Andrew D. W. McKie

Quantitative theory for laser ultrasonic waves in a thin plate

Numerical inversion of the Hankel–Laplace transform has been performed for the case of ultrasonic displacements in an infinite, homogeneous, isotropic plate which is excited thermoelastically by a laser pulse. Values for the elastic moduli and the plate thickness may be extracted when the calculated displacements are compared directly to those obtained experimentally. Previous authors have demonstrated methods for determining the elastic modulus in thick plates; this letter shows that using a different method for the development of the theory allows similar modulus determinations to be made for thin as well as thick plates.

Laser generation of narrow-band and directed ultrasound

Abstract An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.

Generation of narrow‐band ultrasound with a long cavity mode‐locked Nd:YAG laser

A passively mode‐locked, flashlamp‐pumped Nd:YAG laser with a cavity length of 11.19 m has been developed to study the noncontact generation of narrow‐band ultrasound. The individual mode‐locked pulses acted as separate sources of ultrasound, producing a train of acoustic pulses with a repetition rate of about 13.4 MHz. The ultrasound was generated in an aluminum sample and remotely detected with a path stabilized Michelson interferometer. The energy in the multiple pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse.

Modulated laser array sources for generation of narrowband and directed ultrasound

Patterned illuminating sources and appropriate time modulation may be used to enhance certain features of laser-generated acoustic waves in time or frequency as a function of direction. Steerable, narrowband, “toneburst” ultrasonic signals have been generated from a single laser pulse which was spatially modulated by transmission through a lenticular array. In addition, narrowband toneburst ultrasonic waves have been generated from a mode-locked laser pulse train providing spectral narrowing of the laser-ultrasonic signal. Both temporal and spatial modulation of the laser pulse improve the signal-to-noise ratio for laser ultrasonics.

Optical sensing of in-plane ultrasonic transients

An optical sensor has been developed, based on the differential laser Doppler anemometer, for the detection of in‐plane ultrasonic transients. Ultrasonic pulse propagation around a 90° corner has been investigated in an aluminum block. By generating ultrasonic pulses on diametrically opposite sides of the detector a polarity inversion of the ultrasonic pulses detected is demonstrated. The results obtained are as expected for the case of in‐plane displacement measurements.

Dual-beam interferometer for the accurate determination of surface-wave velocity.

A novel dual-beam interferometer has been designed and constructed that enables two beams from a He-Ne laser to probe remotely the surface of a material. The separation of the two He-Ne beams is adjustable in the 15-to- 40-mm range with a spatial resolution of 2 microm. Surface-acoustic-wave measurements have been performed with two different probe separations so that the travel time for the surface waves over a known distance can be determined accurately. With the aid of autocorrelation algorithms, the Rayleigh pulse velocity on 7075-T651 aluminum has been measured to be 2888 +/- 4 m/s. The current precision of the system is limited mainly by the 10-ns sampling rate of the digital oscilloscope used. Rayleigh pulse interactions with a surface-breaking slot, machined to a nominal depth of 0.5 mm, have also been examined and the depth estimated ultrasonically to be 0.49 +/- 0.02 mm. The system may also provide a technique for direct quantitative studies of surface-wave attenuation.

Laser-based ultrasonic inspection of complexly contoured composite and metallic structures for aerospace applications

Previously, the applicability of laser-based ultrasonic techniques for the automated inspection of polymer-matrix airframe structures, having either flat or contoured geometries, has been demonstrated without restriction to large radii of curvature. More recently our LBU inspection capability has been extended to include higher spatial resolution for the inspection of complexly contoured metallic structures. In this paper, progress regarding the application of LBU to the inspection of complex geometry polymer-matrix composite structures and metallic rocket engine components is summarized.

Determination of glass thickness using laser-based ultrasound

Thickness measurements of glass plates and glass bottles using laser-based ultrasound (LBU) are described. Ultrasound in the glass specimens was generated thermoelastically with either a pulsed CO2 laser, or a Q-switched Nd:YAG laser in the case of colored glass filters. The detection of ultrasound was accomplished by one of the following methods; a spherical Fabry-Perot interferometer system or a photo-refractive interferometer based on two-wave mixing. A self-interference effect, utilizing the partial reflection from the front and back faces of a glass plate was also demonstrated to have sufficient sensitivity under certain conditions. The thickness of the glass plates and colored glass bottles was determined using the fundamental reverberation frequency obtained from the time-domain waveform data. LBU results were compared to physical thickness measurements and showed excellent agreement.

Laser-based ultrasonic inspection with a fiber-coupled scanning Cassegrain system

State-of-the-art integrally stiffened composite materials, manufactured for use in the next generation of commercial and military aircraft, are increasingly being used for structural components such as wings and fuselages. However, the complexity of the manufacturing processes can produce small variations in the shape of integrally stiffened composite structures. Thus, a priori knowledge of the nominal part shape often does not provide sufficient accuracy to allow an automated conventional ultrasonic inspection. In contrast, automated inspections of integrally stiffened structures can be performed using laser-based ultrasound techniques since a priori knowledge of the nominal part shape is adequate to scan the laser beams over the structure. This paper addresses the issues associated with the extension of laser-based ultrasonics to inspections in remote and limited access areas, and describes the implementation of a fiber-based remote and limited access LBU inspection system based upon a Cassegrain scanning and optical collection system. The ability to quickly and directly manipulate flexible low mass optical fibers equipped with specialized endoscopic scanning optics make fiber systems an attractive method for the development of limited and remote access inspection systems. The Cassegrain optical system is described in detail and both numerical and experimental validation of the system operational characteristics are presented.

Laser ultrasonics: generation and detection considerations for improved signal-to-noise ratio

It is shown that improvement in the detection sensitivity of laser-ultrasonic systems may be obtained by generating narrowband acoustic signals using both temporal and spatial modulation of the generating laser. A laser-generated acoustic tone burst waveform will have lower peak amplitudes than a single acoustic pulse providing the same system SNR. Consequently, lower power density laser pulses may be used to avoid surface damage.