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Development of Acoustic Emission Sensors with Required Characteristics A.A.Bazhenov, V.I.Yarovikov Russian Federal Nuclear Center - All-Russian Research Institute of Experimental Physics, Sarov, Russia Contact

There is a great variety of acoustic emission transducer (AE transducer) produced by different companies (only Physical Acoustics Corporation has more than 40 different models [1]), nevertheless implementation of the AE technique is constantly expanded. The emergence of new tasks generates a need for the development of new AE transducers most adequate for a specific task. High requirements are imposed on technical characteristics of primary acoustic-emission transducers which determine the resolution (electroacoustic transformation coefficient width of the operating frequency range, variations in frequency response, etc). High-sensitive sensors operating in a frequency range 20-200 kHz are necessary for control over extended objects, e.g. main multiple gas pipelines at sites where they crass rivers, highways, etc. The table presents technical and design characteristics of the developed contact piezoelectric vibroacoustic and acoustic emission transducers [2-5]. Vibroacoustic transducers AII205 and AII206, taken as the basic models of acoustic emission control systems for extended objects have been considered in the work. These transducers offer high coefficient of electroacoustic transformation in the operating frequency range between 20 and 200 kHz. Transducers are made around piezoelements in the form of elongated cylinder, comparable with the received acoustic wave length, they operate in a resonance mode. It is worth noting that when choosing the sensor designs, the authors had to abandon the dampers and selected resonance broad bandwidth transducers. This choice allowed, on the one hand to obtain high transformation coefficient (» 55 dB relative to 1 V/m/s), which is the most important characteristic of diagnostic systems, and, on the other hand, to provide simplicity, technological efficiency and minimal number of joints between different materials, that contribute to attaining needed characteristics of sensors (flatness of the frequency response, reliability, etc). Fig. 1 shows a typical pulse characteristic of the vibroacoustic transducer AII206 on exposure to an acoustic signal, occurring as a rod of the collet pencil is broken (a), and its spectrum calculated by discrete Fourier transform (b).

Fig 1: Typical pulse characteristic of transducer AII206 (a) and its spectrum (b)

Fig. 2 shows frequency responses of transducers AII205 and for comparison those of a widely used in practice analogue R6 (Physical Acoustics Corp., USA) obtained on metrological facility IIfAII-PM (Dal'standard, Khabarovsk, Russia) acted upon by Releight waves (test unit material is steel 30X13).

Fig 2: Typical frequency response of transducers AII205 and R6 on exposure to Releight waves

Fig. 3 presents frequency responses of the same transducers on exposure to longitudinal waves obtained on metrological facility IIfAII-PM (Dal'standard, Khabarovsk, Russia) with a specific acoustic impedance at the point of the transducer mounting z = 31.10⁶ kg/m^2.s.

Fig 3: Typical frequency response of transducers AII205 and R6 on exposure to longitudinal waves

Designing of vibroacoustic transducers, operating in a resonance mode, in low ultrasound area, involves some features related to large longitudinal and transverse size of a sensing element and variations in frequency response.

Transducers	Transducers type
characteristics	AII205	AII206	AII202	AII203	AII204
Type of the received wave	volumetric (longitudinal) and surface waves with a comb periodic structure (Lambar, Releight waves)
Type of a sensitive element	elongated cylinder		short cylinder	ring	disk
Operating frequency range, kHz	20 - 100	50 - 200	100 - 500	100 - 900	300 - 1000
Average electroacoustic transformation coefficient, dB ref to V/m/s	55	55	54	53	50
Operating temperature range, °C	-60...+100
Electric capacitance, pF	200	250	400	500	400
Average resonance frequency, kHz	60	90	180	-
Variations in frequency response, dB	±4		+10 -15	±4
Method of the electric signal output	coaxial
Type of the used piezoceramics	PZT-19
Type of the protector material	high-strength ceramics (Chilymin)
Type of the body material	stainless steel		titanium alloy
Cable lead out of the body	from the side
Method of the body sealing	epoxy adhesive
Attachment to an object	By glue or using contact liquid with simultaneous compression against the object under control
Overall dimensions, mm	Æ36´32	Æ32´28	Æ16´15	Æ22´15	Æ16´15
Table 1:

The problem of these transducers development was solved by mathematics simulation and optimization of design parameters of piezoceramic sensitive elements. Available approximate calculation techniques for such transducers do not enable to have the problem completely solved. Using known methods of piezoelements mated fields [6-9], in the context of deformed media mechanics, a new technique has been developed based on the calculation of the piezoelement electroelasticity at volumetrically stressed sensitive elements (SE), with regard to volumetric waves reflection and passage through the transducer design elements [2-4]. This technique offers some significant distinctions such as consideration of all components of the piezoelement strain tensor, including those which under specific boundary conditions may be zero, as well as considering input circuit parameters of amplifying-transforming equipment.
The proposed technique offers versatility physical evidence, it provides convenient computational algorithm and optimization based upon the requirement of a specific task (obtaining predetermined electroacoustic transformation coefficient, shape and variations of the frequency response).
This technique is used to calculate sensitive elements of any complex shape (disks, rings, multilayer plates, sectional rods, etc.).
As optimization techniques of AE transducers parameters, may serve synthesis of a required frequency response from different modes of the piezoelement vibration (in longitudinal and lateral directions), suppression or isolation of certain oscillation modes, etc. To illustrate the computational technique Fig. 4 shows calculated and experimental frequency responses of the acoustic emission transducer AII203 acted, upon by longitudinal waves.

Fig 4: Calculated and experimental characteristics of acoustic emission transducer AII203 acted upon by longitudinal waves, and its external view

References

1. Acoustic Emission Sensors. Product Bulletin.- Physical Acoustic Corporation, 1997.
2. A.A.Bazhenov, V.I.Yarovikov. On the calculation of piezoelectric ultrasonic receiver based on the electroelasticity equations at volumetric stressed state// Measuring technique, 7, pp. 55-56, 1998.
3. V.Yarovikov, A.Bazhenov. Ultrasonic Piezoelectric Transducers for Leak Detection Systems of Nuclear Power Plant Equipment// Proceedings "7^th European Conference on Non-Destructive Testing ", Copenhagen, 26 - 29 May 1998, Vol. 2, pp. 1556 - 1563.
4. A.A.Bazhenov, V.V.Smirnov, V.I.Yarovikov. Some problems of designing wide band acoustic emission transducers. Proceedings of the International scientific and practical conference «Piezoequipment - 95». Fundamental problems of piezoelectronics. V.3, pp. 98-103, Rostov - on -Don: NII of Physics, 1995.
5. V.I.Yarovikov, A.A.Bazhenov. Piezoelement for receiving band transducers. Application for an invention of the RF 97116557 of Oct. 7, 1997, published in BI 20, 1999 (Positive solution for the RF patent for an invention, dated Dec. 28, 1999.
6. V.M.Baranov. Acoustic measurements in nuclear power engineering. - M.: Energoatomizdat, 1990.
7. V.I.Domarkas, R.I.Kazhis. Control and measuring piezoelectric transducers. - Vilnius: Mintis, 1974.
8. A.I.Trofimov. Ultrasonic systems of control over distortions in fuel in channels of nuclear reactors. - M.: Energoatomizdat, 1994.
9. V.Z.Parton, B.A.Kudryavtsev. Electromagnetoelasticity of piezoceramics and electroconductive bodies. - M.: Science, 1988.

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