tailored and complex geometry parts. The fabrication process is prone to induce various defects that can have detrimental effects on the AM components. Therefore, a reliable technique that enables monitoring the integrity of AM components and in return helping to optimize the fabrication parameters in mission-critical structures is highly demanded. This work presents a fast and high-resolution damage visualization method using laser-ultrasonic (LU) imaging technique for accurately detecting and quantifying the subsurface defects in printed AM components. Specifically, a fully noncontact LU scanning system is implemented to generate and detect high signal-to-noise ratio laser ultrasonic waves using a pulsed laser and laser Doppler vibrometer, respectively. A strategy for fast defect localization using Rayleigh waves with circular scans is firstly proposed. The high-resolution 3D synthetic aperture focusing technique (SAFT) imaging with raster scans is subsequently performed focusing around the located damage areas to stereoscopically visualize and quantify the subsurface defects. The reconstructed images are further processed and improved using Gaussian filter algorithm to obtain accurate defect shapes, sizes, and positions. The feasibility of the proposed method is eventually verified on AlSi10Mg and stainless steel (316L) components containing subsurface defects with various types and dimensions. The measured sizes are well consistent with the designed values, suggesting that it is a reliable inspection method for AM parts to ensure quality control and feedback.

Interaction analysis of different defects with laser ultrasonic sound waves and defect characterization

The paper presented a novel approach for characterization of defects based on laser ultrasonic nondestructive testing. Both flat materials and curved materials are used as experimental samples, and crack-type defects and circular defects with different depths are applied as experimental objects. Through analyzing the interaction between different defects and acoustic waves, a novel characterization algorithm is proposed based on the energy loss theory to realize the characterization of defects. In more specific details, the defects are firstly scanned by laser ultrasonic test platform to obtain the wave field diagram. Then, the interaction between defects and the sound waves is analyzed, and the type of defects is determined based on the shape of the reflected waves. Finally, the interaction analysis and quantitative characterization are carried out through the reflection energy coefficient, and compared with other characterization methods. It was found that for circular defects in materials with different shapes, the reflection energy coefficient increases with the increases of defect depth. But for crack-type defects, when the depth is greater than 0.5 mm, the reflection energy coefficient increases first and then reaches a stable value. Through the study, not only the interaction between defects and sound waves is analyzed and verified, but also the proposed approach can be adopted for qualitative and quantitative characterization of defects.

Laser ultrasonic loading and optical interference detection of closed cracks in K9 glass.

The traditional optical detection method has a limitation in detecting closed cracks in optical elements. In this Letter, a novel, to the best of our knowledge, method has been developed for the detection of closed cracks in transparent samples. Firstly, behaviors of closed cracks under ultrasonic waves loading are demonstrated through the finite-element method (FEM). Simulation results show that the crack can be opened by the laser ultrasonic shear wave (S-wave). Then, a Mach-Zehnder interferometer system is experimentally established to detect closed cracks under ultrasonic waves loading in the sample. It is demonstrated that closed cracks which cannot be detected by interferometry become visible in interferogram images under ultrasonic waves loading. The width of the opened crack can also be quantitatively estimated. Thus the method would be a promising approach to detect initially closed cracks in optical elements.

2D slowness visualization of ultrasonic wave propagation for delamination detection in CFRP laminates

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A0) mode of the Lamb wave: the A0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves.

Nondestructive evaluation of porosity in additive manufacturing by laser ultrasonic surface wave

A noncontact and nondestructive method based on laser ultrasound is proposed for evaluation of porosity in additive manufacturing components. The 304L stainless steel samples with porosities in the range of 0.1%–5.7 % were fabricated using selective laser melting. Laser ultrasonic surface wave was used to characterize the porosity by the wave speed, peak frequency and wavelet packet energy. The result indicates that the surface wave speed, peak frequency and wavelet packet energy are sensitive to the slight variation of porosity in the samples. Good R-squares of fitting between surface wave velocity and porosity were obtained when it considers the pore irregular morphology. The peak frequency of surface wave decreases as the disappearance of high-frequency signals induced by the scattering attenuation. The wavelet packet analysis result shows that the sum of the percentage energy of the ninth layer of wavelet packet decomposition nodes 2–4 is linearly related to the porosity, and exhibits the highest coefficient of determination (R = 0.98) among the proposed characteristics. It is concluded that the laser ultrasonic surface wave based on wavelet packet energy is recommended to be a promising candidate for the quantitative nondestructive evaluation of the porosity of additive manufacturing components.

Assessment of the intrinsic damage to refractory materials at high temperatures using the laser ultrasonic pulse method

Refractory materials play a significant role for high temperature industrial processes. In such harsh environments occurring material damages are a challenge and their assessment is important. In this work refractory material damages have been assessed by laser ultrasonic pulse (LUP) method by investigating changes of Young's modulus and Poisson's ratio up to 1000 °C. The LUP method operates by laser ultrasonic wave generation and detection and is therefore non-contacting and non-destructive. As a consequence of material damages, the propagation velocities of the ultrasonic waves inside the materials are affected. Therefore, the elastic properties change and material damages can be determined up to a microstructural range. A broad variety of refractory ceramics were examined e.g., fused silica, alumina, andalusite and high alumina composite materials. After induced intrinsic damages by pressing and water quenching thermal shocks from 950 °C to 10 °C Young's modulus and Poisson's ratio decreased.

Investigation of the laser ultrasound propagation in K9 glass using optical interferometry

The propagating of laser-generated ultrasonic waves in K9 glass was investigated. Many methods have been developed to detect the laser ultrasound since laser ultrasonic waves can be used to measure material parameters or characterize materials properties. In order to reduce the measuring time, a Mach–Zehnder interferometer, a full field measuring tool, was preferred in this paper. The ultrasonic wave was produced on the K9 glass surface by a Q-switched Nd:YAG laser absorbed in a liquid layer. The interferograms were then taken at various delay times by a CCD camera after single pulse induced laser ultrasonic waves. Ultrasonic waves in the K9 glass can be observed from interferogram images. The results provide an understanding of laser ultrasound propagation in K9 glass in the lifetime.

Analysis on ultrasonic waves generated in anisotropic carbon fiber reinforced plastic laminate by laser incidence from various directions

Laser-ultrasonic waves are used in nondestructive inspections because they can be excited on arbitrarily shaped surfaces by irradiation of a pulsed laser from a distance. During the non-contact scanning of a laser, the waveform of excited ultrasonic wave is changed because the incident angle varies substantially. The change of the waveform will be more obvious for carbon fiber reinforced plastics (CFRP) laminates that have anisotropic properties. In order to guarantee the inspection, in this paper, we investigated the generation of ultrasonic waves in CFRP laminates by a laser irradiation with various incident angles and azimuth angles. Firstly, we constructed a simulation model: the thermal force field caused by the laser absorption is derived from a heat conduction equation, and it is introduced into an FEM software for analyzing the propagation behavior of generated ultrasonic waves. Then, the simulation result is compared with an experiment result. Through the FEM analysis, we found that the amplitude of the excited waves has an obvious directivity because of the strong anisotropy in the thermo-elastic property of the CFRP. In addition, the change in propagation behavior in the case of a small incident angle of 30° was found to be inconspicuous. However, when the incident angle of the laser beam is larger than 60°, the directivity pattern changes depending on the azimuth angle. Thus, it is important to control the incident angle and direction when a laser ultrasonic inspection is applied to CFRP structures.

Application of the Gas-Coupled Laser Acoustic Detection technique to non-destructive monitoring of mechanical components

Non-contact ultrasonic techniques are fundamental to devise online monitoring systems for moving or difficult to access structures. Gas-Coupled Laser Acoustic Detection (GCLAD) is an unestablished, non-contact detection technology which relies on measuring the deviation affecting a laser beam when travelling across an ultrasonic wavefront propagating in a fluid. The aim of the work is to provide in-depth highlights on the principles on which the technique leverages, with a view towards how several laser beam and ultrasonic wave features reflects on the signal acquired by the GCLAD device. By numerical and experimental approaches, parameters needing to be specifically addressed and suitably set during the investigation phase are highlighted, which enable amplitude maximization of the acquired signal. Specifically, effect of the probe laser beam spot size is thoroughly analyzed, as well as the mutual orientation between the beam and the ultrasonic propagation directions. Three test configurations are lastly proposed, providing different results in terms of GCLAD sensitivity to the acoustic waves; such differences are highlighted by applying the technique to a railway axle on which an artificial crack has been machined, providing a first assessment of the GCLAD capabilities in the non-destructive testing field.