Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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A NEW AND VERSATILE SANDWICH TYPE ULTRASONIC TRANSDUCER FOR NDT OF DIFFERENT MATERIALS

C. Miclea [Corresponding author: Cornel Miclea, e-mail: cmic@alpha1.infim.ro], V. Tanasoiu, C. F. Miclea
National Institute for Materials Physics, Atomistilor 105 bis, 76900 Magurele, Bucharest, ROMANIA
A. Gheorghiu, S. Moroianu
Hyperion University, Faculty of Physics, Calea Calarasilor 169, sector 3, Bucharest, ROMANIA

ABSTRACT:

Nondestructive testing of materials and structures with ultrasonic has now become a classical test method for many important influencing factors. The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves and they are reflected both by interfaces and by the internal flaws of the object. The ultrasonic waves must be used in a proper frequency range for each material. The essential tool for ultrasonic detection is the ultrasonic transducer, which simply consists of a piezoelectric element housed in a special case, excited by a very short electrical signal and transmitting an ultrasonic pulse. The same piezoelectric element can in turn generate an electrical signal when it receives an ultrasonic signal. The efficiency of NDT methods depend on the ultrasonic transducer which, in its turn, depend on the piezoelectric element used. The piezoceramic elements for high frequency transducers are easier to make since they consist of a simple disc with thickness lower than 2 mm. But for lower frequencies some difficulties arises concerning the dimensions of the piezoceramic elements which become extremely large and implies high voltage signals. To avoid these difficulties we designed and constructed a rather simple piezoceramic active element of sandwich type, consisting of a given number of discs, glued together with polarization in opposition and electrically connected in parallel. This construction allows the transducer to be made at any desired working frequency simply by choosing the corresponding number of discs with a certain diameter, in order to adequately control the directivity of the ultrasonic beam by increasing the irradiating area. In our case the sandwich piezoactive element was made of five thinner discs of 20 mm diameter and 2 mm thickness, so that the whole construction has the resonant frequency at 60 kHz Transducers made with such sandwich like elements at different low frequencies as well as their performances for NDT are presented and discussed in the work. They can be successfully used for NDT, either in the trough transmission or pulse-echo arrangements.

1. INTRODUCTION

Nondestructive Testing (NDT), can be considered as the science able to identify some physical and mechanical properties or defects in a piece of material or structure, without altering their end use capabilities. For some materials, especially for heterogeneous materials, such as concrete, the NDT can become a difficult work, especially due to the overlap of multiple reflected waves. Therefore in such cases NDT methods encounter a number of difficulties due to the nature of material and can thus be applied only in a limited number of cases and usually when macro-cracking or other damages are involved. Different materials and structures need a proper frequency of the transducer used. Thus, for example, metallic materials require frequencies above 1 MHz while other such as concrete, wood, plastics need lower frequencies. Nowadays the most NDT of materials and structures use piezoelectric transducers which may cover a wide range of frequencies. The essential tool for NDT with piezoelectric transducers is the piezoelectric element, mounted in a special case.

This piezoelectric element is excited by an extremely short electrical signal and consequently it transmits an ultrasonic pulls. In its turns the same piezoelement generates an electric signal when it receives an ultrasonic signal due to the reversibility of piezoelectric effect. The returning signal from the tested object bears all the information about the structure, properties or damages of the material so that it must be correctly "decrypted" in order to gain a complete image of it. In case of high frequency transducers the piezoactive elements are easier to make. But for lower frequencies some difficulties arises concerning the dimensions of the piezoelectric elements which become extremely large and implies high voltage signals. In order to avoid these difficulties we constructed a rather simple piezoceramic active element of sandwich type.

This work presents the construction and performances of such a transducer made with sandwich like piezoceramic element and working at low frequency.

2. EXPERIMENTAL

Ultrasonic technique provides quick and reliable results in NDT examination when ultrasonic properties of materials are measured. One of the most important tools for such investigation is the piezoelectric transducer, which can work either as source or detector of ultrasounds. It can play equally both functions due to the reversibility of the piezoelectric effect and to the independence of the transmission and reflection constants with respect to the direction of the working discontinuities [1-11].

Efficiency for emitters and sensivity for receivers are the fundamental characteristics of ultrasound transducers and these qualities need to be maximized in order to get high efficiency transducers. Both are dependent on electromechanical coupling factor and some other dielectric, piezoelectric and elastic constants of the piezoelectric materials [12-14]. Mechanical quality factor Qm determines the efficiency and bandwidth. Thus, for example, in pulse echo transducers the use of short ultrasonic pulses requires a very low Qm for a compact impulse response. A low Qm also ensures a good transfer of acoustic energy into the load, which implies the matching of acoustic impedance of transducer and the load [15, 16].

2.1. Design and construction of transducer
The most simple piezoelectric transducer consists of a disc (or cylinder) shaped piezoceramic element operating in one direction axis of polarization normal to the parallel faces.

This piezoceramic element is, in fact, the heart of any real transducer, since it produces the high frequency ultrasonic vibration in response to a short electrical pulse and on the other hand it receives the reflected high frequencies sound signals and transforms them into electrical signals. The vibrations of the end faces of such a disc resemble those of a piston like motions with adequate directivity and good separation of harmonics if the disc diameter is sufficiently large, compared to the thickness. On the other hand if lower frequencies are required then the thickness must be large enough so that the disc becomes inconveniently large.

Since the requirement for different materials and structures investigation is a rather low frequency [17-19], because of the high wave attenuation, the dimensions of the piezoceramic element would become so big that it will imply a high power generator to work at high voltages. In order to overcome these difficulties a "sandwich" type element can be proposed as piezoactive element. Such an element, made of a number of thinner discs, glued together and having the polarization directions in opposition is then tightly glued on a steel cylinder playing the role of backing material.

This composite structure made of a sandwich type piezoactive element and a steel cylinder shown in figure 1 has two main advantages. One is that the electrical connection being disposed in derivation, each disc receives the whole emf of the driver, which is much smaller compared to the case of a single thick element and thus makes it possible a considerable simplification of the electronics.

Fig 1: The composite structure of transducer consisting of sandwich type piezoactive elements glued on a steel cylinder.

The second and most important advantage is that one can chose the desired working frequency for the transducer simply by choosing the right number of thin piezoactive elements, since the resonance frequency for such a composite structure is a function of the number of thin piezoactive elements involved, as can be seen from figure 2, which illustrates the dependence of the resonance frequency of the number of thin disc of the sandwich structure. The experimental construction on which the measurements were carried out was made from discs of 20 mm diameter and 2 mm thickness, glued together one by one with the direction of polarization in opposition in a sandwich type structure.

They were next glued on a steel cylinder with a diameter of 20 mm and a thickness of 20 mm. The resonance frequency of the system was measured after each element was stuck. One can see that the resonance frequency decreases with increasing number of elements, following a curve as that shown in figure 2.

Fig 2: The resonator frequency as a function of the number of thin piezoactive elements of the sandwich. The points show the experimental values of the sandwich active elements and the line illustrate the theoretical polynomial fit.

This curve can be described with a good approximation, by a fifth order polynomial of the form:

(1)

(2)

For the construction of the transducer we chose a structure with five piezoceramic active elements corresponding to a resonance frequency of 60 kHz. This proved to be a proper frequency for materials investigations [20]. The schematic view of the transducer is shown in figure 3.

Fig 3: Schematic view of the transducer illustrating the main parts of it.

The transducer produces longitudinal waves, which propagate into the outer space. Its vibration mode is the fundamental longitudinal vibration, proper to a cylinder. The ultraacoustic wave packets produced and emitted through the frontal part, via the protection disc, propagate into the specimen to be investigated and they are received by a similar transducer, attenuated and distorted due to the defects of materials. The changes in the pattern of the emitted waves represent the encrypted structure of materials investigated.

2.2. Characteristics of transducer
The electrical characteristics of this transducer were determined by means of an impedance gain/phase analyzer type HP4194A. The electrical spectra are shown in figure 4.

Fig 4: The characteristics electrical spectra of the transducer showing the phase and impedance versus frequency.

The resonance frequency is situated around 60 kHz with a minimum impedance of 22 ohms and the antiresonance frequency at 71 kHz with a maximum impedance of 28 kohms. Using these values one can simply calculate the quality factor Q by means of the formula:

(3)

where fw - is the working frequency which correspond to the resonance frequency fr and Df = fa - fr is the bandwidth of the oscillation spectrum. The estimated value of Q was 5.5. There is another way to calculate Q by means of the formula:

(4)

where X is the reactance and R the resistance of the electrical branch of the equivalent circuit of the transducer. The reactance is given by:

(5)

where L and Cb are the inductance and capacitance of the electrical branch of the equivalent circuit. The equivalent circuit with its characteristics, as determined by the impedance analyzer gave the following values: R=30.4 W, L= 9.94 mH and Cb = 1.5nF so that the estimated value of Q was 5.44 in good agreement with the value estimated with (3).

As concerns the acoustical characteristics, they were determined by using a pair of transducers, one emitter and one receiver, and measuring the ultrasonic output by means of the gain phase analyzer. The transmitted ultrasonic wave within the working range is shown in figure 5. The shape of the acoustical spectrum is that of a product of two lorentzians, each corresponding to a single mode.

The data from this figure allow calculating the relative bandwidth at 1.5 dB under the resonance peak and the corresponding Q number with the formula:

(6)

Fig 5: The characteristic acoustical spectrum of a pair of transducer working in the trough-transmission arrangement.

Using the experimental values for w and D w 1.5dB we obtained for Q a value of 50 which seems a reasonable value for a pair of transducers working in tandem conditions.

3. SUMMARY

A sandwich type piezoelectric transducer working at 60 kHz was designed and constructed for different materials inspections and NDT investigations. The transducer is a composite structure made of five piezoceramic discs of 20 mm diameter and 2 mm thickness glued together, with polarization in opposite directions and then glued on a steel cylinder which plays the role of backing material. The transducer can work either in the through transmission or pulse-echo arrangement.

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