FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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an additional reverse magnet to the pipe bar magnet to give the magnetic
profiles shown in Figure 3.
These figures show that stress in ferromagnetic structures can produce significant
but complex magnetic effects. : now review the basic results of the
effects of uniform stress on simple sarii£-\es of ferromagnetic material such as
line pipe steel.
Effects of Constant Stress
Figure 4 shows hysteresis loops for simple samples of line pipe steel under
constant tension or compression. The effect of stress is to give a slight
tilt to the hysteresis loops. At low field tension tends to increase the
magnetisation, compression to decrease it although both tend to decrease it
at high field. At first this seems to be explicable by applying Le Chatelier"s
principle (which states that the application of a constraint to a system in
equilibrium tends to shift the position of equilibrium to reduce the applied
constraint) to the magnetostrictive effect (the change in length on magnetisation)
. Figure 5 shows typical magnetostrictive behaviour for iron. The
magnetostrictive coefficient (strain) is dependent on both field and applied
stress. A positive magnetostrictive coefficient i.e. expansion on magnetisation,
would suggest that tensile strain would cause an increase in
magnetisation in order to reduce the applied stress and this seems to be in
good accord with Figure 4. unfortunately the results of changing the stress
in a constant field are more complex.
Effects of Changing Stress in a Constant Field
Figure 6 shows the changes in magnetisation occur ing from the same initial
conditic.is during a tension cycle and a compression cycle under constant
applied field. Both of these cycles result in increased magnetisation and
are therefore contrary to predictions based on Le Chateliei's principle.
Figure 7 shows the changes in magnetisation occuring as a result of similar
stress cycles applied at different fields around a hysteresis loop. The
results depend on the initial conditions and are due to a reduction in the
magnatic hysteresis.
Magnetic hysteresis occurs because magnetic domains are not free to move to
their equilibrium magnetisated but are pinned (2). The effect of stress is
to reduce this domain pinning and to let the magnetisation approach the true
equilibrium (anhysteretic) value. The dotted line in Figure 7 shows the
anhysteretic magnetisation curve, explained in Figure 8. The stress induced
shift towards the anhysteretic magnetisation is further exemplified in
Figure 9 which shows the changes in magnetisation induced by similar stress
cycles. For cycles originating at points on the initial magnetisation curve
the changes appear to be related to .he differences between the initial and
anhysteretic magnetisation curves and this is confirmed by the very small
changes resulting from cycles originating on the anhysteretic curve itself.