This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

A review of the various optical methods to detect ultrasound (bulk and surface waves) at the surface of opaque solids is presented. The most useful techniques are thoroughly analyzed. Their performance when nonideal conditions are encountered, such as vibrations, air turbulence, and rough light scattering surfaces is evaluated. This review includes a description of knife-edge techniques, optical heterodyning, differential interferometry, and velocity (time-delay) interferometry methods, plus a mention of various less-important tech-

Optical Detection of Ultrasound

The laser generation of ultrasound in solids is reviewed with particular emphasis on the application of this unique ultrasonic source. Three regimes for the generation of ultrasound in solids using lasers in the visible near infrared wavelength region exist: thermoelastic, plasma and constrained surface source regimes. The mechanism for ultrasonic generation in each of these regimes is given. Recent experimental investigations into laser-generated ultrasound are also described, including a description of a different mechanism for the generation of ultrasound in solids using a CO2 laser. Finally, the many applications of laser-generated ultrasound are reviewed. These applications range from nondestructive testing to the determination of candle flame parameters. Possible future applications are also outlined.

https://iopscience.iop.org/article/10.1088/0022-3727/26/3/001/pdf

Laser-generated ultrasound: its properties, mechanisms and multifarious applications

Piezoelectric sensing is of increasing interest for high-temperature applications in aerospace, automotive, power plants and material processing due to its low cost, compact sensor size and simple signal conditioning, in comparison with other high-temperature sensing techniques. This paper presented an overview of high-temperature piezoelectric sensing techniques. Firstly, different types of high-temperature piezoelectric single crystals, electrode materials, and their pros and cons are discussed. Secondly, recent work on high-temperature piezoelectric sensors including accelerometer, surface acoustic wave sensor, ultrasound transducer, acoustic emission sensor, gas sensor, and pressure sensor for temperatures up to 1,250 °C were reviewed. Finally, discussions of existing challenges and future work for high-temperature piezoelectric sensing are presented.

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High-Temperature Piezoelectric Sensing

Quantitative experimental measurements have been made in the study of thermoelastic generation of elastic waves in a metal by unfocused laser radiation. A calibrated wide‐band detection system, incorporating a capacitance transducer, has enabled acoustic waveforms to be recorded with a minimum of distortion. From these measurements, a theoretical model has been developed. The transfer function of the metal block has been deconvoluted to give the acoustic source function, which was modeled as a rapidly expanding point volume of material. The thermoelastic source generated longitudinal (L) and (S) waves, but the latter predominated at the epicenter, where, in experiments presented here, both wave amplitudes L and S were proportional to the total absorbed energy in the laser pulse.

Quantitative studies of thermally generated elastic waves in laser‐irradiated metals

The acoustic emission power during deformation of pure aluminium single crystals and polycrystals, and specimens of A1 1·3wt% Mg, has been measured. In pure aluminium the acoustic emission activity increased with grain size to approach the levels of activity from single crystals, whereas the emission activity of the Al 1·3wt% Mg solid solution exhibited a peak at a grain size of ∼80 μm, and samples with large grain size generated almost no detectable emission. A model is presented for acoustic emission by a dislocation, and is used to explain the observed emission dependence upon grain size in both metals.

The origin of acoustic emission during deformation of aluminium and an aluminium–magnesium alloy

Laser generation of ultrasound in metals

The authors describe a capacitance transducer for use as a calibrated detector of energetic acoustic emission events It is displacement-sensitive with a wide frequency response, and typically has a rise-time of 20 ns The electrical output of the transducer has been calibrated as a function of displacement, and the device has been tested in a broad-band acoustic emission system It is envisaged that the transducer could either be used to measure emission transients directly, or act as a reference against which conventional piezoelectric transducers could be compared

A calibrated capacitance transducer for the detection of acoustic emission

The elastic waves, radiated from an isolated elastic microfracture of variable orientation and received at the epicentre of an elastic half-space, have been calculated and used to predict the acoustic emission signals that would be detected on various faces of a compact tension specimen. The displacements due to elastic waves radiated by incremental cleavage extension of precracks in such samples have then been measured, both ahead of and vertically above the advancing cracks. These acoustic emission signals are found to differ from the predicted waveforms because of the presence of precrack free surfaces, along which disturbances propagate from the crack tip at the surface wave velocity. Their effect is to increase the amplitude of the acoustic emission signal and enhance their lower frequency content. The experimental results are consistent with recent elastodynamic calculations of elastic radiation from incremental crack advance.

Elastic wave radiation from cleavage crack extension

Describes a new broad-band approach to the detection of acoustic emission in which a capacitance transducer has been used to record the transient elastic waves generated by crack growth processes as surface displacements. A unique specimen geometry, the 'Yobell' has been developed to reduce interference from internal reflections of the ultrasonic pulse, so that the measured surface displacements can be compared with those calculated for a source buried below the surface of the half-space. A static source model has been used to relate the rise-time and amplitude of transients to the time-scales and magnitudes of incremental crack growth.

C. B. Scruby

Papers

The origin of acoustic emission during deformation of aluminium and an aluminium–magnesium alloy

Laser generation of ultrasound in metals

A calibrated capacitance transducer for the detection of acoustic emission

Elastic wave radiation from cleavage crack extension

A new technique for the measurement of acoustic emission transients and their relationship to crack propagation