·Table of Contents
·Methods and Instrumentation
Electromagnetic Acoustic Transducers (EMATs) Design Evaluation of their PerformancesS. ALIOUANE, M. .HASSAM, A. BADIDI BOUDA & A. BENCHAALA
Centre scientifique et technique en Soudage et Contrôle B.P.64, Route de Dely Ibrahim, Chéraga, Algiers ALGERIA
To improve the flexibility and the rapidity of this method, it would be
useful to generate and receive acoustical waves with a contactless
electromagnetic probe. The contactless electromagnetic-acoustic transducers
(EMATs) offer several advantages, which have an essential importance for the
non-destructive testing with ultrasound
This work consists of EMATs conception and construction for the contactless excitation and detection of different wavetypes. We have conceived and realised receivers and transmitters EMATs.
This paper is a report of experiments results with these different types of EMATs. We examine the different parameters that influence them in order to optimise them and to know the degree of the applicability of this technique in the non-destructive testing of materials in Algeria.
keywords: EMATs, Electrodynamics effect,Magnetostriction, phased array transducer.
|Fig 1: Principle of electromagnetic acoustic generation|
where n0 is the electrons density. For B0 oriented according Ox the Lorentz force is longitudinal, resulting in a variation of the electron charge density along Oz. An internal electrical field E(z) oriented following Oz has to exist to maintain charge neutrality. Its ions come in an compressional oscillations of amplitude x z. B0 is to Oz, the currents j(z, t) are to Oy direction and the Lorentz force act in the Ox direction. The Lorentz Forces act to produce a shearing force along Ox arise shear oscillations of amplitude x x . Thus for these two polarisations , the acoustic wave equation is :
d is the density of the metal and s the speed of ultrasonic wave
Dobbs  demonstrates that when d <<l and at distances z >> d we have the following wave solution: ,where b = q2d 2/2 and q is the ultrasonic wave number
The conversion coefficient h of the electromagnetic acoustic transducer is the ratio of the generated acoustic power P to the electromagnetic power Q entering the surface. It is done by the following relation, .
At 10MHz, for compressional waves in aluminium, B0= 1T we have
h = 5.7x10-5 .
In non- ferromagnetic materials the Lorentz force is the one contribution to the generation of sound. In a ferromagnetic material, additional forces are produced by magnetostrictive stresses. Ultrasonic wave is no more a linear function of the applied magnetic field. According to these principles an EMAT then consists of a means for producing a static bias field, i.e., a permanent magnet or electromagnet, plus a coil of wires carrying a dynamic drive current.
|Fig 2: EMAT prototype||Fig 3: EMAT and Piezoelectric Transducer|
Exciting and receiving coils are realised on printed circuits. The electromagnet used have U-form for longitudinal waves EMATs , and E-form For shear waves.
Influence of the excitation coil geometry
We want to determine the coil geometry influences on the efficiency for the emission and its sensitivity to the detection. For this purpose, several set of coil of different designs are tested: single flat spirals, double spirals and meander coil. Four sets of flat spirals was fabricated as shown in figure 4
|Fig 4: spiral coil as printed circuit||Fig 5: meander, double and single coil for 21mm gap|
We have therefore conceived a set of different characteristic coil (number of turn, distance between turns). Coils were printed on the same rectangles surface 35 X 11 mm2 (Fig.4). We were limited by 9 turns. Small width of the gap plays an important role in generation phenomenon. Indeed, the developed static field is as important as the gap is smaller. Thereby, both influence of these two parameters namely: the turn number and the gap width, present a contradictory character. Figure 5 shows an other set of coils: double spiral, single and meander coil. These coils are destined for operate in a 21mm-gap while precedents operated in a 14 mm-gap. Table 1 gives the best results obtained by each category of coil.
|Coil type||6 turn-single Spiral||2 x 3 turn-double spiral||8 period-meander||9 turn-single Spiral||10 turn-single Spiral|
|Width of gap (mm)||14||21||21||14||21|
|Table 1: coil geometry influence on the echo high|
|Fig 6: magnetic induction influence in ferromagnetic steel||Fig 7: magnetic induction influence in aluminium|
Influence of the lift off
Influence of the lift-off (coil-material surface) is investigated on the shear and longitudinal echoes magnitudes. For both polarisation, the height of transmitting and receiving echoes are inversely proportional to the lift-off value. We obtained exponential tendency curves (figure 8, 9) with a determination coefficient R2=0.995.
|Fig 8: lift off influence in aluminium|
|Fig 9: lift off influence in stainless steel|
Static magnetic field
We established the table 2 where are data optimal intensity values.
|Material||mode||Optimal Induction (Ampère)|
|Table 2 : optimal induction|
Curves (Fig.6, and 7) representing the evolution of the amplitude according to the magnetic induction indicates us that for non ferromagnetic materials, the efficiency of the transduction is linear with the static magnetisation. So, It is imperative to dope electromagnet to the maximum by providing a strong electrical current in the other hand, it is useless to increase the current for ferromagnetic materials especially steel since the response reaches the saturation for the longitudinal mode at relatively low intensity and around the same intensity for the shear mode.
The 6 turn-spiral gave the best result in transmission, nevertheless, it is not only coil form that causes good efficiency recorded. Indeed, the gap in where coil is placed also is a parameter that affects significantly the dispersion of magnetic field lines. The six-turn-coil is printed on a 14 mm-gap while the meander occupies 21mm width. The dispersion of the magnetic field lines increases with the gap width and then diluting the static magnetic power provided by the electromagnet current.
The meander is suitable for generating angled shear waves. One must bear in mind that such coil possesses essential characteristics of a phased array with the limitation that the phases of adjacent array elements are a constant 180° ,. We will see a study that describes the behaviour of a such coil and the way whose one optimise it by acting on the spatial period of the coil. The table 1 shows the best results obtained for each category of coil. We have noted gain, rejection of the ultrasonic unit and the gap width for a 100 % dynamics in the ferromagnetic steel. We observe that in the case of the transmission, the lowest gain and the lowest rate of rejection have been recorded for the single six turn-spiral printed in a 14 mm gap. For the reception, 9 turn-spiral was the most efficient.
|Fig 10: relative efficiency (in dB) of electromagnetic generation of shear waves in variety of metals as function of frequency.|
Nature of the material and excitation frequency
We was not able to make an experimental study on the influence of the exciting frequency. The theoretical study shows the great influence of this parameter on the amplitude of the generated signal. Curves that illustrate the behaviour of the transduction according to the frequency are traced for several materials . Materials are matched to their mechanical and electrical characteristics namely: sound velocity (s) and the skin depth parameter (b ).
We suppose therefore that propagating mediums are electrically and mechanically isotropic. Curves give the amplitude of generated stress as the frequency . According to relation (4), the relative efficiency of the pulse-echo method for generating shear waves in various metals room temperature as a function of frequency is shown in figure10 ,where it is assumed that B and B0 are constant and any attenuation in the metal is negligible. The efficiency in aluminium at 2 MHz is taken as the reference point, positive dB values indicating a higher efficiency (in magnesium, f) and negative dB values a lower efficiency (a-d). it is apparent that the electromagnetic transducer is most efficient in light metals (Al, Mg) and its efficiency is strongly frequency dependent in metals with small acoustic velocities and so large values of b (Sn, a). The calculation for carbon steel with µ=100 (c) is only a crude estimate, since it ignores any ferromagnetic interactions in the generation process.
The excitation coil design and the electromagnet configuration determine modes of the transduction. Being the seat of the excitation current pulse this coil and this magnet can be arranged in many configurations to produce a variety of wave types such as longitudinal and shear bulk waves excited normal to a sample surface, angled shear waves and guided modes (including the difficult to excite horizontal polarisation) . Identification of these waves is made by the measurement of the propagation velocity in a known sample. Longitudinal waves are naturally generated by the spiral coil and parallel configuration of the static magnetic field. We therefore, produce longitudinal waves in ferromagnetic, stainless steel and in the aluminium. the greatest efficiency is recorded for the ferromagnetic steel with a peak corresponding to an current of 0.05 A in the electromagnet. This is caused by the magnetostriction phenomenon that disappears if the material is saturated. In the aluminium, there is a perfect linearity between amplitude and magnetisation. Cylindrical yoke with E-form was made to generate shear waves. Nevertheless, we know that the flat spiral combined to U-form yoke also generates shear and longitudinal modes. Experiment shows that only E-form yoke generates a pure shear mode, while the U-form yoke produces a quasi-compressional waves. With a double spiral coil (figure 5, middle) and meander coil combined to a U-form magnetic yoke, surface waves are generated. The experiments consists to transmitting with an EMAT on a well polished surface and receiving generated surface waves with a conventional transducer with mechanically adjustable angle, beforehand adjusted (at pulse-echo mode) to the second critical angle. The surface waves echoes were important as compared to bulk waves echoes obtained previously. This was observed only on the ferromagnetic steel. Same experiment on the aluminium gave no result. We have concluded that generated stress are from magnetostriction. This hypothesis has been confirmed when a great sensitivity of the Rayleigh wave echo to the static magnetic field was observed.
Phased array EMATs.
Meander coil transducer possesses essential characteristics of a phased array with the limitation that the phases of adjacent array elements are a constant 180°. This explains his angled radiation . Its directional pattern can be compiled from the diagram of the array element and the directivity coefficient of the group by the following relation :
|Fig 11: linear phased array for orientation and focalisation controlling|
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