Papers by Keyword: Laser Ultrasound

Abstract: 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.

Abstract: Based on thermo-elastic regime, ultrasound can be generated by modulated laser pulse. When laser-generated ultrasound technology is applied to measure the thickness of coating and study the coating characters, even analysis the effects of coating during detecting the cracks and voids of the substrate, it is shown that it’s a very efficiency technology. When measuring the coating thickness, it can control the accuracy scope at the nanoscale; the relative error is under 2% for single layer coating, under 10% for double layer coating. As to coating characters, for example, Young modulus, Poisson's ratio etc, according to BP Neural network algorithm, use the nonlinear map relationship between material parameter and laser generated ultrasound wave forms to get these coating characters reversal. Studying the dispersion characteristic of the coating-substrate system can analysis effects of coating to defects detecting that coating absorbs large energy which leads to faster heat conduction speed than uncoated sample, but have smaller surface wave amplitude. All these are shown that laser-generated ultrasound technology can realize high-precise measurement and have an extensive use in the field of nondestructive testing.

Abstract: The design of PVDF transducer is finished and its designing idea and structure frame are presented. The laser ultrasound in nano A1P/SiO₂ is detected by the PVDF transducer and the acoustic signal curve is obtained. The ultrasound waveform thermal elastically generated by a rectangular laser pulse is theoretically presented. The ultrasound pulse waveform obtained in the experiment is in accordance with the one in theory. It is shown that this design method is simple, practical and easy to implement.

Abstract: Disc-shaped specimens of nickel and iron of about 30 mm diameter and a few millimeters thick were prepared by the high pressure torsion (HPT) method [1]. The produced equivalent shear strain ε linearly increased from a minimum at the centre to the periphery. The cold working process multiplied the dislocations and led to the grain fragmentation. The saturation of the grain refinement was approached at the circumference of the disk-specimens yielding a narrow distribution of the grain size with a mean value of about few hundred nanometers that depends on the material purity, temperature of the deformation [2]. The refinement of the microstructure changes the elastic properties of the material such as hardness, strength and has an influence on the attenuation and phase velocity of the ultrasound [3]. The paper reports the measurements of the longitudinal wave velocity and the attenuation in metals with polycrystalline (pc) and ultrafine (uf) microstructure.

Abstract: In this paper, ultrasonic materials testing researches are reviewed. The latest progress of ultrasonic testing technology is introduced, including water-squirting ultrasonic C-scan testing, laser ultrasound, ultrasonic feature scan imaging, signal processing and pattern recognition technology in the application of ultrasonic testing.

Abstract: This paper proposes a non-contact single-mode guided wave technique, where the laser beam illuminated through the arrayed line slits is used as the transmitter, and the air-couple transducer is used as the receiver. The line-arrayed laser illumination has a wavelength-matching effect that can generate only a few modes. The air-coupled transducer detects the leaky wave of the propagated guided wave, and by tuning the detection angle of the transducer we can detect the selected single mode. The proposed technique is fully non-contact and is possible to avoid difficulty in the mode identification. The experimental results for a 1-mm thick aluminum plate prove the usefulness of the proposed method, and most especially, it shows that the method is powerful in the generation and detection of anti-symmetric modes of the lowest order.

Abstract: A laser ultrasound technique (LUT) is reported for nondestructive characterization of hydrogen concentration (HC) in Zr-4 cladding tubes. With the LUT, ultrasonic waves propagating in the Zircaloy tubes with different HC are generated and detected remotely by optical means. By measuring the dispersion spectra with the LUT, relations between the dispersion spectra and the HC of the Zircaloy tubes are established. HC ranging from 0 to 1200 ppm in the Zircaloy tubes are successfully discriminated by the LUT with a resolution of 200 ppm.

Abstract: A new interference-based optical technique called in-plane displacement interferometer (IPDI) is introduced for the detection of in-plane (IP) motion of ultrasonic waves. The IPDI employs a tiny square indentation of about 30 microns in width on the sample surface and a relatively simple optical arrangement for the detection of IP motion of ultrasonics waves. With the IPDI, measurement of IP motions of Lamb waves propagating along a thin brass foil is demonstrated. Dispersion relations for the S0 mode dominated by in-plane motion in the low fd (frequency times thickness) regime is obtained with the IPDI.

Abstract: This paper describes the development of a confocal Fabry-Perot interferometer (CFPI) for non-contact and non-destructive detection of broadband ultrasound generated by a pulsed laser. The operation theory of CFPI is introduced. The transmission and reflection modes of operation were investigated theoretically and verified experimentally. For the present study, a CFPI cavity of 50cm with 95.4% reflectivity spherical mirrors was constructed with associated resonant cavity control and signal detection electronics. The design is capable of providing detection frequency bandwidth from 140kHz to 50MHz. For the first step of verification, the input signal simulated by an electro-optical modulator (EOM) was used for verifying the feasibility of surface wave measurement. Signals obtained from an avalanche detector were compared with the results through theoretical analysis of the CFPI transfer function in a transmission mode. The results show a favorable agreement between the two. Furthermore, transmitted ultrasound signals from a 5MHz contact ultrasound transducer were detected and compared between the CFPI system and a Michaelson interferometer. Patterns of ultrasound arrival and reflection were clearly detected by both. Because an intrinsic transfer function is embedded in the operation of CFPI, the output signal will be distorted when measuring surface displacement. A digital filtering process was considered for compensation for the surface displacement signal. From the comparative results, it was further concluded that the present CFPI design has a displacement resolution of 0.05nm. Future studies will be focused on the reflection mode operation for fully utilization of non-contact laser ultrasound generation and detection.

Abstract: We developed a laser TOFD (Time of flight diffraction) algorithm which utilizes not only longitudinal wave but also shear wave. This algorithm made it possible to obtain accurate flaw depth without knowing the specimen velocity and probe distance previously. We constructed the laser TOFD system and applied it to estimate the slit depth of aluminum alloy plate. Time of flight of lateral wave, flaw tip diffraction waves and mode converted shear wave at flaw tip were used to estimate the slit depth using new algorithm.