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ACFM
in Detail
Defect Sizing
Theoretical modelling
of the interaction between a unidirectional current and a planar
defect was carried out at University College London during the 1970's.
This work allowed surface-breaking defects of any realistic shape
(e.g. semi-elliptical, circular, arc etc.) to be sized from measurements
of surface voltages using the ACPD
technique.
In the 1980's this work was extended (for the case of a semi-elliptical
defect in a metal where the skin depth is small compared to the
defect depth) to model the resulting magnetic fields above the
surface. The model produced tables of values of normalised magnetic
field strength for various combinations of defect length, depth
and sensor lift-off.
In order to provide a defect size (length and depth) the ACFM
software requires input of four parameters:
- The background magnetic field strength (Bx measured away from
the defect).
- The field strength at the deepest part of the defect (Bx minimum
value).
- The distance between the Bz signals (peak & trough) associated
with the crack ends.
- The sensor lift-off (normally 0 unless the surface is coated).
The length 3, (measured on the sample, or from an encoder) is
converted to actual defect length.
The software then uses
the ratios of 1 and 2 - a dimensionless number, which is independent
of instrument gain, material permeability or conductivity (provided
the skin depth is small) - and compares this with values in the
look-up tables for the given lift-off to give defect depth.
Crack
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