T.O. 33B-1-14-50volume. A small crack has a proportionately greater effect on a small probe field than on a large probe field. Ingeneral, cracks whose lengths are less than half the diameter of the coil are difficult to detect. In the event encirclingcoils or inside coils are used, short or narrow coils are preferred for inspection of small localized conditions. Spacingof the coils must be considered when determining the resolution required. The coil or probe must match the frequencyrange and output impedance of the instrument being used. Ingeneral, cracks whose lengths are less than half thediameter of the coil, are difficult to detect and will indicate a 50% decrease in amplitude.4.5.2.3 ProbeHousings.The housing for most general purpose surface probes are cylindrical in configuration and from 1/8 to 3/8 inch indiameter. Probes can be shielded with either mu metal or ferrite to concentrate the field. When defect detection aroundfasteners, in radii, or adjacent to edges is required, it is often advantageous to have a pointed or small rounded tip at theend of the probe. The pointed end allows the probe to be inserted closer to the inspection surface, or edge, and permitsbetter visibility of probe coil position. The advantages of a pointed probe for these applications are illustrated in Figure4-41. For inspection of bolt holes, special probes are manufactured that permit contact with the side of the hole at anydesired level in the hole. For inspection areas where accessibility is a problem, or where probe positioning is critical, itis often desirable to fabricate a special probe housing as an aid in performing the inspection. The use of specialhousings can greatly decrease the loss of sensitivity associated with probe wobble and lift-off during scanning. Whenlarge quantities of parts are to be inspected, special probes present a distinct advantage if they enable per unitinspection time to be reduced. Test procedures and Technical Orders for the eddy current inspection of specific aircraftcomponents may specify the design anduse of special fixtures when required.Figure 4-41. Advantages of Pointed and Radiused Probes for Eddy Current Inspection.
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