Laser Ultrasonic Technique Research Articles

Monitoring of Thermal Aging of Aluminum Alloy via Nonlinear Propagation of Acoustic Pulses Generated and Detected by Lasers

Nonlinear acoustic techniques are established tools for the characterization of micro-inhomogeneous materials with higher sensitivity, compared to linear ultrasonic techniques. In particular, the evaluation of material elastic quadratic nonlinearity via the detection of the second harmonic generation by acoustic waves is known to provide an assessment of the state variation of heat treated micro-structured materials. We report on the first application for non-destructive diagnostics of material thermal aging of finite-amplitude longitudinal acoustic pulses generated and detected by lasers. Finite-amplitude longitudinal pulses were launched in aluminum alloy samples by deposited liquid-suspended carbon particles layer irradiated by a nanosecond laser source. An out-of-plane displacement at the epicenter of the opposite sample surface was measured by an interferometer. This laser ultrasonic technique provided an opportunity to study the propagation in aluminum alloys of finite-amplitude acoustic pulses with a strain up to 5 × 10−3. The experiments revealed a signature of the hysteretic quadratic nonlinearity of micro-structured material manifested in an increase of the duration of detected acoustic pulses with an increase of their amplitude. The parameter of the hysteretic quadratic nonlinearity of the aluminum alloy (Al6061) was found to be of the order of 100 and to exhibit more than 50% variations in the process of the alloy thermal aging. By comparing the measured parameter of the hysteretic quadratic nonlinearity in aluminum alloys that were subjected to heat-treatment at 220 °C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, and 1000 h), with measurements of yield strength in same samples, it was established that the extrema in the dependence of the hysteretic nonlinearity and of the yield strength of this alloy on heat treatment time are correlated. This experimental observation provides the background for future research with the application goal of suggested nonlinear laser ultrasonic techniques for non-destructive evaluation of alloys’ strength and rigidity in the process of their heat treatment.

Laser ultrasonic inspection of additive manufactured components

Additively manufactured components are gaining popularity in aerospace, automotive and medical engineering applications. Additive manufacturing (AM) offers tremendous cost advantages over traditional manufacturing methods. However, inter- and intra-layer defects are observed in AM components. Moreover, the lack of appropriate testing methods for assessing the integrity of AM components deters its use, despite the several functional advantages it has to offer. Non-destructive testing (NDT) forms the most common and convenient way of inspecting parts. In this paper, a laser ultrasonic technique for the inspection of AM components is proposed. The results demonstrate laser ultrasonic testing (LUT) as a promising method for the non-contact inspection of additive manufactured components. Furthermore, the results were validated using X-ray computed tomography (CT) and ultrasonic immersion testing (UIT). The sample used in this study was manufactured through selective laser melting (SLM) AM process with built-in holes representing defects.

Numerical simulations of infected-grain detection based on laser�ultrasonic technique

Numerical simulations of infected-grain detection based on laser�ultrasonic technique

Establishment of a high-temperature laser ultrasonic detection technique using an optical fiber sensor

Establishment of a high-temperature laser ultrasonic detection technique using an optical fiber sensor

3P2-1 Laser ultrasonic technique to detect cracks on directed energy deposition (DED) process

3P2-1 Laser ultrasonic technique to detect cracks on directed energy deposition (DED) process

Ultrasonic study on the change in elastic properties of cementite with temperature and Mn content using nearly full density polycrystalline bulk samples

The elastic properties of cementite were studied using a laser-ultrasonic technique. The temperature dependence and effect of Mn alloying were investigated using nearly full density polycrystalline bulk cementite samples. The Young’s modulus (E), shear modulus (G), bulk modulus (B), and Poisson’s ratio (ν) were obtained from the measured longitudinal and shear velocities (VL and VS, respectively). The elastic properties of cementite changed substantially with the temperature, especially around its Curie temperature (TC). The change in longitudinal velocity around Curie temperature was larger than the change in shear velocity. Around Curie temperature, a small decrease was observed in Young’s modulus and shear modulus and sharp minima were observed in bulk modulus and Poisson’s ratio. Mn addition to cementite increased the sound velocity and elastic stiffness and decreased Curie temperature. The present study revealed the temperature dependence of the elastic constants of cementite across Curie temperature for the first time.

Inspection of cracks with focused angle beam laser ultrasonic wave

In this work, in order to improve the detecting ability of laser ultrasonic testing technique for inner cracks in metals, an arc-line-focused laser source is proposed to generate focused angle-beam bulk waves in thermoelastic regime. The arc-line-focused laser source is achieved by passing an expanded and collimated laser beam through a convex and axicon lens off-center. The distribution of the shear waves generated by the arc-line laser source were detected by a laser interferometer to evaluate the focal area. Finally, the measurement of artificial cracks at the bottom surface of aluminum specimen with using the focused angle-beam bulk waves is investigated by experiment.

Laser ultrasonic damage detection in coating-substrate structure via Pearson correlation coefficient

This study explores the feasibility of using laser generated ultrasonic wave to detect the interfacial damage of coating-substrate structure. Two Pearson correlation coefficient based post processing algorithms are respectively modified and used in order to evaluate the damage-induced waveform variation and thus position the damage accurately. In addition, the ultrasonic signals of frequency-domain and time-domain are separately studied to optimize the damage visualization performance of the two approaches. The proposed method helps to facilitate nondestructive examination of laser ultrasonic technique.

Review of NDT and process monitoring techniques usable to produce high-quality parts by welding or additive manufacturing

The main objectives of applying NDT techniques are to ensure the quality of an assembly or a part according to a given specification including known acceptance criteria. It generally enables not only to detect an indication, but also to classify it (size, position, nature…). Many non-destructive testing (NDT) techniques are effective in testing welded components. Radiography, ultrasonic testing, penetrant testing and magnetic particle testing are widely used and standardised. Phased arrays, TOFD and multi-elements eddy current are more and more extensively applied. Tomography, acoustic emission, ultrasonic guided waves, laser ultrasonic and optical techniques continue to be a strong topic of interest. Each of these techniques is based on different physical principles to detect defects on the surface of the part or over its whole volume. However, the geometry, physical and material properties of the part being tested are key factors in the applicability and performance of a given NDT technique. To date, the development of reliable NDT methods for additive manufacturing (AM) parts is still a major challenge. The process may generate various defects such as cracks, voids, inclusions and porosities. NDT techniques need to be optimised or developed to address singular features of the AM processes: complex geometry, special internal structures, anisotropic material properties, typical defects. Knowledge of the potential occurring imperfections produced by the various AM process needs to be improved in order to be able to select the best suited NDT techniques.

Mode conversion detection in an elastic plate based on Fizeau fiber interferometer

Based on laser ultrasonic techniques, the mode-converted waves in an aluminum plate with a rectangular groove have been investigated by a Fizeau fiber interferometer. A Q-switched Nd:YAG laser was used for ultrasonic generation, and mode conversions occurred when ultrasonic waves interacted with the groove. Increasing the distance between the detection point and the groove could lead to the obvious changes in time domain signals. Experimental results showed that the ultrasonic waves induced by the pulsed laser were extremely sensitive to the surface-breaking cracks due to elastic wave mode conversions, and as the distance increased, the discrimination between different mode-converted waves was gradually enhanced. Furthermore, the depth of the groove was quantitatively measured through the analysis of the mode-converted Rayleigh waves. Our interferometer exhibited an outstanding performance in detecting elastic waves, which could be used not only in exploring the mechanism of mode conversions but also in evaluating structure cracks.

Estimation of δ and C13 of organic-rich shale from laser ultrasonic technique measurement

The laser ultrasonic technique (LUT) has many advantages in dealing with the anisotropy of organic-rich shale over the transducer ultrasonic contacting measurement. We have developed a systematic procedure to estimate anisotropic parameter [Formula: see text] and elastic parameter [Formula: see text] in organic-rich shale from LUT measurement. A novel filtering method called the structural-oriented space-varying median filter (SOSVMF) is proposed for removing the erratic noise from the recorded data. We apply the proposed algorithm onto one synthetic example and real recorded data in artificial bakelite and organic-rich shale by using LUT to show the successful performance. Compared with the original recorded data, the denoised waveform by SOSVMF will also help to obtain more accurate parameter estimations of the anisotropic parameter [Formula: see text] and elastic parameter [Formula: see text]. More importantly, the presented systematic procedure may extend the application of LUT to the study of attenuation anisotropy in organic-rich shale and scale physical modeling. In addition, the surface-modification method may be an effective method to enhance the laser-ultrasonic conversion efficiency.

Detection of disbonds in multi-layer bonded structures using the laser ultrasonic pulse-echo mode

The laser ultrasonic technique with a pulse-echo mode has been investigated to detect disbonds in a multi-layer bonded structure and a quantitative method has been proposed to evaluate the defect sizes. The simulations were carried out to analyze influences of spot sizes on the characteristics of laser ultrasonic reflected waves, the interaction of laser ultrasonic reflected waves with disbonds, and quantitative characterization on disbonds. A noncontact laser ultrasonic inspection system has been established to perform a series of experiments to verify the theoretical results. Laser ultrasonic C-scans based on reflected shear waves can clearly redraw appearances of disbonds in the adhesive bond. The proposed quantitative method can evaluate the sizes of disbonds within a reasonable error range. Moreover, the experimental data are in good agreement with the simulation results. Therefore, a combination of the laser ultrasonic pulse-echo mode and the proposed quantitative strategy is practical for accurate detection of disbonds in multi-layer bonded structures.

Estimation of Local Elastic Moduli of Carbon-Containing Materials by Laser Ultrasound

The elastic properties of two carbon-containing materials are investigated. Shungite rock is a natural nanocomposite; isotropic pyrolytic graphite is an artificial material. Precision measurement of the local velocities of longitudinal and shear waves propagating in shungite and isotropic pyrographite samples was performed by laser ultrasonic techniques. Young's modulus, the shear modulus and Poisson's ratio are calculated, and the chemical composition of the samples is given.

Applications of laser ultrasonic technique on nondestructive testing and evaluation of materials

Nondestructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. As one of unconventional NDT technology, laser ultrasonic technique (LUT) have played an important role in inspecting the materials in some extreme environments, such as high temperature, high pressure, corrosion, radioactive and etc. LUT uses lasers to generate and detect ultrasonic waves in a noncontact way. Thus, compared with the traditional ultrasonic technique, LUT has the advantages of high precision, high sensitivity without requirement for geometry of target material. So far, LUT has been well applied to composite inspections for the aerospace industry and on-line hot tube thickness measurements for the metallurgical industry. This talk describes the application of LUT on flaws detection in rail and thermal barrier coatings.

Combined laser ultrasonics, and Raman scattering in diamond anvil cell system operating in the transmission configuration

Recently it has been demonstrated that it is possible to detect both longitudinal and shear waves as well as measure their velocities for a non-transparent iron layer compressed in a diamond anvil cell (DAC) to pressures approaching 23 GPa by using laser ultrasonics (LU) technique. These experiments were conducted in the reflection configuration when point source and point receiver were located at the same side of the specimen. The purpose of the present study is to examine the generation and detection of the acoustical waves in the LU-DAC system operating in the transmission configuration. The experimental results obtained at 16.4 GPa presented in this article demonstrate that the LU-DAC technique can be applied for longitudinal wave velocity measurements in small specimens under high pressure in the transmission configuration.

Laser ultrasonic quantitative recognition based on wavelet packet fusion algorithm and SVM

With the development of industrialization and modern technology, laser ultrasonic technique is more and more used in aerospace, machinery and electronics, measurements, metallurgy chemical engineering, materials science, railway transportation, bridge engineering, etc. In order to maintain the excellent characteristics of new materials (such as thermal properties, mechanical properties, chemical properties and optical properties, material structure must be early diagnosed and monitored before properties change. Nondestructive Testing technology plays a great role in monitoring reliability of industry products. This paper proposes a new feature selection method based on wavelet packet algorithm, and applies SVM (support vector machine) for quantitative classification on the ultrasonic echo data generated by cracks in Laser ultrasonic experiment. By combining the nondestructive device and dimension reduction method in machine learning, this paper analyses the scatter plot of two cracks in 2d and the fitting surface in 3d and give the quantitative index for determining the performance of used methods.

Orientation imaging of macro-sized polysilicon grains on wafers using spatially resolved acoustic spectroscopy

Due to its economical production process polysilicon, or multicrystalline silicon, is widely used to produce solar cell wafers. However, the conversion efficiencies are often lower than equivalent monocrystalline or thin film cells, with the structure and orientation of the silicon grains strongly linked to the efficiency. We present a non-destructive laser ultrasonic inspection technique, capable of characterising large (52×76mm2) photocell's microstructure - measurement times, sample surface preparation and system upgrades for silicon scanning are discussed. This system, known as spatially resolved acoustic spectroscopy (SRAS) could be used to optimise the polysilicon wafer production process and potentially improve efficiency.

The Use of Flexible Ultrasound Transducers for the Detection of Laser-Induced Guided Waves on Curved Surfaces at Elevated Temperatures.

In this study, a flexible ultrasonic transducer (FUT) was applied in a laser ultrasonic technique (LUT) for non-destructive characterization of metallic pipes at high temperatures of up to 176 °C. Compared with normal ultrasound transducers, a FUT is a piezoelectric film made of a PZT/PZT sol-gel composite which has advantages due to its high sensitivity, curved surface adaptability and high temperature durability. By operating a pulsed laser in B-scan mode along with the integration of FUT and LUT, a multi-mode dispersion spectrum of a stainless steel pipe at high temperature can be measured. In addition, dynamic wave propagation behaviors are experimentally visualized with two dimensional scanning. The images directly interpret the reflections from the interior defects and also can locate their positions. This hybrid technique shows great potential for non-destructive evaluation of structures with complex geometry, especially in high temperature environments.

Comparison Study between RMS and Edge Detection Image Processing Algorithms for a Pulsed Laser UWPI (Ultrasonic Wave Propagation Imaging)-Based NDT Technique.

In this study, a non-contact laser ultrasonic propagation imaging technique was applied to detect the damage of plate-like structures. Lamb waves were generated by an Nd:YAG pulse laser system, while a galvanometer-based laser scanner was used to scan the preliminarily designated area. The signals of the structural responses were measured using a piezoelectric sensor attached on the front or back side of the plates. The obtained responses were analyzed by calculating the root mean square (RMS) values to achieve the visualization of structural defects such as crack, corrosion, and so on. If the propagating waves encounter the damage, the waves are scattered at the damage and the energy of the scattered waves can be expressed by the RMS values. In this study, notch and corrosion were artificially formed on aluminum plates and were considered as structural defects. The notches were created with different depths and angles on the aluminum plates, and the corrosion damage was formed with different depths and areas. To visualize the damage more clearly, edge detection methodologies were applied to the RMS images and the feasibility of the methods was investigated. The results showed that most of the edge detection methods were good at detecting the shape and/or the size of the damage while they had poor performance of detecting the depth of the damage.

Directional dependence of aluminum grain size measurement by laser-ultrasonic technique

A blade method was used to identify the relative position of pulsed and detect lasers when measuring aluminum grain size via laser-ultrasonic technique. Aluminum specimens received a variety of treatments to obtain different grain sizes. Electron backscatter diffraction data were used to obtain the parameters of the grain size calculation model. The finite element method model was established by COMSOL Multiphysics to simulate the generation and propagation of ultrasonic waves in aluminum. Both simulation and experimental data revealed that directional pattern has a significant effect on the results of grain size measurement when unpredictable deviations occur. The direction angle of the first and third ultrasonic echoes differs at the detection point when deviation occurs. Thus, the error of grain size measurement using laser-ultrasonic technique will increase when deviation increases. For the grain size measurement of aluminum using laser-ultrasonic technique, the accuracy is acceptable when deviation is no more than 0.2mm when the thickness of the specimen and diameter of the pulsed laser are set to 2mm and 1mm respectively.