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An Experimental-numerical Investigation of the Face-to-face Sensor Characterization Technique

The face-to-face acoustic emission sensor response characterization technique has been widely used as an alternative to other calibration procedures (for example, absolute calibration, secondary calibration, reciprocity, and so on) because of its simplistic procedure. The results of the acoustic emission sensor response characterization are reported on a dB scale (referenced to 1 V/μbar) as a function of frequency. This type of result has migrated to several ASTM International documents; for example, the requirement for the acoustic emission sensor sensitivity in ASTM E 1419 states, “Sensitivity shall be greater than −77 dBV (referred to 1 V/μbar, determined by face-to-face ultrasonic examination) within the frequency range of intended use.” This work investigates the output of the driving transducer used in the face-to-face characterization procedure via the means of a transfer block experiment. Experimental measurements were made of the absolute surface displacement of a transfer block caused by a driving transducer as a function of frequency. The experimental results, coupled with validated multi-physics transient dynamic finite element simulations of the propagating stress waves, show that the driving transducer’s output (pressure) has a strong dependence on frequency. Furthermore, it is shown that the frequency dependence changes when a driving transducer with an altered center frequency is used. Thus, providing a sensor characterization result versus frequency on a dB scale referenced to 1 V/μbar is shown to be arbitrary. If the true transient pressure output (as a function of frequency) from the driving transducer when coupled to a solid material could be measured, then a more relevant characterization might be obtained. To overcome this inconsistency, and provide a means of producing equivalent characterization curves among all acoustic emission sensors, a possible normalization approach is considered.

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