Table of contents
where Tik= BiHk - 1/2 ikHkHk - 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 . 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:
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 .
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.
International Conference "Computer Methods and Inverse Problems in
Nondestructive Testing and Diagnostics", 21-24 November 1995, Minsk, Belarus
Contact for the proceedings: DGZfP Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Motardstraße 54 , 13629 Berlin, Germany
Telephone: + 49 (030) 386 29 911, Fax:.. /29 918,
|| UTonline | |Top ||