Elsevier

Ultrasonics

Volume 42, Issues 1–9, April 2004, Pages 431-438
Ultrasonics

A conical piezoelectric transducer with integral sensor as a self-calibrating acoustic emission energy source

https://doi.org/10.1016/j.ultras.2003.12.039Get rights and content

Abstract

The experimental results of a conical piezoelectric transducer with integral backing sensor as a self-calibrating simulated acoustic emission (SAE) energy source are presented. It has been shown that there is a negative linear relationship between the energy of SAE signal detected by the backing sensor and the relative strength (signal energy) of SAE source in the structure detected by a reference sensor under different transducer-to-structure coupling efficiencies, with AC drives of the same wave packet, frequency and peak amplitude to excite the conical transducer as a SAE source at all the investigated frequencies over the frequency range of interest in AE measurements (nominally from 50 kHz to around 1 MHz). This should enable the relative strength of the SAE source in a structure to be determined using the SAE measurement from the backing sensor for the selected electrical inputs to the conical transducer, and hence to remove the concerns about inconsistent transducer-to-structure coupling affecting the relative strength of the SAE source for calibration.

Introduction

The conical piezoelectric transducer was originally developed at the National Institute of Standards and Technology (NIST) in the USA for use as a high-fidelity broadband acoustic emission (AE) receiver [1], [2]. Typically the transducer has a flat amplitude response within ±3 dB over a frequency range from 50 kHz to 1 MHz. The transducer consists of a cone-shaped piezoelectric element (approximately 1 mm contact tip diameter and 2.5 mm height) fixed to a small flat area on the end face of a brass cylinder (37 mm diameter and 25 mm height) at the back. The contact tip is small compared to the AE wavelength over the frequency range of interest (nominally 50 kHz to 1 MHz). A device of this type can also be used as a transmitter for generation of simulated acoustic emission (SAE) source for AE calibration in structures. But as with all surface coupled piezoelectric transducers, their outputs are very sensitive to the coupling between the transducer and the structure. This means that the coupling must be very carefully controlled or assessed if reliable SAE calibration sources are to be generated and be considered in AE system calibration, if reliable AE measurements are to be made.

It has been shown that a piezoelectric pin-shaped sensor (2 mm diameter, referred as backing sensor) permanently bonded into the tungsten-epoxy backing of a conical piezoelectric transducer (transmitter for SAE source generation) could be used to assess the coupling variations between the conical piezoelectric transducer and the structure [3]. It was found that the amplitude of the first arrival SAE signal detected by the pin-shaped backing sensor is correlated with the coupling efficiency. However, with this particular design, increasing the coupling efficiency of the conical transducer to the structure has inconsistent effects on the backing SAE signals at excitation frequencies under and above 500 kHz. At an excitation frequency of 140 kHz, there is a negative linear relationship between the backing SAE signal amplitude and the coupling efficiency; contrarily at an excitation frequency of 850 kHz, there is a positive relationship between the backing SAE signal amplitude and the coupling efficiency. It was thought that the variation of backing SAE signal amplitude with coupling is controlled by the effect of coupling on the resonance frequencies of the transducer [3]. The transducer has a resonance at around 500 kHz. The resonance frequency increases as the impedance of the surface to which the transducer is coupled increases, and increasing the coupling efficiency effectively increases the load impedance. The effect of a slight increase in the resonance frequency is to decrease the response of the transducer below 500 kHz and to increase the response of the transducer above 500 kHz. This causes reversal effects on the backing SAE signal amplitude at excitation frequencies under and above 500 kHz [3]. The variation of the sensitivity of the backing sensor response to changes in coupling efficiency with frequency from a negative correlation to a positive correlation through zero means that the device will not work at all frequencies of interest in AE measurements, nominally from 50 kHz to 1 MHz. But importantly, it was found that the acoustic impedance of the structure material is the controlling factor affecting the relationship between the backing SAE signal and the transducer coupling efficiency.

More recently, a large contact surface piezoelectric sensor (the crystal is in a complex shape with an active element diameter in the range of 10–18 mm [4], referred as backing sensor) has been permanently bonded to the brass backing of a conical piezoelectric transducer (transmitter for SAE source generation) for assessing the coupling efficiency between the conical transducer and the structure, with the use of SAE signal energy to facilitate the correction of coupling variations [5]. A negative linear relationship has been found to exist between the SAE signal energy detected by the backing sensor and the transducer coupling efficiency to the structure for AC drives of the same wave packet, frequency and peak amplitude to the transmitter at all the investigated excitation frequencies between 50 kHz and 1 MHz. A self-calibrating conical piezoelectric transducer for coupling efficiency was produced with consistent characteristics over the frequency range of interest in AE measurements. The idea behind the self-calibration is that the SAE energy detected by the backing sensor will be related to the SAE energy that goes into the structure to form an acoustic field, i.e. the SAE source. Under AC drives of the same wave packet, frequency and peak amplitude to the conical piezoelectric element, the more efficient is the transducer-to-structure coupling, the more SAE energy will be transmitted into the structure and the less SAE energy will be detected by the backing sensor. Therefore, measurement of the SAE signal energy from the backing sensor should be an indication of the degree of coupling between the transducer and the structure. The use of a brass backing and a large contact surface piezoelectric sensor as the backing sensor is to facilitate the SAE energy transmission and pick-up for coupling assessment, compared to the highly damping tungsten-epoxy backing and small contact-tip pin-shaped transducer as used in Ref. [3]. The analysis of signal energy of the whole SAE event instead of signal amplitude of the first arrival is to facilitate the signal processing. The best single overall measure of AE signal strength is the signal energy [6]. This paper reports on experimental results of the self-calibrating characteristics of the device under AC drives of variable peak amplitudes to the transmitter over the frequency range of interest in AE measurements.

Section snippets

Construction and performance as a transmitter

The basic construction of the self-calibrating conical piezoelectric transducer presented here is similar to that used in the NIST conical transducer, except that a backing sensor is introduced for assessing the coupling variations. The construction is also a sister version of the self-calibrating transducer as reported in Ref. [3], except that a brass backing as in the original NIST design is employed instead of a tungsten-epoxy backing, and a large contact surface piezoelectric sensor instead

Self-calibrating SAE energy source

The self-calibrating tests were performed on a 6 mm thick stainless steel flat plate of dimensions 300 mm × 200 mm, supported at the corners by four 6 mm thick 50 mm × 50 mm rubber pads. The plate represents a typical geometry of the structures for AE applications and Fig. 3 shows the experimental arrangements. The conical piezoelectric element of the self-calibrating transducer was exited by 10-cycle Hanning windowed tone bursts with peak amplitudes between 50 and 200 V. The backing signal,

Conclusions

Piezoelectric transducers are used as both transmitters and sensors in acoustic emission testing applications. However, the transducer outputs are significantly affected by the coupling between the transducer and the structure. This means that the coupling must be very carefully controlled or assessed if reliable SAE calibration sources are to be generated and be considered in the AE system calibration, if reliable AE measurements are to be made. This paper shows that by integrating a

Acknowledgements

The authors wish to acknowledge the support of EPSRC (Grant GR/M44200) and the industrial collaborators within the Intersect Intelligent Sensing Faraday Partnership Flagship Project (1998–2002) entitled “Acoustic Emission Traceable Sensing and Signature Diagnostics (AESAD)” (website: http://www.brunel.ac.uk/research/bcmm/aesad/index.htm). The authors would also like to acknowledge Professor Peter Cawley, Imperial College for providing the contact source for conical transducer prototypes and

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