Fundamental to the development of wideband waveform-based acoustic emission technology are wideband, acoustic emission sensor/preamplifier combinations with high sensitivity. We believe we have developed a practical, wideband, acoustic emission sensor/preamplifier design, which has demonstrated signal-to-noise characteristics comparable to commercial resonant sensor/preamplifiers. This situation is in contrast to the fact that many early wideband sensor designs and many present "off the shelf" commercial wideband sensors are typically an order of magnitude or more less sensitive than resonant systems.

Sensitivity is the result of two factors:

1. the magnitude of the output response voltage of the sensor/preamplifier combination for a given input displacement signal; and
2. the corresponding electronic noise characteristics of the sensor/preamplifier combination.

The titles, abstracts, references and in some cases additional information from our relevant papers in this subject area are listed below.

Research leading to a significant improvement in the signal-to-noise sensitivity of wideband acoustic/ultrasonic contact displacement sensors for wood and polymers is described. Design principles for such high-sensitivity sensors are reviewed. Comparisons of response between ceramic and polymer piezoelectric elements are made on low modulus specimens. A new, practical high-sensitivity sensor is characterized and its signal-to-noise sensitivity is compared to that of an existing commercial wideband displacement sensor. The comparisons were made for polymer, maple, and redwood samples. Optimization of the piezoelectric element in the new sensor is considered. The typical increased sensitivity of the new sensor is about 30 dB over the existing commercial sensor.

A series of research efforts has examined the optimization of the sensitivity (signal-to-noise ratio) of wideband acoustic/ultrasonic contact displacement sensors for low modulus specimen materials such as wood and polymers. It was found that conflicts between mechanical and electrical design optimization prevent attaining a better response sensor using available piezoelectric ceramic element. In spite of the lack of optimization of the polymeric piezoelectric sensor, it was found that a newly developed practical version of a mass-backed conical PZT-5A sensor element combined with a low noise closely coupled field-effect-transistor results in significant sensitivity increases over an existing commercial wideband sensor/preamplifier on low modulus samples of wood and polymers. This new sensor will enable development of wideband acoustic/ultrasonic NDE techniques.

The development of a series of wideband acoustic emission (AE) sensor/preamplifier systems is described. Key design factors are discussed along with the actual design and characterization of the sensors. These new sensors with integral amplification are out-of-plane, displacement response sensors nearly independent of frequency over a range from as low as 30 to 50 kHz up to 1.2 MHz. The sensor design includes electromagnetic shielding and mechanical protection of the sensitive elements. More importantly, these practical sensors have signal-to-noise sensitivity that is equivalent to typical commercial resonant sensor/preamplifier systems operating from 100 kHz to 300 kHz. A mounting fixture for the sensors has also been developed.

A practical, wideband, displacement AE sensor nearly independent of frequency has been developed at NIST/Boulder. The sensor is protected form electromagnetic noise and mechanical damage. The sensor-system sensitivity has been improved to be essentially equivalent to typical commercial resonant sensors and is easily mounted on surfaces of any orientation. It is not insensitive to typical signals detected by AE, as was the case with past wideband sensors. It can be used on conducting as well as nonconducting specimens.

Previous work has compared the relative performance of various wide-band ultrasonic transducers used as receivers. Studies have also been made comparing the merits of various optical sensors and evaluating their applicability to Acoustic Emission (AE). In this paper, the calculated and measured sensitivities of such transducers are compared with AE signal levels as estimated by Scruby, Wadley, and Simmons. While optical sensors appear to provide many practical advantages over contact sensors, particularly at very low frequencies, it is found that they cannot meet the sensitivity requirements for wide-band AE detection in metals. Furthermore, it is found that a new transducer, recently developed at NIST, has sufficient sensitivity for such applications. In particular, the NIST HFHS sensor is found to exhibit sensitivity which approaches the "thermal rattle" in aluminum to within l0 dB over the 250 kHz to 1 MHz region. Also, it is shown that the new transducer's noise floor is well below both the AE detection threshold and the sensitivity limits of both optical and airborne-sound transducers. Furthermore, its performance is in good agreement with the computer model used in its design.

Based upon an analysis of published results and our own calculations and measurements, we find that a contact piezoelectric transducer (NIST HFHS) exhibits the sensitivity required for observing AE events in metals and ceramics. In particular, we have found that our NIST HFHS transducer, a practical, rugged, shielded transducer with internal gain, exhibits sensitivity well below (10-16 dB) the AE detection threshold. Using a Michelson interferometer we have verified the performance of the NIST HFHS transducer and found it to be less than l0 dB above the "thermal rattle" limit of the aluminum substrate.

The current NIST HFHS transducer design appears to be well-suited for wide-band AE studies on high modulus materials such as aluminum, steel and ceramics. Future work will seek to extend the capability of these sensors to lower modulus materials.

Polymeric piezoelectric sensors potentially provide a better acoustic impedance match for polymer composites and wood. This class of sensors was studied as a wideband sensor for polymer composites and wood. The response of such sensors was compared to that from a flat with frequency conical displacement sensor (NIST/SRM). A lead break source was applied to the top surface of a large thin polymer plate with the reference sensor and the polymer sensors both at a distance of 0.254 m from the source. Sensor variables such as PVDF versus copolymer and film thickness were examined. The effect of sensor aperture size was examined as well. Two preamplifier approaches were used. The NIST/SRM preamplifier as well as a physically close field-effect-transistor. Results indicate the need for a mass backed, relatively small diameter sensor to obtain wideband, flat, out-of-plane displacement response. Results also show that even with the impedance mismatch, the conical sensor is a better choice.

1. Copolymer and PVDF (over a more limited frequency range ) mass backed sensors have out-of-plane displacement response on a low modulus polymer plate specimen over a range of 40 - 300 kHz.
2. The response of these sensors is approximately flat with frequency with the best sensor ( 3 x 500 mm copolymer) having essentially equal response sensitivity as a PZT conical sensor on a low modulus specimen.
3. All the polymer sensors show some improvement relative to the PZT conical sensor when they are on the polymer plate rather than the aluminum plate.
4. The low dielectric constants of PVDF and copolymer prevent optimization of sensor geometry to reduce the sensor mechanical stiffness to levels which might significantly increase response.
5. More innovative sensor designs are required that maintain sensor capacitance and decrease sensor stiffness.

A study has been carried out concerning the suitability of polymeric piezoelectric film sensors for wideband acoustic emission (AE) research. The responses of such sensors were compared to that of a flat with frequency conical reference displacement sensor. A lead-break source was applied to the top surface of a large 3.1 mm thick aluminum alloy plate with the reference sensor and the polymer sensors both at a distance of 0.254 m from the source. Several sensor variables were examined: (1) PVDF versus copolymer; (2) film thickness; (3) film area; and (4) in-plane bonded versus out-of-plane mass-backed. The effect of sensor aperture was also examined both experimentally and computationally using a dynamic finite element calculation for an out-of-plane lead break on an aluminum plate. Two preamplifier types were used. The NIST-SRM preamplifier as well as a physically close field-effect-transistor. Results indicate the need for a mass backed, relatively small-diameter sensor to obtain wideband, flat, out-of-plane displacement response.

1. At sufficiently small apertures (a few mm) bonded polymer film sensors exhibit good fidelity but very poor response to in-plane displacements on the 3.1 mm aluminum.
2. Polymer disk sensors with a small aperture and sufficient mass backing can exhibit wideband approximately flat with frequency out-of-plane displacement response up to at least 500 kHz on the 3.l mm aluminum.
3. The best polymeric piezoelectric out-of-plane wideband sensor has a response sensitivity which has a mean level some 6 to 12 dB below that of the NIST-SRM conical sensor.
4. The relatively low dielectric constant of the polymeric films does not allow use of the optimum geometrical design of a small diameter and thick element.
5. A three-high sensor stack did not show the expected improvement in output response. This result is probably due to the complicated nature of the stack design.

A computer model has been used to evaluate the suitability of five piezoelectric materials for use in broadband (10 kHz-2 MHz) acoustic-emission (AE) "point" sensors. The computer model accounts for the effects of the electrical loading of the piezoelectric material by the input impedance of the preamplifier and the mechanical loading by the specimen and backing materials. We compare the model predictions with results of measurements, which are normalized using the NIST Standard Reference Material (SRM) "conical" transducer. The effects of selecting different piezoelectric materials (PZT-5A, Lead Metaniobate, X-cut Quartz, 36"Y-cut LiNbO₃ and PVDF) and transducer configurations ("pinducer" vs. "conical") are evaluated experimentally and analytically. It is shown that materials exhibiting the highest dielectric constant, e₃₃^S, are best suited for high-performance AE sensor applications. In addition, it is concluded that field-effect transistors exhibiting very small noise currents, less than 1. 9 fA/(Hz)^1/2, are required to maximize the low-frequency signal-to-noise performance of PZT-5A sensors.

Piezoelectric materials having high "transmitting" constants, h₃₃, and very high dielectric constants are best for constructing sensitive, broadband AE sensors. PZT-5A belongs to this class of materials. Pinducers were used in this study to evaluate the receiving properties of various piezoelectric materials. However, they are not well suited for use in sensitive, broadband AE sensors, because they exhibit unwanted acoustic resonances, which cause signal distortions. Also, the masses of their brass backing materials are too small to permit sensitive operation below 100 kHz. Conically-shaped elements, heavily backed, are more suitable for use in sensitive, broadband AE sensors. A good example of a rugged AE "conical" sensor is described in detail in Ref. 89.

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