FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

- 82 -
reinitialise the stress dependence so that the next application of stress
will again cause relatively large irreversible changes in magnetisation.
It should be noted that the magnetisation changes occuring in a complex case
such as a non uniformly magnetised, non uniformly stressed pipe, indicated by
external leakage field measurements as shown in Figure 13 are well understood
in terms of the M-H diagram of Figure 14 even though it derived from the behaviour
of a small sample with uniform magnetisation and stress.
The technique shows promise for monitoring the maximum stress that a pipeline
has been subjected to since last magnetisation (usually by magnetic inspection
pigging). It might therefore be used to detect dangerous anomalous bending
stresses due to foundation subsidence in pipelines constructed over permafrost
or river crossings etc. Other applications to high performance steel structures
not subject to periodic remagnetisation such as offshore drilling platforms
may be simpler, particularly for statically loaded structures where stresses
increase steadily without cyclic variations.
Conclusions
Stress has long been known to have major effects on ferromagnetic properties.
For many N.D.T. techniques this is an undesireable complication although it
can also be exploited both to enhance defect signals and as a potential
technique for monitoring stress. This requires some knowledge of the basic
effects of stress on a ferromaghet and until recently the situation has been
confusing. It is now clear that stress causes both reversible and irreversible
changes in magnetisation and that these are not directly related to magnetostriction
as had generally been supposed. The irreversible changes, which are
relatively large and occur mainly during the initial application of stress,
are due to overcoming magnetic domain pinning and thereby reducing the
hysteresis. The reversible effects are related to the stress dependence of
the reversible (anhysteretic) magnetisation. We hope that with this recent
improvement in understanding it will be possible to devise and develop
further techniques for exploiting the effects of stress on ferromagnetic
behaviour to augment current N.D.T. methods.
References
(1) D.L. Atherton and D.C. Jiles, 'Effects of stress on the magnetisation of
steel', IEEE Trans, on Magnetics, Vol MAG-19, pp. 2021-2023 Sept. '83.
(2) D.C. Jiles and D.L. Atherton, "Theory of ferromagnetic hysteresis',
J. Appl. Phys., Vol. 55 pp. 2115-2120 March '84.
(3) L. Dobranski, "The effects of uniaxial isostress on the anhysteretic
and initial magnetisations of various pipeline steels", M.Sc. (Eng.)
Thesis, Queen's University at Kingston, April '84.
(4) D.L. Atherton, L.W. Coathup, D.C. Jiles, L. Longo, C. Welbourn and
A. Teitsma, 'Stress induced magnetisation changes of steel pipes -
Laboratory tests', IEEE Trans, on Magnetics Vol MAG-19, pp. 1564-1568
July '83.
(5) D.L. Atherton, C. Welbourn, D.C. Jiles, L. Reynolds and J. Scott-Thomas,
'Stress induced magnetisation changes of steel pipes - Laboratory tests.
Part II', IEEE Trans, on Magnetics Vol MAF-20, November "84.