Stress measurements in ferromagnetic steel are relative to a previous
measurement, preferably taken with known stress level, e.g. no stress.
Calibration curves can be established and quantification of actual
stress level is thus possible.
Plasticity, permanent changes in the material caused by stress
exceeding yield, is detected. This is e.g. observed when monitoring
areas with residual stress. Changes of residual stresses related to
welds are measured, e.g. when monitoring fatigue in welds.
The steel ″remembers″ previous external loads, i.e. the load has changed
the magnetization of the steel. This is measured by FEMM and is called
remanent stress, which shows the highest stress since the last
measurement. This magnetization is wiped out by the measurement, and
the permanent change can then be measured.
Differentiation between these changes are related to the analyses of
changes of permeability and resistance and the depth of these changes.
This has been verified by several high cycle fatigue tests.