Electromagnetic Testing

For deep subsurface crack detection (more than 5mm) the lower frequency would
be required. This test might be performed with hybrid (driver/pick-up) coil to
improved detection of the low amplitude responses from smaller discontinuities
deeper in a product.
For detection of very small stress or fatigue crack in a near surface inspection
process the higher frequency range could improve sensitivity to smaller cracks.
The compromise at very high frequencies is the issue of skin effect or surface
noise. Special probe or scanning process may be required for this type of test
also.
Figure 6.9 shows a block diagram for a multifrequency instrument operating at
three frequencies simultaneously. In modern systems this is referred to as
simultaneous injection. This diagram shows three dedicated frequency modules
but more recent adaptations use multiple variable frequency circuits. In Figure 6.9,
excitation currents at each selected frequency are impressed across the coil at
the same time. You will recall from earlier chapters that the electromagnetic
envelope around an alternating current driven coil is very dynamic. It is very
difficult to model what the combined electromagnetic flux pattern would look like
with more than one frequency affecting the coil at a given moment in time.
Charlie Chong/ Fion Zhang