Three - University of Arkansas Physics Department

The Portable Holographic Interferometer for residual stress
measurement and nondestructive testing (NDT) of the pipelines
Ywiy Onishchenko*, Anatoli Kniazkov, Jon Shulz, Gregory J. Salamo
University of Arkansas, 226 Physics Bld., Fayetteville, AR 72703, USA
ABSTRACT
Introducing a small scratch (10-25p depth) on the surface of a part containing residual stresses produces a small
change in displacements around of the scratch on the surface. When the displacements are measured as a function of
the depth of a scratch, a very small depth releases displacements of about U1O.The present paper shows that
introduction of an additional faze shift permits determination of very small displacements and also presents the
portable interferometer and the technique for measurement of residual stress in field conditions.
Keywords: residual stress, portable interferometer, NDT, pipe lines.
1. Introduction
Holographic interferometry is an inspection technique that proved very useful in nondestructive testing. There is no
physical contact with the test object. It can be applied to test objects of a wide variety of sizes and shapes, and to any
type of solid materials, optical transparent or opaque. A high degree of sensitivity may be obtained, which makes
possible the deteaion of low levels of displacements during inspection, resulting in interference patterns. Most
procedures for measurement of residual stress near the surfsce involve removal of the material layer or drilling a
hole. X-ray measurement is widely used but may be difficult to implement for certain geometric configurations and
materials. The feasibility of employing interferometric techniques to measure the change in dioplacement occuning
in the vicinity of the zone of the defoct or of the removal material (drilled hole, thin cut, etc.) has been recently
investigated [1,2,3.4,9,10,11]. For good accuracy in counting the interference fringes (good accuracy of the
determination of the residual stress) it is necessary to create a number of them. This number depends on depth of the
zone of the removed material (0.5mm and more). In this case, we have to question the use of this technique for the
investigation of the real structures. Ifthe depth of the zone of the removed material is very small (10-25p) one can
get a number of interference less than 1/2. This depth is compared with the structural relief of the structure and can be
very efasily removed from the structure if it is necessary. For determination of the number of interference less than '/1
it is necessary of use the additional phase shift. This paper reports a technique for measuring residual stress using
the portable holographic interferometer which has been mounted on the real construction to make measurement in
field conditions.
2. Background of the Complience Method (Optical Part)
It is well known that holography is a linear process in the sense that two or more opt~cal waves can be recorded
sequentially in time and later can be reconstructed simultaneously. Therefore the sum, difference, or even time
average of a sequence of waves can be formed. In applications to interferometry, Ul(qy) represents the light
scattered or transmitted to the hologram plane by some object, and U2(qy) represents light &om the same object
after it has been slightly deformed or changed in some manner. Slight deformation or changes of the object
primarily affect the phase of U1, so we write
U ~ Y = 4hy)erp ) { -i+(&y) I,
The irrad'iance of the rcconsrructed wave then becomes:
U~(&Y) = a(%y)erp { -iIMx,y) + A$(&Y)I 1. (1)
'~m-ce:
Email: P&U@&&&&~B ; Telephone: 501 575 7978; Fax: 501 575 4580
part of the SPIF Conference on Nondestructive Evaluation of Utilities and Pi~elines Ill
Newoort Beach. California March 1999 SPlE Vol. 3588 0277-786X1991810.00