Abstract

The method of direction diagram control is present. The magnetic contact fluide use as active element for forming the direction diagram. The control are made by external magnetic field. It give the possibility to change the structure of magnetic fluid volume.

Table of contents

1. Introduction
2. Theoretical analysis
3. Experimental results
4. Conclusions
5. References

1 Introduction

The expansion of the ultrasonic flaw detection information space is connected to development of transducers with controlled directional diagram. Various types of transducers, enabling to generate the predetermined directional diagram are known. In all cases, on a surface of ultrasound input it is necessary to create distribution of acoustic pressure determined by the to predetermined directional diagram. In the last years there were attempts of development of transducers by synthesis under the given directional diagram by analogy from problems, soluble in a microwaves [1]. However, to realise predetermined distribution in practice inconveniently, as on a surface of radiator it is necessary to create sharply changed on amplitude and phase distribution of pressure. The given work ~s devoted to the decision of a question of practical realisation of predetermined distribution of pressure on a radiating surface.

2 Theoretical analysis

The location of magnetic liquid in a backlash between surfaces of a transducer and object of ultrasound input is defined by volumetric force [2]:

(1)

where T_ik= B_iH_k - 1/2 _ikH_kH_k - Maxwellian stress tensor, B- a magnetic induction, _ik - the Kroneker symbol, M - magnetisation of a magnetic liquid, H - intensity of a magnetic field. Thus, the change of a gradient and magnitude of a magnetic field determines change of distribution of a magnetic liquid in the field of acoustic contact. At the same time, ultrasound is transmitted in researched object only through the area, filled by contact magnetic environment. In turn, the directional diagram depends on amplitude-phase distribution of oscillatory speed of particles on a surface. Thus, operating a magnetic field in the area of acoustic contact there is the opportunity to operate by the directional diagram of acoustic radiator. Magnetisation of a magnetic liquid M is determined by the Langeven law [2]. At achievement by a magnetic field H of certain magnitude, saturation of magnetisation comes and the acting volumetric force is determined only by a gradient of magnetic field. Proceeding from it should be executed and account of the forms of volume of magnetic liquids.

The second method of directional diagram control is connected to the phenomenon of magnetostatic instability of magnetic liquids volumes in magnetic fields. The magnetic liquid represents the super paramagnetic system. At effect of a magnetic field, separate particles begin to state along field lines. Thus, between them th; force of dipole-dipole interaction is arise, which aim to "break off volume". The stabilising factor are forces of a surface tension, on magnitude inversely proportional to radius of a surface curvature. At achievement of certain magnitude of a external magnetic field, magnetic forces begin to exceed surface forces and that to save balance reorganisation of volume in the party of a increase of curvature of a surface occurs. Farther, the increase of intensity of a magnetic field leads to modification form of magnetic fluid volume up to saturation. The threshold of magnetic liquid volume stability is defined from a condition of a minimum of its thermodynamic potential, including potential energy in a field of forces of weight, surface energy of free borders and energy of a magnetic field:

(2)

where = (x,y) - displacement of a surface of a magnetic liquid from a initial situation, h - initial thickness of a layer of a liquid, - a factor of a surface tension, S - area of free borders. From expression for force, acting on volume of magnetic liquid to follows, that volume of a magnetic liquid in a backlash will be located in the field of a maximum of intensity of a magnetic field. In absence of limiting walls the maximum of fields coincides a hydrostatic pressure maximum. This area should be used for input of elastic oscillations. However, the free lateral surface of a flat layer in tangent to this surface direction is unstable concerning formation edge

structure. There is a number of critical significance H* and VH* [3,4] determining occurrence of instability. The magnitudes of these significance is defined, basically, magnetisation of a magnetic liquid layer and factor of a surface tension [5].

3. Experimental results

The example of realisation of controlled of directional diagram by offered way is shown on a fig. 1. In submitted case of measurement were made by a transducer with a corner of a prism 7°. As a source of acoustic pressure was used piezoelectric element. The magnetic system was placed in damper and consisted from two permanent magnets in a form of rectangular parallelepiped. The magnetic contact liquid is located in a backlash between surfaces of a acoustically transmitance element (prism) and investigated object.

As a investigat ed object a standard steel sample in a form of half of a cylinder was used. On a fig. 2 the directional diagrams of a transducer concerning the acoustic axis z are shown. At the first case, this is the wide diagram with two maxima directed under corners 8° and 21°. The width of this diagram on a level 0.707 is _0.707 ~ 25°. After turn of magnets on 45° diagrams has one maximum with width _0.707 ~ 12° and relatively high level of side lobes ~ 0.45. At further rotation of magnets, under the corner 90°, the main maximum narrows to width _0.707 ~ 9°. Besides, the direction of a maximum is moved from 19° to 15°.

Fig. 1. The design of transducer for control of direction diagram. x-acoustic axis; I-piezoelement; 2-magnetic fluid; 3-acoustic prism; 4-magnets; 5-sample; 6-demper; 7-frame. <= Fig.2 The directional diagram of ultrasonic transducer with 7° acoustical prism for different direction of acoustical axis x relatively of areas with magnetic fluid 1.

4. Conclusion

Thus, the magnetic liquids present a convenient tool for development of new types of acoustic transducers with the controlled directional diagrams for the purpose of non-destructive testing. In particular, the described system was intended for automation of the ultrasonic control of axes of carload wheel pairs with a transducer placed on an end. Such transducers can be used in the installation for automated testing, under the distance testing, inspection in hard-to reach places and so on.

References

1. Karpelson A.E. Ultrasonic transducer, forming predetermined directional diagram //Sov. Journal Defectoscopy. 1989. No2. P.50-57.
2. Shliomis M.l. Magnetic fluids//Syv. Journal UFN. 1974, 112, 3, 426-458.
3. Baev A.R., Mayorov A.L. Experimental investigation of magnetostatic instability of magnetic fluid, In a book: "Structural propitious and hydrodynamics of magnetic colloids", pp 64-67, Sverdlovsk 1986.
4. Baev A.R., Mayorov A.L. Confinement of magnetic fluid between ultrasonic transducer and ferromagnetic sample. In a book: "Magnetic fluid. Sciences and applied investigations': pp 62-70. Minsk, 1983.
5. Author's sertificate No1647378 (USSR),

Authors

International Conference "Computer Methods and Inverse Problems in
Nondestructive Testing and Diagnostics", 21-24 November 1995, Minsk, Belarus
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