FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

Irreversible and Reversible Changes
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Le Chatelier's principle cannot be directly applied to ferromagnetic mate-ials
since they exhibit hysteresis and are therefore not in true reversible
equilibrium. The first effect of stress applied to a ferromagnet is to cause
a reduction in the hysteresis, i.e. a shift toward the anhysterstic magnetisation.
This effect is irreversible and is generally the dominant effect during
the initial application of stress. The second effect is that the anhysteretic
curve is itself stress dependent (3) as shown in Figure 9. This effect is
essentially reversible. It could therefore be correlated with the magnetostrictive
effect except that magnetostrictive coefficients are generally
measured along the initial magnetisation curve which is irreversible and are
therefore not directly applicable.
The two effects appear to be approximately addative. In steel, the reduction
in hysteresis is the dominant effect during the initial stressing but during
subsequent stress cycles the reversible shift of the anhysteretic becomes
relatively more significant.
In stress monitoring applications both these effects may need to be considered.
Stress Enhancement of Far Side Corrosion Signals
A simple example showing the exploitation of stress effects is illustrated by
Figure 10. The signal obtained from magnetic flux leakage detectors are
greater for near side defects such as corrosion pits than for far side defects
because the anomalous leakage fluxes are concentrated near the defect. If
however a gas pipeline with external corrosion is stressed by the gas pressure
external corrosion pitting will cause anomalous highly stressed regions on
the inside of the pipewall. These may be detected using active or residual
leakage flux techniques and used to enhance the far side corrosion signals
as shown. The optimum procedure needs rather careful consideration as
discussed below.
Bending Stress Monitoring
We have built (4,5) a laboratory scale demonstration of pipe bending stress
monitoring using two 3.4 m long 11 cm diameter pipes sprung together. The
apparatus is shown in Figure 11. Changes in magnetisation as indicated by
the peak to peak amplitude of the external radial field profile logged with
a fluxgate magnetometer one pipe diameter from the wall, are shown during
two stress cycles in Figure 12. There are large irreversible changes occuring
during the initial stressing after magnetisation and smaller reversible
changes occuring thereafter. Figure 13 shows similar changes in magnetisation
occuring during five bending stress cycles followed by remagnetisation and
further stress cycles. The processes are shown conceptually on an M-H
diagram in Figure 14. The effect of remagnetisation is basically to