Ultrasonic Generation by Pulsed Lasers

A noncontact all‐optical method for surface photoacoustics is described. The surface acoustic waves (SAWs) were excited employing a KrF laser and detected with a Michelson interferometer using a 633‐nm HeNe laser. Due to an active stabilization scheme developed for the interferometer a surface displacement of 0.2 A could be detected. The materials investigated included pure materials such as polycrystalline aluminum, and crystalline silicon; films of gold, silver, aluminum, iron, and nickel on fused silica; and a‐Si:H on Si(100). In the case of pure materials the shape of the acoustic pulse and the phase velocity were determined. The dispersion of the SAW phase velocity observed for the film systems was used to extract information on the film thickness, density, and transverse and longitudinal sound velocity. Models for the theoretical treatment of film systems and the calculation of dispersion curves are presented.

Laser generation and detection of surface acoustic waves: Elastic properties of surface layers

Surface acoustic wave (SAW) device coverage includes delay lines and filters operating at selected frequencies in the range from about 10 MHz to 11 GHz; modeling with single-crystal piezoelectrics and layered structures; resonators and low-loss filters; comb filters and multiplexers; antenna duplexers; harmonic devices; chirp filters for pulse compression; coding with fixed and programmable transversal filters; Barker and quadraphase coding; adaptive filters; acoustic and acoustoelectric convolvers and correlators for radar, spread spectrum, and packet radio; acoustooptic processors for Bragg modulation and spectrum analysis; real-time Fourier-transform and cepstrum processors for radar and sonar; compressive receivers; Nyquist filters for microwave digital radio; clock-recovery filters for fiber communications; fixed-, tunable-, and multimode oscillators and frequency synthesizers; acoustic charge transport (ACT); and other SAW devices for signal processing on gallium arsenide. Shallow bulk acoustic wave (SBAW) device applications include gigahertz delay lines, surface-transverse-wave resonators employing energy-trapping gratings and oscillators with enhanced performance and capability. >

Applications of surface acoustic and shallow bulk acoustic wave devices

A noncontacting technique has been developed to measure the thickness of thin metal sheet by using a pulsed laser to generate both symmetric and antisymmetric Lamb waves. These have been detected with a laser interferometer. Analysis of the waveforms allows an estimation of the sheet thickness and accuracies to within 2% are attainable on sheets as thin as 27 μm.

Estimation of the thickness of thin metal sheet using laser generated ultrasound

In the non-destructive testing of concrete structures, the use of Rayleigh waves shows some advantages to characterise surface cracks: easiness of excitation and recording, access to only one surface of the structure required, great spectral sensitivity to the propagation medium…. But the behaviour of Rayleigh waves on surface defects in concrete is difficult to perceive in the field, even if the dependence of the diffraction pattern on the crack's geometrical features is significant. A numerical model is adapted from earth physics in order to better understand the influence of the crack geometry on Rayleigh-wave propagation. This model, based on an indirect boundary element method, calculates the three-dimensional seismic response of two-dimensional structures. Synthetic seismograms are obtained for the propagation of a Rayleigh wave across various crack geometries. The variations of spectral ratios between the transmitted and incident waves are studied as a function of the crack depth. They are used to design an efficient procedure for the determination of crack depths.

Characterization of surface cracks with Rayleigh waves: a numerical model

Abstruct-The interaction of surface acoustic Rayleigh pulses with surface-breaking defects has been experimentally studied. The pulses were generated using a Q-switched Nd:YAG laser, and were detected by either a ball capacitance transducer or a laser interferometer. It is shown that reflected waveforms from a defect possess two major components. The first arises from direct reflection of a Rayleigh pulse from the top of the defect, while the second appears to arise from a shear pulse originating from the base of the defect, which is then mode-converted to a Rayleigh pulse on reaching the metal's surface. This model is supported by further experiments which examined interaction with 90" and 270" corners, as well as down-steps. The results offer a new laser technique for measuring defect depths in the range 0.1 to 5 mm.

Surface Acoustic Wave Interactions with Cracks and Slots: A Noncontacting Study Using Lasers

Nanosecond pulses from a Nd:YAG laser are used to excite clamped aluminum disks to resonance. The first two flexural vibration modes are shown to be in agreement with classical plate theory. This theory is then applied to the case of flat‐bottomed holes in aluminum plates, when the remainder of the plate below the hole may be said to be partially clamped. It is shown that the theory can still be used over a well‐defined regime. In addition, the depth of the hole can be obtained from the ring‐around longitudinal ultrasonic pulse generated by the laser. A combination of these two techniques, therefore, allows a sizing of circularlike flaws in laminations lying close to a sample’s surface.

R. A. Crosbie

Papers

Surface Acoustic Wave Interactions with Cracks and Slots: A Noncontacting Study Using Lasers

Flexural resonance measurements of clamped and partially clamped disks excited by nanosecond laser pulses