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Receiving displacement sensitivities (Rx) of ultrasonic transducers and acoustic emission (AE) sensors are evaluated using sinewave packet excitation method and compared to the corresponding data from pulse excitation method with a particular emphasis on low frequency behavior below 20 kHz, down to 10 Hz. Both methods rely on the determination of transmitter displacement characteristics using a laser interferometric method. Results obtained by two calibration methods are in good agreement, with average spectral differences below 1 dB, indicating that the two calibration methods yield identical receiving sensitivities. At low test frequencies, effects of attenuation increase substantially due to increasing sensor impedance and Rx requires correction in order to evaluate the inherent sensitivity of a sensor, or open-circuit sensitivity. This can differ by more than 20 dB from results that used common preamplifiers with ~10 kΩ input impedance, leading to apparent velocity response below 100 kHz for typical AE sensors. Damped broadband sensors and ultrasonic transducers exhibit inherent velocity response (Type 1) below their main resonance frequency. In sensors with under-damped resonance, a steep sensitivity decrease occurs showing frequency dependence of f2~f5 (Type 2), while mass-loaded sensors exhibit flat displacement response (Type 0). Such behaviors originate from sensor characteristics that can best be described by the damped harmonic oscillator model. This model accounts for the three typical behaviors. At low frequencies, typically below 1 kHz, receiving sensitivity exhibits another Type 0 behavior of frequency independent Rx. Seven of 12 sensors showed this flat region, while three more appear to approach the Type 0 region. This appears to originate from the quasi-static piezoelectric response of a sensing element. In using impulse method, a minimum pulse duration is necessary to obtain spectral fidelity at low frequencies and an approximate rule is given. Various factors for sensitivity improvement are also discussed.

This review introduces several areas of importance in acoustic emission (AE) technology, starting from signal attenuation. Signal loss is a critical issue in any large-scale AE monitoring, but few systematic studies have appeared. Information on damping and attenuation has been gathered from metal, polymer, and composite fields to provide a useful method for AE monitoring. This is followed by discussion on source location, bridge monitoring, sensing and signal processing, and pressure vessels and tanks, then special applications are briefly covered. Here, useful information and valuable sources are identified with short comments indicating their significance. It is hoped that readers note developments in areas outside of their own specialty for possible cross-fertilization.

In order to quantify the wave motion of guided ultrasonic waves, the characteristics of piezoelectric detectors, or ultrasonic transducers and acoustic emission sensors, have been evaluated systematically. Such guided waves are widely used in structural health monitoring and nondestructive evaluation, but methods of calibrating piezoelectric detectors have been inadequate. This study relied on laser interferometry for the base displacement measurement of bar waves, from which eight different guided wave test set-ups are developed with known wave motion using piezoelectric transmitters. Both plates and bars of 12.7 and 6.4 mm thickness were used as wave propagation media. The upper frequency limit was 2 MHz. Output of guided wave detectors were obtained on the test set-ups and their receiving sensitivities were characterized and averaged. While each sensitivity spectrum was noisy for a detector, the averaged spectrum showed a good convergence to a unique receiving sensitivity. Twelve detectors were evaluated and their sensitivity spectra determined in absolute units. Generally, these showed rapidly dropping sensitivity with increasing frequency due to waveform cancellation on their sensing areas. This effect contributed to vastly different sensitivities to guided wave and to normally incident wave for each one of the 12 detectors tested. Various other effects are discussed and recommendations on methods of implementing the approach developed are provided.

This study examined the behavior of AE sensor couplants for through-transmission of acoustic signals or out-of-plane motion. In order to provide controlled conditions, face-to-face transmitterreceiver arrangement was utilized. All liquid and gel couplants are found to give satisfactory service with careful installation, producing the minimum couplant thickness of 5 to 8 μm. Their performance is nearly identical to 1.2 MHz. With normal installation procedures, however, the loss can be 3 to 5 dB with viscous resins and gels, as couplant thickness can become 10 to 15 μm and high frequency components are lost more than the peak amplitude reduction. Honey turns out to be an excellent couplant because of its higher acoustic impedance, while commonly used silicone grease can be a poor choice at higher frequencies. Higher frequency transmission loss increases with thickness as predicted by theory of through-gap transmission with approximately linear dependence on frequency.

This study reports the characteristics of acoustic emission (AE) waveguides using displacement pulse of about 1-μs duration from an ultrasonic transducer, driven by a short impulse. By using one of broadband sensors as a receiver, improved characterization of waveguides is possible over 50 to 2000 kHz. Here, we obtained responses of circular rod waveguides of different diameters from 1.6 to 12.7 mm with lengths varying from 60 to 900 mm. It is found that the lowfrequency part of L(0,1)-mode rod waves is dominant in thin rod waveguides. This mode is increasingly suppressed above 6-mm diameter. This mode is also responsible for stretching a shortduration (~1 μs) source into a long pulse of more than 100 μs as its velocity is reduced at higher frequencies. At larger diameters, slower L(0,3)-mode becomes the dominant one, and its peak is found at its highest velocity over a narrow range of frequency. To keep the pulse duration short, it is necessary to keep the waveguide diameter below 3 mm, preferably at 1.6 mm or less so that the relatively non-dispersive L(0,1)-mode prevails over the frequency range of interest. Diameter reduction, however, leads to reduced sensitivity unless small aperture sensors are used. Tube waveguides are found to offer a significant advantage of spectral smoothness. In tubes, L(0,2) mode plays an important role of providing nearly constant velocity and spectral flatness. Effects of threading are also evaluated. Three practical waveguide designs are suggested. More extensive exploration of tubular waveguides is highly recommended.

This paper reviews progress in methods of signal analysis used in acoustic emission (AE) as applied to materials research field. The achievement and inadequacy in understanding of AE from materials during the deformation, fracture and other processes are examined systematically. New goals for the future are also discussed in view of new analytical tools and vastly advanced instrumentation.

We have obtained the transfer functions of a wide range of AE sensors commonly available and utilized. These were determined by a pulse-laser excitation in conjunction with a laser interferometer and a deconvolution procedure typically in the frequency domain. Using typical source waveforms and a convolution procedure, one can then visualize waveforms expected out of these AE sensors. In turn, one can also deduce approximate source waveforms from AE signal waveforms, which a similar sensor has detected. Some sensors showed displacement response, while another gave velocity response. Some unexpected results are found, including a mixed response of a small sensor and location sensitivity of otherwise a well-behaved sensor.

Techniques of acoustic emission (AE) were used during the over-straining applied on 10-m length straight pipes and elbows of a polyethylene reactor in a factory. Over-straining is done before the pipes were put in operation and also periodically during service. AE enables us to monitor the stability of the deformation process and indicates the zones, which will require further detailed examinations by other NDT techniques; it should also improve safety during this process through early detection of critical damage. We examined several groups of pipes using a pattern recognition program to separate the signals due to cracking, confirmed by other NDT techniques. We will report on the attempt to establish new criterion.