FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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THE INFLUENCE OF STRESS ON THE INSPECTION OF STEEL WITH PARTICULAR
REFERENCE TO GAS PIPELINES
V.L. Athenian, U.C. Jil&i and C. Wzlbouin
Q.ueew'4 Un.lve.mIty, Kingiton, Ontanio
*Research supported in part by the Department of Energy, Mines and Resources
and the National Research Council (IMRI).
ABSTRACT
It is commonly recognized that stress induced permeability changes are a factor
capable of giving signals or spuria in magnetic or electromagnetic non destructive
inspection of steel. In fact the situation is more complex because ferromagnetic
materials exhibit hysteresis with the result that their magnetic
behaviour is dependent not only on the ambient stress and magnetic fields but
also on both the magnetic and stress histories.
Although stress effects usually complicate N.D.I, they can also be exploited,
for example, far side corrosion signals can be enhanced by the application of
stress which is concentrated in the regions of wall thinning. Magnetometer
surveys of pipelines also show stress anomalies and changes in stress can be
detected. The technique appears promising for monitoring stresses in other
high performance steel structures such as offshore drilling rigs.
Introduction
It is well known that stress alters the magnetic behaviour of ferromagnetic
materials and that changes in permeability give signals or spuria in magnetic
or electromagnetic non destructive testing of steel. Generally permeability
variations are an unwanted complication, particularly in the case of eddy
current inspection, but they can be exploited when properly understood. The
effect of stress on the magnetic behaviour of ferromagnetic materials is more
complex than had been generally recognized until recently (1) because ferromagnetic
materials are hysteretic with respect to both magnetic field and
stress. Their magnetic behaviour therefore depends not only on the ambient
stress and magnetic fields but also on both the magnetic and stress histories.
It is possible that magnetic monitoring may therefore t' used to indicate
ambient stress and stress history.
Figure 1 shows as an example a log of a magnetometer survey above a gas pipeline.
There are large fluctuations in the magnetic fields and some of these
are attributed to the stresses introduced in the line at cold bends made in
the field in order to follow undulating topography. Principally, however
the fluctuations are due to the individual joints from which the line is
constructed behaving as randomly orientated individual bar magnets as shown in
Figure 2. The effect of stress introduced at cold bend deformations is to add