Electromagnetic Testing Chapter 3- Electromagnetic Testing

If the diameter of a surface coil is much larger than the defect size, the defect
will not be reliably detected. Likewise, if the vertical coil length is longer than
the vertical component of the magnetic field, the excess coil length is not
utilized. If the vertical coil length is much shorter than the vertical component
of leakage flux, a weaker signal will be induced into the coil and the signal to
noise will be lower. A greater electromotive force (emf) will be induced into
the coil if the number of turns of wire is increased; this increases the
inductance, coil impedance, and signal-to-noise ratio of the system. There are,
of course, practical limitations to the number of turns of wire for optimum coil
design. Improved flaw detection can result from using a soft iron or powdered
iron core in the direction of the coil. The iron core tends to concentrate the
lines of flux more effectively, coupling them into the coil and favorably
increasing both coil impedance and output voltage. Finally, coil orientation is
as important as defect orientation with regard to detecting flux leakage from
defects. Surface coils are particularly effective for detecting flux leakage
because they are normal to the surface and all surface defects produce a
strong vertical or normal flux leakage pattern (see Figures 3.10 and 3.11).
Charlie Chong/ Fion Zhang