Electromagnetic Testing

Figure 1.8 is again useful to illustrate discontinuity response because of
current distribution. As an example, consider testing a nonferromagnetic tube
at a frequency that establishes a standard depth of penetration at the
midpoint of the tube wall. This condition would allow a relative current density
of about 20% on the far surface of the tube. With this condition, identical near
and far surface discontinuities would have greatly different responses. Due to
current magnitude alone, the near surface discontinuity response would be
nearly five times that of the far surface discontinuity.
Discontinuity orientation has a dramatic effect on response. As seen earlier,
discontinuity response is maximum when eddy currents and discontinuities
are at 90 degrees or perpendicular. Discontinuities parallel to the eddy
current flow produce little or no response.
The easiest technique to ensure detectability of discontinuities is to use a
reference standard or model that provides a consistent means of adjusting
instrumentation.
Charlie Chong/ Fion Zhang