Probes - Shielding & Loading
of the challenges of performing an eddy current inspection is
getting sufficient eddy current field strength in the region of
interest within the material. Another challenge is keeping the
field away from nonrelevant features of the test component. The impedance change caused by nonrelevant features
can complicate the interpretation of the signal. Probe shielding and loading are sometimes used to
limit the spread and concentrate the magnetic field of the coil.
Of course, if the magnetic field is concentrated near the coil,
the eddy currents will also be concentrated in this area.
shielding is used to prevent or reduce the interaction of the
probe's magnetic field with nonrelevent features in close proximity
of the probe. Shielding could be used to reduce edge effects when
testing near dimensional transitions such as a step or an edge.
Shielding could also be used to reduce the effects of conductive
or magnetic fasteners in the region of testing.
Eddy current probes are most often shielded
using magnetic shielding or eddy current shielding. Magnetically
shielded probes have their coil surrounded by a ring of ferrite
or other material with high permeability and low conductivity.
The ferrite creates an area of low magnetic reluctance and the
probe's magnetic field is concentrated in this area rather than
spreading beyond the shielding. This concentrates the magnetic
field into a tighter area around the coil.
Eddy current shielding uses a ring of highly
conductive but nonmagnetic material, usually copper, to surround
the coil. The portion of the coil's magnetic field that cuts across
the shielding will generate eddy currents in the shielding material
rather than in the nonrelevent features outside of the shielded
area. The higher the frequency of the current used to drive the
probe, the more effective the shielding will be due to the skin effect
in the shielding material.
Probe Loading with Ferrite Cores
coils are wound around a ferrite core. Since ferrite is ferromagnetic,
the magnetic flux produced by the coil prefers to travel through
the ferrite as opposed to the air. Therefore, the ferrite core concentrates
the magnetic field near the center of the probe. This, in turn,
concentrates the eddy currents near the center of the probe. Probes
with ferrite cores tend to be more sensitive than air core probes
and less affected by probe wobble and lift-off.