Table of contents

Based on the use of laser as a coherent and intense light source, the photo–acoustics originated from the discovery made by Alexander Graham Bell was extended to laser–ultrasonics (LU), and it has been applied to wide area of ultrasonics, optics, material characterization and nondestructive inspection.

In 1996, a research group for LU was started in the Japanese Society for Nondestructive Inspection (JSNDI), and researches on LU and related topics such as noncontact measurements and elastic wave theories were discussed. Similar activities were pursued also in North America and in Europe. The international symposium on LU was started in Montreal, Canada in 2008 by Jean Pierre Monchalin in order to offer a forum for involved with basic researches and industrial applications of LU. In the second symposium in Bordeaux, France nearly 120 papers were presented. It is our honor to have organized the third symposium, LU2013 on 25–28 June in Yokohama, Japan. The articles published here provide a sample of achievements presented there.

In LU2013, we focused on the laser generation and/or detection of acoustic waves, application to nondestructive testing, ultrafast–optoacoustics and innovative instruments. Research achievements in biomedical applications, advanced sensing including noncontact, micro/nanoscale or nonlinear measurements, as well as theory and simulation of ultrasound were also included, considering the interdisciplinary nature of this field.

We enjoyed very excellent and informative 3 plenary talks, 11 invited talks, 81 oral and 41 poster presentations with 168 attendees. According to requests, we organized a post deadline poster session to give an opportunity to present recent achievements after the deadline. Contributions of the participants, the scientific and organizing committees are highly appreciated. The conference tour was a dinner cruise to the Tokyo bay, and we hope this experience will remain as a pleasant memory in attendees. As decided in the meeting for the next symposium, the next symposium will be held in Chicago, USA in 2015. We would be happy if the articles in this issue work as a trigger to attend LU2015.

Kazushi Yamanaka

Ikuo Ihara

Makoto Ochiai

Further conference information and photographs are found in the pdf.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We report on a contactless method to focus laser generated bulk and surface ultrasound waves in the thermo-elastic coupling mode by annular shaped illumination. By using a spatial light modulator (SLM) the beam profile of a pulsed picoseconds laser was shaped to annular forms flexibly and further rings with a thickness of 50 μm and a generation energy as low as 2 mJ were generated on the surface of aluminum plates. The annular shapes have been used to focus acoustic waves toward the center. In this work, a photorefractive adaptive interferometer set up based on Two-Wave Mixing in a fast BSO crystal was used to probe and detect the converging acoustic pulses at the center of the laser generated rings. By moving the detection point about 1 mm out of the ring epicenter, the amplitude of bulk and surface waves drop quickly which shows the converging evidence of the acoustic waves in the ring center. For a 3 mm thick aluminum plate, the ring size from 1 mm to 10 mm was scanned. The optimum ring diameter and the focal length of the acoustic waves along the central axis were found. Applications of this technique in subsurface defects detection as well as sample thickness measurement are investigated.

Incorporating Laser-Ultrasonic systems for excitation and acquisition of ultrasonic waves into NDT&E systems establishes the benefits of contact free methods to high-frequency ultrasonic measurements. Techniques using this huge potential were investigated for a long time but in most cases with focus to surface waves. To study the material bulk properties strong surface damaging laser pulses were mostly used. However, by shortening the pulse length down to some nano-seconds at moderate intensities one can excite ultrasonic waves in the bulk of the material without any damage.

The work presented deals with the prediction of the acoustic fields shape based on arbitrary energy distribution across the excitation laser spot and its use in ultrasonic inspection. An algorithm is discussed which uses the laser beam profile, measured surface normal displacements and bulk wave speeds as input. Comparing simulated an measured data reveals basic agreements but also still some discrepancies caused by additional thermal diffusion in the excitation region. Using this algorithm already offers to analyse the presumed location and shape of longitudinal and transversal wave foci. Further, its capability for optimizing laser-acoustic systems is demonstrated.

A new approach of generating acoustic waves utilizing evanescent light is presented. The evanescent light is a non-propagating electromagnetic wave that exhibits exponential decay with distance from the surface at which the total internal reflection of light is formed. In this research, the evanescent light during total internal reflection at prism surface is utilized for generating acoustic waves in aluminium and the feasibility for ultrasonic measurements is discussed. Pulsed Nd:YAG laser with 0.36 J/cm² power density is used and the incident angle during the total internal reflection is arranged to be 69.0° for generating the evanescent light. It has been demonstrated that the amplitude of the acoustic waves by means of evanescent light is about 1/14 as large as the one generated by the conventional pulsed laser. This reveals the possibility of using a laser ultrasonic technique with near-field optics.

The knife edge detector—also known as optical beam deflection—is a simple and robust method of detecting ultrasonic waves using a laser. It is particularly suitable for detection of high frequency surface acoustic waves as the response is proportional to variation of the local tilt of the surface. In the case of a specular reflection of the incident laser beam from a smooth surface, any lateral movement of the reflected beam caused by the ultrasonic waves is easily detected by a pair of photodiodes. The major disadvantage of the knife edge detector is that it does not cope well with optically rough surfaces, those that give a speckled reflection. The optical speckles from a rough surface adversely affect the efficiency of the knife edge detector, because 'dark' speckles move synchronously with 'bright' speckles, and their contributions to the ultrasonic signal cancel each other out. We have developed a new self-adapting sensor which can cope with the optical speckles reflected from a rough surface. It is inelegantly called the SKED—speckle knife edge detector—and like its smooth surface namesake it is simple, cheap, compact, and robust. We describe the theory of its operation, and present preliminary experimental results validating the overall concept and the operation of the prototype device.

In this work, a phased array laser ultrasound system with using fibre optic delivery and a custom-designed focusing objective lens has been developed for enhancing the ultrasound generation. The fibre-phased array method is applied to improve the sensitivity and detecting ability of the laser-EMAT system for defect inspection.

The excitation of SAW was detected on diamond with built-in ion-implanted graphitized layer under its illumination with femtosecond laser pulses. The spectral width of a SAW pulses were in the range of 1.5÷2 GHz. It was found out that the anisotropy of the SAW propagation was practically absent. The increase in the implantation dose from 4·10¹⁵ cm⁻² to 12·10¹⁵ cm⁻² was shown to give rise to a dispersion of SAW propagation.

Boundary integral equations are formulated to investigate nonlinear waves generated by a debonding interface of bi-material subjected to an incident plane wave. For the numerical simulation, the IRK (Implicit Runge-Kutta method) based CQ-BEM (Convolution Quadrature-Boundary Element Method) is developed. The interface conditions for a debonding area, consisting of three phases of separation, stick, and slip, are developed for the simulation of nonlinear ultrasonic waves. Numerical results are obtained and discussed for normal incidence of a plane longitudinal wave onto the nonlinear interface with a static compressive stress.

A material property measurement system for steel sheets using laser-based ultrasonics was developed. The system consists of a pulsed Nd:YAG laser for ultrasonic generation and multi-channel interferometer coupled with a CW single frequency laser for ultrasonic detection. The system can measure the frequency of the S1 Lamb wave mode of zero group velocity (S1f) as well as the longitudinal resonance frequencies without ablative damage to the steel surface. It was confirmed that Poisson's ratio could be directly obtained by combining the measured S1f value and the longitudinal resonance frequencies. To evaluate the applicability of this system in an actual steel production setting, the system was installed in hot rolling pilot plant that produces steel samples. As a result, it was demonstrated that the system can measure dynamic changes in Poisson's ratio values within steel sheets, even in the hot rolling pilot plant environment. Material property data, such as Poisson's ratio, during the thin sheet production process will be very useful for manufacturing high value-added steel, such as sheets with uniform quality.

Our group had previously proposed a generation laser scanning system for visualizing ultrasound propagation on an object as an animate image, which provided visible and quick flaw inspection. Recently, we improved this system to make it completely non-contact by employing an air-coupled ultrasound transducer as a receiver instead of a contact transducer, and demonstrated the successful visualization of Lamb waves propagating on aluminum and carbon fiber reinforced plastic plates, as well as the detection of flaws. In this research, we applied this system to the non-contact visualization of circumferential guided waves on aluminum pipes. It was shown that circumferential guided waves propagating in opposite directions could be visualized separately, and that a flaw such as a slit or thinning on the inside surface of the pipe could be successfully detected even when it existed outside the scanning area.

As an improvement in the imaging technique for the low S/N ratio of crack tip echo in ultrasonic inspection, we developed the Subharmonic Phased Array Crack Evaluation (SPACE) system. Although the pulser of conventional SPACE can generate large, over tens of nm, displacement ultrasound at a crack, it is effective only for closed cracks and not effective for most industrial cracks. For general use of the SPACE system in industrial inspection, we need to develop larger displacement ultrasound incidence equipment considering crack openings of the order of several nm to sub μm. In this study, we developed a high voltage excitation SPACE and larger amplitude ultrasound incident to a crack using a high voltage proof transducer. The suitability of the developed system was investigated using typical models of cracks.

A high sensitivity EMAT system using chirp pulse compression technique was developed. The system uses a high power gated amplifier having 2kVpp output to transmit chirp waves. Pulse compression of the received signals are performed digitally in a PC after amplification and analog-to-digital conversion. A 20dB improvement of the signal-to-noise ratio was achieved by chirp pulse compression and synchronous averaging. A new surface cooling technique was also developed to improve the signal amplitude of the bulk shear wave with hot steel, and its effectiveness was demonstrated. An actual plant test of crater end detection by the developed EMAT system was conducted at a continuous caster, and clear detection by non-contact EMATs was achieved.

This study aims to find the development for the evaluation of the surface roughness by the Acoustic Emission (AE) method with air blowing. We paid attention to the AE wave due to air blowing on the specimen plate with different surface roughness. The relationship between the AE wave and surface roughness of specimen plates was investigated. As the result, there is large and continuous difference in the Root Mean Square (RMS) value of their AE waveform. The RMS value decreases by increasing of the surface roughness of specimen plates. It suggested that this characteristic has the possibility to establish a new method of nondestructive surface roughness testing.

We report the results of an impulsive stimulated scattering measurement conducted on a 550 nm film of Ni₂MnGa on a MgO substrate, aiming at the direct experimental observation of the highly temperature dependent elasticity. The phase velocity of the surface acoustic waves excited on the surface of the coating-substrate shows an anomaly around the phase transition temperature of Ni₂MnGa, showing that it is possible to use impulsive stimulated scattering as a characterization tool to determine temperature dependent elasticity of a thin film.

Laser acoustic emission (AE) method is a unique in-situ and non-contact nondestructive evaluation (NDE) method. It has a capability to detect signals generated from crack generation and propagation, friction and other physical phenomena in materials even in high temperature environment. However, laser AE system has lower signal-to-noise ratio compared to the conventional AE system using PZT sensors, so it is difficult to apply this method in noisy environment. A novel AE measurement system to detect events in such difficult environments was developed. This system could continuously record all AE waveforms and enable unrestricted post-analyses. Noise reduction filters in frequency domain coupling with a new AE event extraction using multiple threshold values showed a good potential for AE signal processing. This system was successfully applied for crack monitoring of plasma spray deposition process of ceramic coating.

A new ultrasonic method to monitor the temperature distributions of a thick-walled hollow cylinder whose inner surface is heated is presented. This method basically consists of laser ultrasonic measurements and a heat conduction analysis of the heated hollow cylinder. Both longitudinal wave (LW) and surface acoustic wave (SAW) are used for estimating the temperature distribution of the cylinder. To demonstrate the validity of the proposed method, a numerical simulation and an experiment are carried out. In the numerical simulation, a steel hollow cylinder (the inner and outer diameters are 20 mm and 100 mm, respectively) is uniformly heated by a constant heat flux at the inner surface. It has been verified in the simulation that temperature distributions estimated by the proposed method completely agreed with the theoretical results. In the experiment, an aluminum hollow cylinder (the inner and outer diameters are 20 mm and 50 mm, respectively) is heated by pouring molten metal at 300 °C into the hollow cavity of the cylinder. A laser ultrasonic system is used for measuring both LW and SAW of the heated cylinder. It has also been demonstrated that the proposed method can measure the change in the temperature distribution during heating.

An approach for characterizing paper surface quality by ultrasound as an alternative non-contact method is presented. In this work, an air-coupled ultrasound at frequency range from 0.3 MHz to 4.2 MHz has been applied to surface roughness characterization, where a series of sandpapers and pure papers having random and relatively wide range of root-mean-square of roughness Rq from 2.0 to 92.8 are employed as specimens. The amplitude of reflected wave from each specimen is measured with pulse-echo configuration at normal incidence. A Kirchhoff-based scattering model is used to express the scattering phenomena from random rough surfaces and the relations between the normalized amplitude of the reflected wave and surface roughness parameters are then examined. It has been shown through the experiments that high frequency air-coupled ultrasound up to 4 MHz is useful to characterize surface roughness in the order of few microns of Rq. In addition, it has been suggested that an irregularity of paper surface geometry such as skewness could be characterized from the deviation of the normalized amplitude.

Material characteristics in metals such as strength, stiffness and fracture resistance are strongly related to the underlying microstructure. The crystallographic structure and orientation are related to the ultrasonic properties through the stiffness matrix. In individual grains it is possible to analytically determine the ultrasonic velocity from the orientation and stiffness, or determine the stiffness from the known orientation and measured velocity. In this paper we present a technique for imaging the crystallographic orientation of grains in metals using spatially resolved acoustic spectroscopy (SRAS) and a novel inverse solver that can determine the crystallographic orientation from the known stiffness matrix for the material and the SRAS velocity measurement. Previously we have shown the ability of this technique to determine the orientation on single crystal nickel samples; we extended the technique to multigrain industrial metals, such as aluminium, nickel and Inconel. The comparison between SRAS and electron backscatter diffraction (EBSD) on the nickel sample is presented. SRAS is a fast, accurate, quantitative and robust technique for imaging material microstructure and orientation over a wide range of scales and industrial materials.

Laser welding of aluminium generally creates embedded welding defects, such as porosities or cracks. Non Destructive Inspection (NDI) after processing may ensure an acceptable weld quality by defect detection. Nowadays, NDI techniques used to control the inside of a weld are mainly limited to X-Rays or ultrasonics. The current paper describes the use of a Laser Ultrasound (LU) technique to inspect porosities in 2 and 4-mm thick sheet lap welds. First experimentations resulted in the detection of 0.5-mm drilled holes in bulk aluminium sheets. The measurement of the depth of these defects is demonstrated too. Further experimentations shows the applicability of the LU technique to detect porosities in aluminium laser welds. However, as the interpretation of raw measures is limiting the detection capacity of this technique, we developed a signal processing using Time-Reversal capabilities to enhance detection capacities. Furthermore, the signal processing output is a geometrical image of the material's inner state, increasing the ease of interpretation. It is based on a mass-spring simulation which enables the back-propagation of the acquired ultrasound signal. The spring-mass simulation allows the natural generation of all the different sound waves and thus enables the back-propagation of a raw signal without any need of filtering or wave identification and extraction. Therefore the signal processing uses the information contained in the compression wave as well as in the shear wave.

Air-coupled ultrasound is used as non-contact ultrasonic testing method. For wider application of air-coupled ultrasonic technique, it is required to know situation of ultrasonic propagation between air and solid. Transmittance of the ultrasonic waves from air to solids is extremely small with 10⁻⁵ however it was revealed that, by using computer simulation methods based on the two-stage elastic wave equation in which two independent variables of stress and particle velocity are used, visualization calculation of ultrasonic propagation between air and solid was possible. In this report, the calculation of air-coupled ultrasound using the new Improved-FDM for computer simulation of ultrasonic propagation in solids is shown. Waveforms obtained by 1-dimensional calculation are discussed for principle and performance of the calculation. Visualization of ultrasonic incidence to cylindrical steel pipe is demonstrated as an example to show availability for ultrasonic testing.

As a first step to investigate mechanism of nonlinear ultrasonic generation at closed cracks, computer simulation for ultrasonic propagation in 1 -dimensional solid including closed interface was examined using Improved-FDM. Fundamental calculation model which described interaction between open / closure motion of the interface and ultrasonic stress was developed. In the model, compression stress is distributed over the entire solid, as motive force for closure of the interface. The interface is exhibited by the small region, and its open / closure are determined using calculated strain of the region. As a result, motion of the interface causing generation of saw-tooth like displacement waveform was observed. Amplitude modulation of displacement waveform was also observed, and it indicated possibility that small fluctuation of open / closure timing caused the modulation of the amplitude.

Conventional ultrasonic imaging based on the difference in acoustic impedance fails to detect and visualize small heterogeneities and local plastic deformation in metals. Nonlinear ultrasonic imaging technique visualizes higher harmonic amplitudes which are generated at the heterogeneities by finite amplitude sinusoidal burst waves, therefore, it can be applied for detecting small non-metallic inclusions, local plastic deformation and micro cracks. By transmitting 35 MHz sine burst waves and receiving harmonics of 105 MHz in the maximum, non-metallic inclusions in stainless steel of some ten in size and crack tip plastic zone of 2 mm in diameter are visualized.

Gas-coupled laser acoustic detection (GCLAD) was primarily developed to sense laser-generated ultrasound in composite materials. In a typical setup, a laser beam is directed parallel to the material surface. Radiated ultrasound waves deflect or displace the probe beam resulting from changes in the air's index of refraction. A position-sensitive photodetector senses the beam movement, and produces a signal proportional to the ultrasound wave. In this paper, we discuss three applications of GCLAD that take advantage of the unique detection characteristics. Directivity patterns of ultrasound amplitude in water demonstrate the use of GCLAD as a directional hydrophone. We also demonstrate the sensing of waveforms from a gelatin. The gelatin mimics ultrasound propagation through skin tissues. Lastly, we show how GCLAD can be used as a line receiver for continuous laser generation of ultrasound. CLGU may enable ultrasound scanning at rates that are orders of magnitude faster than current methods.

Bonding conditions such as jointing agent thickness and bond strength (quality) at the interface between two components is a critical concern in an assembled product because the jointing area is often the weakest component in products. Using a laser ultrasonic technique, zero-group-velocity (ZGV) Lamb waves were utilized to characterize the thickness of an adhesive layer for an epoxy-bonded sample and the bond quality for brazed samples. Using two modes of the ZGV Lamb waves, the thickness of the adhesive layer ranging from 0.2 to 0.8 mm can be estimated with high accuracy. In bond quality measurements, poor bond quality lowered the frequency of ZGV Lamb waves. The decrease in the frequency depended on the amount of shear stress along the interface caused by ZGV Lamb waves.

Although the easiest way to enhance ultrasonic energy generated with pulse laser is to increase laser output, excessive laser output causes damage of the surface. This study introduced an alternative way to generate burst signals without any damages at the surface using a newly developed high repetition pulse laser controlled by galvano mirrors. The calculation results using two-dimensional elastodynamic finite integration technique coupled with thermoelastic effect proved that burst wave of 1 MHz and its higher harmonics were generated while supressing excessive temperature rise using this technique. Moreover, significantly large displacements at the frequency range sufficiently lower than laser repetition rate were observed of the same order of displacements generated with one single shot with the same input energy.

In ultrasonic pulse-echo measurement with a long buffer rod (waveguide), it is required to prevent the generation of spurious echoes (often called trailing echoes) accompanying with a main echo in the buffer rod. In this work, new method to prevent such trailing echoes in the rod is proposed and the effectiveness of the method has been demonstrated experimentally and numerically. In the method the cross-sectional shape of the rod perpendicular to the axial direction is a polygon having sides any one of which is not parallel to any of the other sides, so that trailing echoes are hardly generated during the propagation of pulsed ultrasonic wave in the rod. Three-dimensional numerical simulations based on a finite different method are performed to examine the behaviours of ultrasonic pulse-echoes including trailing echoes for several types of buffer rods having different cross-sectional shapes such as a circle, triangle, square, pentagon, hexagon and heptagon. Based on the results, experiments with several buffer rods are carried out at frequency 5 MHz. It has been found that heptagon may be the suitable shape for effectively eliminating trailing echoes and improving the signal-to-noise ratio of the measured pulse-echo.

In the development of medical ultrasound techniques, fast and accurate pressure field measurement is important. The most common method to measure an ultrasound pressure field is mechanically scanning a hydrophone, which takes a long time and might disturb the acoustic field. In this study, we used an optical shadowgraph method. To perform this method quantitatively, it is important to define the optical propagation length precisely. For this purpose, a holographic diffuser was used as the imaging screen. Combined with a computed tomography (CT) algorithm, a pressure field was reconstructed, and the result was compared with that of hydrophone measurement. By using two shadowgraph data from short and long propagation lengths, the pressure field was successfully reconstructed even at a pressure level for high intensity focused ultrasound (HIFU) treatment.