Holographic visualization of flaws using magnetic stressing

Holographic visualization of flaws using magnetic stressing P.C. MEHTA, D. MOHAN, C. BHAN, P. LAL, R. H R A D A Y N A T H A method of magnetic stressing is described. The results of holographic non-destructive testing using magnetic stressing are also reported. KEYWORDS: non-destructive testing, holographic interferometry, magnetic stressing Introduction

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Holographic interferometry has been widely accepted as a viable tool for nondestructive testing of materials. It permits the qualitative and quantitative study of minute changes in the object contours by comparing each point on its surface with itself before and after a change takes place. The success of holographic nondestructive testing of a material, however, depends upon the stressing technique adopted. The stressing should deform the body under test in such a manner that the 'good' areas are distinguished from the 'bad' areas simply by studying the interference generated on the holographic interferogram. The stressing techniques used are normally mechanical, thermal, pressure and vibrational ~. In the present communication we have applied holographic interferometry for observing the effect of a magnetic field on metals. It is shown that magnetic stressing can also be adopted for holographic nondestructive testing of certain materials.

Experimental procedure All the holograms were recorded by the conventional two beam off-axis technique. As the available magnet/electromagnet produced only a weak field, the real-time method revealed that the fringe controlled device was not necessary in the recording set-up.

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Sample location and electromagnet

study were, therefore, not clamped directly to the electromagnet but were fixed in a vice on the table, in isolation from the electromagnet. The effect of the magnetic field is to induce net magnetism in the sample wherein each molecule may be supposed to

The effect of a magnetic field on aluminium and iron bars (150 mm x 50 mm x 10 mm) was studied. Flaw detection was carried out on two magnetic bases (Indian make, holding force 15 kg) having an on/off lever. The holograms were recorded on Agfa Ho~o test 10E75 plates using a Jodon HN-7 HeNe laser at 6328 A, and developed in Kodak D-19 developer for 5 minutes. The holograms were bleached in a ferric chloride bath. To study the effect of the magnetic field, the samples were positioned between the pole-pieces of an electromagnet (holding force 25 kg) as shown in Fig. 1. First a doubleexposure hologram of the electromagnet without the sample was recorded to check its stability. During the 2_rst exposure the current did not flow in the coil (magnet off), but current was passed (magnet on) during the second exposure. The reconstructed hologram revealed a number of fringes superimposed on the magnet (Fig. 2) showing that various components of the magnet had moved due to the action of the magnetic field. The samples under The authors are at the Instruments Research and Development Establishment, DEHRA DUN-248008, India. Received 2 February 1982.

Fig. 2

Reconstructed image of the electromagnet without sample

0030-3992/82/050269-03/$03.00 © 1982 Butterworth & Co (Publishers) Ltd OPTICS AND LASER TECHNOLOGY. OCTOBER 1982

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undergo a process of alignment. The net effect of the magnet on the body is the aggregate of the attractive forces acting on the different particles of the body. All magnetic substances possess some power of magnetic induction and in turn are affected by mechanical forces under the action of a magnetic field. This, however, depends upon the material of the body. Magnetic induction is generally extremely feeble except in iron, steel, nickel and cobalt. The holographic results recorded are in conformity with this. Fig. 3 shows the reconstructed images of the iron and aluminium bars obtained by double-exposure holograms. The fringes in the case of iron are much more closely spaced than those for aluminium showing that the effect of the magnetic field on iron is considerably greater. In order to use a magnetic field as a stressing technique for holographic nondestructive testing, we selected a magnetic base consisting of an iron box containing a permanent magnet. The body of the iron enclosure contained a brass strip. A double-exposure hologram of the magnetic base with the magnet in 'off' and 'on' positions during the two

Fig. 4 Reconstructed image of a magnetic base showing the presence of a strip of 'foreign' material

Fig. 5 Reconstructed image of a magnetic base showing the presence of flaws

Fig, 3 Reconstructed images showing the effect of the magnetic field: a -- on the iron bar; b -- on the aluminium bar

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exposures was recorded. The reconstructed image (Fig. 4) shows the presence of the brass strip in the iron box at the site where the fringes curve abruptly. On another magnetic base the magnetic stressing revealed casting faults (Fig. 5) which may be due to the presence of an 'air bubble' or any other foreign material in the iron enclosure of the magnetic base. The same magnetic base was examined by mechanical and thermal stressing methods. Mechanical stressing did not reveal the flaw even using the fringe control technique.

OPTICS A N D LASER T E C H N O L O G Y . OCTOBER 1982

Thermal stressing, however, gave results in agreement with magnetic stressing.

Conclusion Some results of holographic visualization of flaws have been reported using a magnetic field as the stressing technique. The experimental results illustrate the use of holography to detect faults in ferromagnetic crystals of large dimensions.

OPTICS A N D L A S E R T E C H N O L O G Y . O C T O B E R 1982

Holography can also be exploited to detect transient magnetic phenomena in ferrite rods such as the passage of acoustic disturbances leading to variation in magnetic absorption 2 .

References 1 2

See for example, HolographicNondestructive Testing, Ed. R.K. Erf, AcademicPress (1974) White,R.L. JApplPhys 31 (1960) 86 S

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