C. B. Scruby

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

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 measurements of laser‐generated acoustic waveforms

The generation of acoustic waves in metals by pulsed laser irradiation over a wide range of material conditions has been studied. Capacitance transducers have been used to obtain quantitative measurements of the amplitude of bulk acoustic waveforms where the laser beam was directed onto free metal surfaces in the presence and absence of surface plasmas, and onto modified metal surfaces. The application of acoustic wave propagation theory has allowed theoretical waveforms to be determined. By combining data for thermoelastic and normal force sources, waveforms have been produced that follow closely those measured experimentally.

Laser generation as a standard acoustic source in metals

Absolute acoustic waveforms, generated by laser irradiation of aluminium and mild steel samples, have been detected at epicenter with a wide‐band capacitance transducer. Comparison has been made with theoretical waveforms, utilizing theory for wave propagation within a plate and assuming certain source characteristics. Results have shown that laser generation is able to produce three types of standard acoustic source, namely a horizontal force dipole and a normal force monopole, each with step‐function time dependence, and a normal force monopole with δ‐function time dependence. Underlying generation mechanisms are discussed.

Elastic wave radiation from cleavage crack extension

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.RésuméOn a calculé les ondes élastiques qui émanent dune micro-rupture élastique et isolée dorientation variable, et qui aboutissent à lépicentre dun demi-espace élastique. On a utilisé ces calculs pour prédire les signaux démission acoustique qui seraient détectés sur les diverses faces dune éprouvette de traction compacte. Les déplacements associés aux ondes élastiques et créés par une extension progressive des clivages dans la zone de préfissuration de ces éprouvettes ont été mesurés, à la fois en avant de la fissure en propagation, et à la verticale de celle-ci. On trouve que les signaux démission acoustique diffèrent des formes dondes prédites en raison de la présence de surfaces libres de préfissuration le long desquelles se propagent des perturbations avec comme point de départ lextrémité de la fissure et avec comme vitesse celle de londe de surface. Leffet en est daccroître lamplitude du signal démission acoustique et daugmenter le contenu en fréquences faibles. Les résultats expérimentaux sont en accord avec les calculs élasto-dynamiques récents démission dondes élastiques à partir dune fissure en propagation progressive.

An assessment of acoustic emission for nuclear pressure vessel monitoring

Abstract Recent research has greatly improved our understanding of the basic mechanisms of deformation and fracture that generate detectable acoustic emission signals in structural steels. A critical review of the application of acoustic emission (AE) to the fabrication, proof testing and in-service monitoring of nuclear pressure vessels is presented in the light of this improved understanding. The detectability of deformation and fracture processes in pressure vessel steels is discussed, and recommendations made for improving source location accuracy and the development of quantitative source assessment techniques. Published data suggest that AE can make an important contribution to weld fabrication monitoring, and to the detection of defects in lower toughness materials during vessel proof testing. In high toughness materials, however, the signals generated during ductile crack growth may frequently be too weak for reliable detection. The feasibility of AE for continuous monitoring has not yet been adequately demonstrated because of high background noise levels and uncertainty about AE signal strengths from the defect growth processes that occur in service. In-service leak detection by AE shows considerable promise. It is recommended that further tests are carried out with realistic defects, and under realistic conditions of loading (including thermal shock and fatigue) and of environment.

The performance of thick piezoelectric transducers as wide-band ultrasonic detectors

Abstract A Q-switched Nd:YAG laser has been used to generate reproducible acoustic displacements in metallic samples. The acoustic waveforms were initially detected by an absolute displacement-sensitive, capacitance transducer. The capacitance transducer was then replaced in turn by two piezoelectric ceramic transducers of different thickness and diameter. Comparison of waveforms from the two types of detector indicate that thick piezoelectric transducers are useful in the detection of fast rise-time displacement waveforms at metal surfaces. Their performance is optimized by making them as thick as possible, with a small area of contact with the surface.

Relationship between microstructure and acoustic emission in Mn-Mo-Ni A533B steel

AbstractThe sources of acoustic emission in a steel of the A533B composition with low sulphur content have been investigated using two different recording techniques. First, systematic narrow band (0.1–1.0 MHz) measurements were made of acoustic emission activity during uniaxial tensile tests of specimens with a range of controlled microstructures. Mechanical properties and fracto graphic data were analysed, and the most probable deformation and fracture mechanisms deduced for these microstructures. It appears that only a few processes generate detectable emission in this material. For instance, high-velocity dislocation motion can be detected during the yield of slow-cooled or long-tempered specimens, and cleavage fracture in rapidly quenched specimens. A model is presented to relate the magnitude of the emission to the dynamic operation of dislocation and fracture sources. Second, very wide band (25 MHz) calibrated measurements were made of the most energetic emission sources in rapidly quenched materia...

Some Applications of Laser-Generated Ultrasound in Metals

A Q-switched Nd:YAG laser has been used by the authors to generate elastic waves at metal surfaces. As a result of this work, various applications have been highlighted, and are described. These include use as a remote method of acoustic generation for nondestructive evaluation, as a standard acoustic source with excellent repeatibility, and as a method for liquid depth estimation.

Laser‐generated ultrasonic waveforms in metals

A Q‐switched, Nd:YAG laser has been used to generate elastic waves within aluminum and mild steel disks. A single 24‐ns pulse from the laser irradiated one face of each disk, and acoustic displacements were detected at the other face following propagation through the substrate. A capacitance transducer was used as the detector, providing undistorted displacement waveforms over a wide bandwidth. The displacement waveform depended markedly on the irradiation conditions. At low laser power densities, such that generation occurred primarily by thermoelastic expansion, a common waveform was produced whose amplitude varied linearly with optical energy. At higher power densities in the presence of material ablation, the longitudinal amplitude increased substantially whereas that of the shear decreased. Wave propagation theory in a plate has been applied to the problem, and theoretical waveforms in good agreement with experiment have been produced; this leads to conclusions concerning the nature of the acoustic s...