Laser Ultrasonic Technique Research Articles

Estimation via Laser Ultrasonics of the Ultrasonic Attenuation in a Polycrystalline Aluminum Thin Plate Using Complex Wavenumber Recovery in the Vicinity of a Zero-Group-Velocity Lamb Mode

In this paper, we present a method to recover the complex wavenumber dispersion relations using spatial Laplace transform from experimental spatiotemporal signals measured by laser ultrasonic technique. The proposed method was applied on zero-group-velocity Lamb modes in order to extract the ultrasonic attenuation in a polycrystalline aluminum plate of about 70 μm thickness. The difference between the experimental and theoretical Laplace Fourier transforms was minimized in the least square sense to extract the complex amplitudes and complex wavenumbers of the modes at about 40 MHz. The experimental results were compared to values reported in the literature that were measured by other means and those estimated by using the quality factor extracted from a single temporal signal.

Application of all-optical laser ultrasonics for characterization of sub-mm layers in multilayer structure

The last decades have seen the rapid development of laser ultrasonic (LU) techniques for remote inspection for composite structures. However, it is still challenging for current LU approaches to characterize the sub-mm layers in a multilayer structure. This study aims to apply the all-optical (AO) LU technique for the characterization of the sub-mm layers in a multilayer structure. A specific AOLU system is developed to acquire the transmitted longitudinal waves from the metallic composites. Following the acquisition of the transmitted signals, the spectrum analysis is utilized to overcome the ultrasonic signal aliasing in the time domain. The least-squares method is subsequently adopted to estimate the thicknesses of the layers simultaneously. This study has carried out a successful application of the AOLU technique for spectrum analysis of the sub-mm multilayer structure with the advantages of non-contact remote detection, reliable single-mode acquisition, and overcoming waveform aliasing.

Simple method of measuring thicknesses of surface-hardened layers by laser ultrasonic technique

We proposed a simple non-contact inspection method using a laser ultrasonic technique to measure the thickness of a surface-hardened layer. This measurement is based on the dependence of a surface-wave velocity on the thickness of the hardened layers. In this case, it is essential to measure the surface-wave velocity with a wavelength comparable to the thickness of a hardened layer. However, it is not easy to selectively generate the surface waves with a desired wavelength. Thus we proposed the method where the surface waves with a desired wavelength are extracted using bandpass electrical filters. This method is simpler than the conventional one based on dispersion because the surface-wave velocity is directly obtained from measured temporal waveforms. We applied the method to measure the thickness of hardened layers and showed that the results were in good agreement with that obtained by the conventional method and numerical results.

In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

ABSTRACT Laser ultrasonic technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding, and it is effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is a prototype product (1064 nm, 20 mJ/pulse, 1.5 ns, 20 Hz), was used to generate ultrasonic, and a laser interferometer (532 nm, 1 W) was used to detect the surface micro-vibration caused by the arrival of ultrasonic wave and was scanned using an uniaxial stage. After acquiring the B scope, which is composed of a generation point and multi-detection points, the synthetic aperture focussing technique (SAFT) is applied. SAFT image obtained by in-process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during gas metal arc (GMA) welding process in the mid-pass of a multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in the process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in the process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

An optimized laser ultrasonic synthetic aperture imaging method using differential technique

Laser ultrasonic synthetic aperture focusing technique (SAFT) is susceptible to multiple modes of ultrasonic generated by laser pulse. In this work, we propose an optimized synthetic aperture imaging method based on the differential technique, resulting in the effective detection of internal defects. The laser ultrasonic SAFT finite element model is established, and the models are calculated in the case of existing internal defect or not. Thereby, the original ultrasonic data containing the defect reflected waves and the differential data are obtained, and the internal defects are reconstructed by the delay superposition algorithm. The results show that the defect imaging is submerged in the high-amplitude background noise superimposed by the surface acoustic wave; when only the original data are used. However, the optimized SAFT imaging based on the differential technique can not only reduce the interference of other mode waves on the imaging area significantly, but also retain the defect reflected waves effectively. Moreover, the imaging and precise locating of multiple defects are realized, which pave the way for enhancing the internal defects detection ability of laser ultrasonic.

Research on the influence of heat treatment on residual stress of TC4 alloy produced by laser additive manufacturing based on laser ultrasonic technique

During the laser additive manufacturing (LAM) process large temperature gradients can lead to high level of residual stress. The residual stress can have irreversible effects such as warping and cracking of parts during and post manufacturing. Heat treatment is an effective method to control and eliminate residual stress. In this paper, the TC4 parts are prepared by laser additive manufacturing, and the influence of heat treatment process on residual stress is researched. Laser ultrasonic technology, as an advanced nondestructive testing method, is applied to measure the residual stress under different heat treatment processes for the first time. The surface wave generated by laser is used to evaluate the residual stress. The results show that laser ultrasonic method can complete the in-situ evaluation of residual stress in additive manufacturing components. The residual stress in TC4 deposited specimen is large, the longitudinal stress is obviously greater than the transverse stress, and the maximum residual stress is about half of the yield strength. The residual stress increases gradually from the upper surface to the bottom layer near the substrate before heat treatment. After heat treatment, the residual stress is reduced to low stress level and a small compressive stress appears. The cooling rate and solution temperature are the main factors affecting the residual stress, and the residual stress increases with the increase of cooling rate and solution temperature. The effect of aging temperature and aging time on residual stress is not obvious. The study serves as useful guidelines for engineers to assessment and regulation of residual stress reasonably in LAM.

Defect measurement using the laser ultrasonic technique based on power spectral density analysis and wavelet packet energy

AbstractThe laser ultrasonic technique (LUT) enables the analysis of complex signals and the extraction of the features of defects. The time‐domain features can be used to quantify various defects, but are negatively affected by the coupling efficiency, measurement noise, and surface roughness during LUT. This paper describes defect analysis methods based on the power spectral density and wavelet packet energy. Defects are characterized using the power of low‐frequency components after wavelet decomposition and the wavelet packet energy, and then linear relationships are established for two defect types. The R2 values of the linear fitting for the characteristic variables of the defect are greater than 0.96. The experimental results show that the proposed methods provide effective and quantitative characterization of the defect size from laser ultrasonic signals.

Determination of fiber volume fraction in a cured laminate

Propagation of ultrasonic wave in Carbon Fiber Reinforced Polymer (CFRP) is greatly influenced by the material’s matrix, resins and fiber volume ratio. Laser ultrasonic broadband spectral technique has been demonstrated for porosity and fiber volume ratio extraction on unidirection aligned CFRP laminates. Porosity in the matrix materials can be calculated by longitudinal wave attenuation and accurate fiber volume ratio can be derived by combined velocity through the high strength carbon fiber and the matrix material with further consideration of porosity effects. The results have been benchmarked by pulse-echo ultrasonic tests, gas pycnometer and thermal gravimetric analysis (TGA). The potentials and advantages of the laser ultrasonic technique as a non-destructive evaluation method for CFRP carbon fiber volume fraction evaluation were demonstrated.

Fatigue damage characterization for composite laminates using deep learning and laser ultrasonic

In this work, we propose a novel deep learning (DL) approach to directly process and characterize the guided wave information measured from an interrupted tensile-tensile fatigue test on fiber-reinforced polymer (FRP) laminates using laser ultrasonic technique. The DL model is constructed with artificial neural networks that consists of a convolutional autoencoder (CAE) for wave feature extraction, a recurrent neural network (RNN) for fatigue loading condition recognition and a neural ordinary differential equation (Neural ODE) to model the evolutional behavior of the extracted wave feature. The training of the neural network utilizes an unsupervised strategy in order for the DL model to learn the underlying dynamics of the wave feature for fatigue damage characterization. Meanwhile, in order to improve the interpretability of the learned DL model, domain knowledge is infused into the training process as physics-informed constraints. The model is trained with laser-excited ultrasonic guided wave signals obtained from acoustic emission (AE) sensors installed on the surface of fatigue specimens, where the raw signal is minimally processed to avoid human intervention as much as possible. A high-fidelity finite element (FE) model is also built to generate virtual samples to be used in a transfer learning strategy. The results show that the DL model has learned to characterize fatigue damage through latent wave features, which can be further interpreted as a meaningful physical quantity with physics-informed constraints applied. Finally, a parametric study is given to demonstrate the capacity of the learned characterization model by the DL approach.

Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring

Structural health monitoring (SHM) is an important aspect of the assessment of various structures and infrastructure, which involves inspection, monitoring, and maintenance to support economics, quality of life and sustainability in civil engineering. Currently, research has been conducted in order to develop non-destructive techniques for SHM to extend the lifespan of monitored structures. This paper will review and summarize the recent advancements in non-destructive testing techniques, namely, sweep frequency approach, ground penetrating radar, infrared technique, fiber optics sensors, camera-based methods, laser scanner techniques, acoustic emission and ultrasonic techniques. Although some of the techniques are widely and successfully utilized in civil engineering, there are still challenges that researchers are addressing. One of the common challenges within the techniques is interpretation, analysis and automation of obtained data, which requires highly skilled and specialized experts. Therefore, researchers are investigating and applying artificial intelligence, namely machine learning algorithms to address the challenges. In addition, researchers have combined multiple techniques in order to improve accuracy and acquire additional parameters to enhance the measurement processes. This study mainly focuses on the scope and recent advancements of the Non-destructive Testing (NDT) application for SHM of concrete, masonry, timber and steel structures.

Evaluation of the Quality of BGA Solder Balls in FCBGA Packages Subjected to Thermal Cycling Reliability Test Using Laser Ultrasonic Inspection Technique

In modern integrated circuit (IC) packaging, it is difficult to perceive and evaluate the quality of the solder ball interconnections due to their complex hidden physical configuration and miniaturization. Laser ultrasonic inspection (LUI) technique is a novel, nondestructive, and noncontact method that has achieved great success in evaluating the quality of interconnections in chip-scale and ball-grid array (BGA) packages. In this article, an enhanced dual-fiber array LUI system is used to evaluate the quality of BGA solder balls in advanced industrial flip-chip BGA packages subjected to thermal cycling reliability tests. LUI results are validated with electrical testing, scanning electron microscope (SEM), and dye-and-pry results. LUI results are well correlated with the intermetallic compound cracks in BGA solder balls observed using SEM, and dye-and-pry. This study demonstrates the feasibility of using the LUI system for one of the modern IC packages and the potential to use the LUI system for future advanced IC packages such as 3-D packaging.

Non-destructive laser-ultrasonic Synthetic Aperture Focusing Technique (SAFT) for 3D visualization of defects

The Laser Ultrasonic (LU) technique has been widely studied. Detected ultrasonic signals can be further processed using Synthetic Aperture Focusing Techniques (SAFTs), to detect and image internal defects. LU-based SAFT in frequency-domain (F-SAFT) is developed to visualize horizontal hole-type defects in aluminum. Bulk acoustic waves are non-destructively generated by irradiating a laser line-source, and detected using a laser Doppler vibrometer at a point away from the generation. The influence of this non-coincident generation-detection on the equivalent acoustic velocity used in the algorithm is studied via velocity mappings. Because the wide-band generation characteristic of the LU technique, frequency range selections in acoustic wave signals are implemented to increase Signal-to-Noise Ratio (SNR) and reconstruction speed. Results indicate that by using the LU F-SAFT algorithm, and incorporating optimizations such as velocity mapping and frequency range selection, small defects can be visualized in 3D with corrected locations and improved image quality.

Ultrafast laser-induced guided elastic waves in a freestanding aluminum membrane

Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.

In-situ prediction of α-phase volume fraction in titanium alloy using laser ultrasonic with support vector regression

A laser ultrasonic based non-contact detection method combined with support vector regression (SVR) was proposed for the first time for in-situ predicting the volume fraction of α-phase in titanium alloy (Ti-10V-2Fe-3Al). The alloy specimens were heated with incremental temperatures from 820 to 1100 °C to obtain different microstructures and volume fraction of α-phase, and the nonlinear relationship between the parameter of Rayleigh ultrasonic wave (velocity, amplitude and peak frequency) and the volume fraction of α-phase was established and analyzed by the combination of laser ultrasonic experiment and metallographic analysis. The SVR, with ultrasonic parameters and heat treatment parameters (heating and cooling rate, holding time and holding temperature) as input features, was implemented to predict the volume fraction of α-phase, achieving a relative mean error less than 5%. The results indicate that the SVR-based laser ultrasonic technique can be applied as a reliable and effective method for in-situ characterization of volume fraction of α-phase in titanium alloy.

In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

Laser Ultrasonic Technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding and it’s effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is prototype product (1064nm, 20mJ/pulse, 1.5ns, 20Hz), was used to generate ultrasonic, and a laser interferometer (532nm, 1W) was used to detect the surface micro vibration caused by arrival of ultrasonic wave and was scanned using a mechanical stage. After acquiring the B-scope, which is composed of a generation-point and multi detection-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during Gas Metal Arc (GMA) welding process in the mid-pass of multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

High-resolution microscopy through optically opaque media using ultrafast photoacoustics.

We present a high-resolution microscope capable of imaging buried structures through optically opaque materials with micrometer transverse resolution and a nanometer-scale depth sensitivity. The ability to image through such materials is made possible by the use of laser ultrasonic techniques, where an ultrafast laser pulse launches acoustic waves inside an opaque layer and subsequent acoustic echoes from buried interfaces are detected optically by a time-delayed probe pulse. We show that the high frequency of the generated ultrasound waves enables imaging with a transverse resolution only limited by the optical detection system. We present the imaging system and signal analysis and demonstrate its imaging capability on complex microstructured objects through 200 nm thick metal layers and gratings through 500 nm thickness. Furthermore, we characterize the obtained imaging performance, achieving a diffraction-limited transverse resolution of 1.2 μm and a depth sensitivity better than 10 nm.

Laser ultrasonic imaging of wavefield spatial gradients for damage detection

This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

Corner inspection method for L-shaped composite structures using laser ultrasonic rotational scanning technique

Composite primary structures that can sustain high loads are used to reduce the weight of structures in various industries. Composite primary structures that are L-shaped or T-shaped have a right-angled corner, and phased-array ultrasonic inspection or water squirt robotic C-scan is mainly used for the corners. However, using a customized transducer has restrictions depending on the size and shape of the corner. In this paper, we propose a novel approach based on rotational laser ultrasonic inspection for even narrow corner sections regardless of size and angle. In the rotational scanning method, the target structure is rotated for inspection, which is different from conventional scanning methods in which transducers or a scanning head are moved along the inspection area. The rotational scanning method is designed to solve the problem of signal-to-noise ratio deterioration that occurs when the ultrasonic sensing laser scans a curved surface. The corner sections of two L-shaped specimens fabricated by autoclave curing and resin transfer molding were inspected using the rotational scanning method. Using the proposed method, all of the artificial defects inserted into the corner section of the specimens were clearly visualized. Then, the inspection results were compared with water squirt C-scan inspection results. The test results showed that the proposed method is highly reliable and useful for inspecting the corner sections.

Artificial Intelligence-Based Bolt Loosening Diagnosis Using Deep Learning Algorithms for Laser Ultrasonic Wave Propagation Data

The application of deep learning (DL) algorithms to non-destructive evaluation (NDE) is now becoming one of the most attractive topics in this field. As a contribution to such research, this study aims to investigate the application of DL algorithms for detecting and estimating the looseness in bolted joints using a laser ultrasonic technique. This research was conducted based on a hypothesis regarding the relationship between the true contact area of the bolt head-plate and the guided wave energy lost while the ultrasonic waves pass through it. First, a Q-switched Nd:YAG pulsed laser and an acoustic emission sensor were used as exciting and sensing ultrasonic signals, respectively. Then, a 3D full-field ultrasonic data set was created using an ultrasonic wave propagation imaging (UWPI) process, after which several signal processing techniques were applied to generate the processed data. By using a deep convolutional neural network (DCNN) with a VGG-like architecture based regression model, the estimated error was calculated to compare the performance of a DCNN on different processed data set. The proposed approach was also compared with a K-nearest neighbor, support vector regression, and deep artificial neural network for regression to demonstrate its robustness. Consequently, it was found that the proposed approach shows potential for the incorporation of laser-generated ultrasound and DL algorithms. In addition, the signal processing technique has been shown to have an important impact on the DL performance for automatic looseness estimation.

Laser Ultrasonic Technique to Non-Destructively Detect Cracks on a Ni-Based Self-Fluxing Alloy Fabricated Using Directed Energy Deposition (DED)

Laser Ultrasonic Technique to Non-Destructively Detect Cracks on a Ni-Based Self-Fluxing Alloy Fabricated Using Directed Energy Deposition (DED)