Electromagnetic Testing Chapter 3- Electromagnetic Testing

For longitudinal flaw detection, both rotating and fixed magnetic yoke heads
can be used to establish circular magnetic fields as illustrated in Figure 3.27.
The rotational yoke at the top is fed with direct current as it rotates around the
tube, thus producing a low-frequency alternating current field capable of
penetrating wall thickness of 0.6 in. or more.With this system, both inner and
outer flaws are scanned from the outside surface. The scanning head glides
on the surface of the tube to assure constant lift-off and sensor signals are
transmitted over slip rings to the electronics unit for evaluation. As many as
16 sensors can be used with this configuration to achieve throughput speeds
of 6 fps.With the fixed yoke at the bottom similar test results can be obtained
by rotating the tube while the inspection head is moved down the tube to
achieve a 100% inspection of the tube surface.
Charlie Chong/ Fion Zhang