Abstract
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.
Highlights
In ultrasonic testing (UT) and acoustic emission (AE) testing, transducers and sensors play key roles in detecting weak signals that arrive with various noise
The standard deviation values are less than 2 dB. These results indicate that the two calibration methods yield identical receiving sensitivities
Receiving displacement sensitivities obtained by two calibration methods, sinewave excitation and impulse excitation methods, are compared and a good agreement is found
Summary
Detected signals are affected by these detectors, having their characteristics is important, especially for system modeling of test procedures [1]. Recent studies using pulse excitation and laser interferometry have characterized the transmission and receiving sensitivities of UT transducers. Sensors 2018, 18, 3861 and AE sensors [3,4] This approach is a variation of ISO16063-11 [2] by identifying the transmission behavior of UT transducers and using it for receiving sensitivity (Rx) determination. The studies [3,4] resulted in the frequency dependence of Rx of more than a dozen UT/AE detectors. An AE sensor in Group D, aimed toward research and specialized applications, has backing material behind the sensing element, represented here by V103
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