volume. A small crack has a proportionately greater effect on a small probe field than on a large probe field. In
general, cracks whose lengths are less than half the diameter of the coil are difficult to detect. In the event encircling
coils or inside coils are used, short or narrow coils are preferred for inspection of small localized conditions. Spacing
of the coils must be considered when determining the resolution required. The coil or probe must match the frequency
range and output impedance of the instrument being used. In general, cracks whose lengths are less than half the
diameter of the coil, are difficult to detect and will indicate a 50% decrease in amplitude.
The housing for most general purpose surface probes are cylindrical in configuration and from 1/8 to 3/8 inch in
diameter. Probes can be shielded with either mu metal or ferrite to concentrate the field. When defect detection around
fasteners, in radii, or adjacent to edges is required, it is often advantageous to have a pointed or small rounded tip at the
end of the probe. The pointed end allows the probe to be inserted closer to the inspection surface, or edge, and permits
better visibility of probe coil position. The advantages of a pointed probe for these applications are illustrated in Figure
4-41. For inspection of bolt holes, special probes are manufactured that permit contact with the side of the hole at any
desired level in the hole. For inspection areas where accessibility is a problem, or where probe positioning is critical, it
is often desirable to fabricate a special probe housing as an aid in performing the inspection. The use of special
housings can greatly decrease the loss of sensitivity associated with probe wobble and lift-off during scanning. When
large quantities of parts are to be inspected, special probes present a distinct advantage if they enable per unit
inspection time to be reduced. Test procedures and Technical Orders for the eddy current inspection of specific aircraft
components may specify the design and use of special fixtures when required.
Figure 4-41. Advantages of Pointed and Radiused Probes for Eddy Current Inspection.