Laser Ultrasonic Technique 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.

Subsurface defect detection of high-temperatured components with laser induced surface wave

Abstract Subsurface defects are detrimental to the safety and integrity of critical components in various fields, particularly for those used in high-temperature environments. For this reason, a reliable non-destructive evaluation (NDE) method for in-situ inspection of subsurface defects of high-temperatured components is highly desired. Laser ultrasonic technique offers a promising potential to accommodate the demands in virtue of non-contact generation and detection of ultrasonic waves. In this work, laser ultrasosnic inspection of subsurface defects in high-temperatured components is proposed, which exploits the modal conversion of surface longitudinal wave to Rayleigh wave. The modal conversion was firstly investigated with finite element simulation, from which the phenomenon of modal conversion can be readily identified. Thereafter, an experimental setup is built to verify the feasibility and effectiveness of proposed method. Notably, a delay-and-sum method based on a scanning procedure is conducted to coherently increase the detection sensitivity of subsurface defects, since the surface longitudinal wave rapidly vanishes with propagation distance., and the temperature-dependant velocity variation can be adaptively compensated. This work provides a viable route for in-situ inspection of subsurface defects in high-temperatured components where conventional ultrasonic method fails, and it would find potential applications in broad fields, such as coating quality evaluation in fabrication, bearing assessment under load, and in-situ monitoring for additive manufacturing.

Laser Ultrasonic Technique for Simultaneous Measurement of Thickness, Slope, and Wave Velocities for Slope Plate in the Thermoelastic Regime

Laser Ultrasonic Technique for Simultaneous Measurement of Thickness, Slope, and Wave Velocities for Slope Plate in the Thermoelastic Regime

Study on the interaction mechanism of laser-generated Rayleigh waves and subsurface inclined cracks

Abstract In this paper, the finite element method was used to study the reflected and transmitted waves of laser-generated Rayleigh waves from subsurface inclined cracks, the propagation paths and mode conversion mechanisms of different characteristic waves are determined. The Rayleigh wave will interact with the crack top tip and propagate back and forth along the crack surface, and be converted to shear waves at the crack top tip. The shear waves will mode-convert to Rayleigh waves at the free surface when the incidence angle of the shear wave is larger than 60°. Moreover, for the Rayleigh wave interacting with the crack bottom tip, when the crack inclined angle is less than 60°, some Rayleigh waves will travel along the crack surface to the crack top tip. When the crack inclination angle is greater than 60°, in addition to the Rayleigh waves propagating upwards along the crack surface, some Rayleigh waves convert to shear waves at the crack bottom tip and then incident on the free surface of the workpiece. Experiments were carried out to validate some of the Rayleigh wave propagation paths. The experimental results matched the theoretical arrival time well, thus verifying the reliability of the analytical wave path. The results are helpful for the quantitative detection of subsurface inclined cracks using laser ultrasonic techniques.

Simultaneous measurement of thickness and ultrasonic wave velocity using a combination of laser-generated multiple wave modes in the thermoelastic regime

ABSTRACT This paper introduces a reliable laser ultrasonic technique without damage to the surface in the thermoelastic regime that provides a one-side access method for simultaneously determining the thickness and ultrasonic wave velocity by employing multiple wave modes. Initially, this study determines the optimal separated distance between two laser beams to achieve an effective signal-to-noise ratio of the employed wave modes. Subsequently, a pre-established ratio of 0.95 for the velocity of skimming longitudinal waves and longitudinal waves enables the simultaneous calculation using a combination of the skimming longitudinal wave, reflected shear wave, and mode-converted longitudinal wave in a single measurement. Experiments were carried out on various plane plates made of aluminium alloy 6061, ranging in thickness from 4 to 20 mm, demonstrating a high accuracy in thickness measurement, with a maximum absolute error of 0.05 mm. Compare the results of ultrasonic wave velocity measurements with theoretical calculations, the maximum absolute error is 32 m/s and 80 m/s for longitudinal wave and shear wave, respectively.

Effects of thickness variation due to presence of roller wake on the thickness measurement using laser ultrasonic technique

Effects of thickness variation due to presence of roller wake on the thickness measurement using laser ultrasonic technique

Acoustoelastic characterization of aluminum plates using zero group velocity Lamb modes

Acoustoelasticity, a characteristic of material anharmonicity, gives rise to a link between wave propagation velocity and the stress state in materials. Ultrasonic techniques to monitor this coupling, particularly with high sensitivity and in a noncontact manner, can have widespread application both in the quantification of applied and residual stress and in the characterization of nonlinear material behavior through measurement of higher order elastic constants. Here, we use a laser ultrasonic technique to excite and detect zero group velocity (ZGV) Lamb wave resonances in aluminum plates under uniaxial loading. A laser line source is used to excite these resonances at different orientations with respect to the applied load and the signals are detected using an interferometer. The effects of stress and source orientation on ZGV resonance frequencies are validated using the theory of acoustoelastic Lamb wave propagation. In addition, a model-based inversion technique is used to extract Murnaghan’s third-order elastic constants from measurements of the stress dependence of the first two ZGV modes generated parallel and perpendicular to the applied load. Laser generation and detection of ZGV resonances is shown to be an effective and powerful approach for the noncontact and nondestructive acoustoelastic characterization of elastic waveguides.

Research on the Method of Absolute Stress Measurement for Steel Structures via Laser Ultrasonic

Accurate measurement of the stress in steel structures is crucial for structural health monitoring. To achieve this goal, a novel technique, the laser ultrasonic technique, was used in absolute stress measurement in this study. The feasibility of this technique has been verified through theoretical analysis and finite element (FE) analysis. A stress measurement experiment in steel specimens was conducted and the relationship between ultrasonic relative wave velocity and stress was explored. The results revealed that there is a similar linear correlation between the ultrasonic relative wave velocity and absolute stress. The stress can be obtained based on ultrasonic relative wave velocity. According to the stress measurement results, it was found that the absolute error between the measured stress and theoretical stress was largest when the stress level was low, and that the measured error of stress gradually decreased with increases in the applied stress. The relative error between the measured stress and the theoretical stress was within 10% when the stress was higher than 100 MPa. This further verifies the reliability of the laser ultrasonic technique under high-stress conditions. Additionally, the impact of temperature and surface roughness on stress measurement was analyzed. The stress error in stress measurement increased similarly linearly with the increase in temperature and increased non-linearly with the increase in roughness. The corresponding compensation methods were proposed to effectively improve the accuracy of measurement.

Acoustoelastic characterization of plates using zero group velocity Lamb modes

Acoustoelasticity, a characteristic of material anharmonicity, gives rise to a link between wave propagation velocity and the stress state in materials. Ultrasonic techniques to monitor this coupling, particularly with high sensitivity and in a noncontact manner, can have widespread application both in the quantification of applied and residual stress and in the characterization of nonlinear material behavior through measurement of higher order elastic constants. Here, we use a laser ultrasonic technique to excite and detect zero group velocity (ZGV) Lamb wave resonances in aluminum plates under uniaxial loading. A laser line source is used to excite these resonances at different orientations with respect to the applied load, and the signals are detected using an interferometer. The effects of stress and source orientation on ZGV resonance frequencies are validated using the theory of acoustoelastic Lamb wave propagation. In addition, a model-based inversion technique is used to extract Murnaghan's third-order elastic constants from measurements of the stress dependence of the first two ZGV modes generated parallel and perpendicular to the applied load. Laser generation and detection of ZGV resonances is shown to be an effective and powerful approach for the noncontact and nondestructive acoustoelastic characterization of elastic waveguides.

Ultrasound-assisted material characterization: An innovative method for the non-destructive in-situ detection of microstructural changes of high-strength aluminum alloys

The use of aluminum alloys in conjunction with thermal-assisted forming techniques permits the production of high-strength and geometrically complicated components without failure. By using innovative, temperature-dependent production techniques, it is possible to manufacture components with tailored properties. So far, however, it is not possible to characterize the microstructural behavior under thermal and mechanical load in real-time. Using a novel characterization method, which is based on the principle of laser ultrasonic testing, it has become possible to record contact-free and non-destructive microstructural changes in-situ during thermo-mechanical treatment. This allows the interaction-free detection of thermal as well as mechanical effects during thermal-supported forming operations. In this research work, the temperature-dependent behavior during the quenching and aging process of the precipitation-hardenable aluminum alloy AA6082 for a tailor-quenched forming process is recorded in real-time using laser ultrasonic techniques. First, the effects of different tool temperatures on quenching rates and material properties were investigated using conventional characterization methods, including temperature measurement, hardness and tensile tests. It was shown that a die temperature of 325 °C leads to the lowest material hardness and strength. Subsequently, the results were compared with the novel, non-destructive in-situ testing method and validated using microstructural analysis. It was proven, that microstructural changes can be identified during varying quench and aging operations. Thus, the influence of different process parameters on the microstructural and mechanical development of aluminum components can be detected in-situ for the first time.

Difference in Thickness Measurement by Laser Ultrasonic Technique Using Time of Flight Methods

Difference in Thickness Measurement by Laser Ultrasonic Technique Using Time of Flight Methods

Nondestructive testing of internal defects by ring-laser-excited ultrasonic

ABSTRACT To improve the excitation efficiency, detection utilisation rate, and signal-to-noise ratio of the laser ultrasonic non-destructive testing of internal defects, the spatial distribution of the excitation light source was changed to a ring-shaped laser source to excite ultrasound. Finite element analysis method was used to simulate and analyse the ultrasonic characteristics excited by the ring laser. The superposition of shear waves on the central axis of a ring laser exhibits considerable advantages for internal defect detection. Consequently, a method was proposed for detecting internal defects using a ring laser to excite and receive ultrasonic waves at the concentric and opposite positions of the ring laser. A ring-source laser ultrasonic detection system was designed to scan and detect circular hole defects with different diameters as well as crack defects with different depths, lengths, and inclination angles. The experimental results showed that the ring-source laser ultrasonic technique could effectively detect defect positions and depths and describe the defect length and external contour under certain conditions. Moreover, it could achieve more complex detection tasks. This study provides an effective reference for the application of laser ultrasonic non-destructive testing of internal defects.

A sensitivity-enhanced all-optical probe for non-contact laser ultrasonic inspection

Non-contact laser ultrasonic technique has been increasingly implemented for non-destructive inspections in harsh environments, high-temperature fields, and components having complex geometries. However, the poor signal-to-noise ratio and low amplitude of laser generated ultrasonic signals under a thermoelastic regime severely restrict its applications. Here, a sensitivity-enhanced all-optical probe was proposed for laser ultrasonic non-destructive testing. It consists of an optical sensor and an ellipsoidal acoustic cavity, where an optical sensor is placed at one focus of the cavity, and the detection point is set at another focus. The ultrasound signals are focused through the cavity and detected by the optical sensor. Side-by-side comparison experiments were carried out, and the results show that the probe can improve the signal amplitude by about 7.8 times compared to using a traditional optical sensor alone. The probe can make laser ultrasound detect defects with lower laser energy, which is of great significance to improve the efficiency of non-contact defect detection.

Study on non-contact measurement method of resistance spot weld nugget diameter using laser ultrasonic technique

Resistance spot welding can instantaneously join two or more plates by the resistance heating of the metal and is in high demand owing to its high productivity and high-work efficiency. For the quality control of resistance spot welding, the size and quality of the welded part, called the nugget, are important. Therefore, this study aims to establish a highly efficient and high-speed resistance spot-weld inspection method that can be applied to whole-lot inspection, which is currently a difficult process. The objective of this study is to measure the nugget diameter using laser ultrasonic technique that enable remote, non-contact ultrasonic inspection. An investigation of the available ultrasonic waves using simulated test specimens demonstrated the feasibility of estimating the distance from the generation/detection point to the joint using the diffraction of the Lamb wave, which can propagate long distances in a thin plate. By measuring the actual resistance spot welding specimens, it was determined that differences in the nugget diameter of approximately 0.5 mm could be clearly distinguished from the arrival time of the diffraction waves. It was also inferred that the nugget diameter could be calculated by determining the propagation velocity of the diffraction wave with a similar accuracy to that of the measurement using a contact-type probe.

Numerical study of length and angle gauging for subsurface-inclined cracks based on reflected surface waves with laser ultrasonic technique

ABSTRACT To achieve quantitative detection of subsurface-inclined cracks, the reflected surface waves from subsurface cracks with different lengths and angles were obtained by the finite element method (FEM). The relationship between the time difference of each feature wave in the reflected waves and the length and angle of the subsurface inclined crack was analysed. The arrival time difference between the lowest trough and the first trough of the reflected wave is independent of the angle of the subsurface crack and approximately a linear function of the crack length. Furthermore, when the crack lengths remain unchanged, the arrival time difference between the highest peak and the second trough of the reflected wave is a quadratic function with the angle of the subsurface crack. Thus, a quantitative measurement method for gauging the length and angle of subsurface inclined cracks is proposed based on the above phenomena. The relationships between the length and angle of the subsurface inclined crack and the arrival time difference of the reflected surface waves were obtained by curve fitting. The proposed method was verified by simulation data. The maximum relative error of the crack length obtained by the method was 7.76%, and the maximum relative error of the obtained inclination angle was 11.61% when the crack angle was large. The proposed approach will open the way for simultaneous measurement of the length and angle of subsurface inclined cracks and structures.

Laser-generated surface waves for quantitative detection of inclined surface cracks based on finite element analysis

ABSTRACT In this paper, transmitted surface waves are used to quantitatively detect the length and angle of inclined surface cracks based on laser ultrasonic technique. The interaction between laser-generated surface waves and inclined surface cracks is analysed, and the propagation path and mode conversion process of the transmitted surface waves are considered. Then, the propagation paths of some peaks and troughs of the transmitted surface waves are obtained. Based on the obtained path and modal conversion information, a quantitative detection method for the length and angle of inclined surface cracks is proposed. The finite element method is used to verify the propagation paths and proposed method, and the results show that the arrival time of the theoretical path is consistent with that of the simulation. The relative measurement errors of the surface crack length and angle are less than 15% and 10%, respectively, which verifies the rationality and correctness of the proposed method.

Non-contact subsurface thermography for high-temperature metallic structures using laser ultrasound and wave-equation-based inversion

Temperature distribution is an important factor influencing the formation of defects in engineering components operating at high temperatures. A non-contact thermographic method based on the laser-ultrasonic technique and the wave-equation-based inversion (WEBI) algorithms is proposed. Three algorithms, which are full waveform inversion (FWI), traveltime waveform inversion (TWI), and double-difference traveltime waveform inversion (DDTWI), are used to reconstruct the temperature maps of the subsurface region of high-temperature metals. The performances of the algorithms are investigated both numerically and experimentally. Among these algorithms, FWI shows the highest accuracy and the largest reconstruction depth but is also susceptible to noise. TWI and DDTWI show similar performances in terms of the reconstruction depth and accuracy, while TWI has better resistance to noise.

High temperature elastic properties of sub-stoichiometric yttrium dihydrides

Yttrium hydrides are considered as candidate materials for neutron moderation applied in microreactors (akin transportable miniaturized nuclear reactors) owing to their superior thermal stability and hydrogen retention. The evolution of elastic properties of these materials at elevated temperatures, needed for predicting the thermomechanical response and performance of the moderator during in-service reactor conditions, however, is lacking. Here, we report the Young’s and shear elastic moduli of three stoichiometries of bulk yttrium hydride (YHx, x = 1.61, 1.82, and 1.84) from room temperature to 1000 °C. In situ temperature-dependent measurements of the longitudinal and shear wave velocities were performed using a laser ultrasonic technique while heating the sample in a vacuum-pumped heating stage. The elastic moduli increased linearly with increasing hydrogen content and decreased by ∼10 % during heating from room temperature to 1000 °C in the three YHx compositions. The linear relationship between the elastic moduli and the hydrogen content in yttrium hydride was verified by atomistic calculations based on density functional theory (DFT). The absence of abrupt changes in the temperature-dependent measurements of elastic modulus of the YHx samples suggested negligible loss of hydrogen at elevated temperatures. Excellent agreement was found between the measured and calculated dependence of the elastic moduli on the stoichiometry, thereby providing a new approach for investigating the effects of fabrication-induced parameters (such as porosity) on the elastic moduli. This study demonstrates the utility of the combined approach involving DFT-based atomistic calculations and measurements of the elastic moduli for the informative development of metal hydrides and can be used as a metric for novel moderator materials investigations for emerging microreactors and beyond.

Study on Blowhole Detection in Fillet Welded Sheet of Lap Joint by Laser Ultrasonic Technique

Study on Blowhole Detection in Fillet Welded Sheet of Lap Joint by Laser Ultrasonic Technique

Laser ultrasonic imaging of complex defects with full-matrix capture and deep-learning extraction

Phased array-based full-matrix ultrasonic imaging has been the golden standard for the non-destructive evaluation of critical components. However, the piezoelectric phased array cannot be applied in hazardous environments and online monitoring due to its couplant requirement. The laser ultrasonic technique can readily address these challenging tasks via fully non-contact inspection, but low detection sensitivity and complicated wave mode conversion hamper its practical applications. The laser-induced full-matrix ultrasonic imaging of complex defects was displayed in this study. Full matrix data acquisition and deep learning method were adapted to the laser ultrasonic technique to overcome the existing challenges. For proof-of-concept demonstrations, simulations and experiments were conducted on an aluminum sample with representative defects. Numerical and experimental results showed good agreement, revealing the excellent imaging performance of proposed method. Compared with the total focusing method based on ray-trace model, the deep learning method could create superior images with additional quantitative information through end-to-end networks, which use the hierarchical features and generate details from all the relevant imaging and physical characteristics information. The proposed method may help assess defect formation and development at the early stage in a hazardous environment and understand the in-situ manufacturing process due to its couplant-free nature.