magnitude of crack response as scanning speed is increased. The reduction in needle def lection with
increased speed of scanning is shown in Figure 4-46. Manufactures may produce equipment manuals that
suggest a fast scanning speed, however, scanning speeds using manual scanning techniques SHALL not
exceed 1/4 to 1/2 inch per second.
Automatic or Semi Automatic Equipment.
Automatic eddy current equipment in conjunction with high speed recorders is capable of operation at
extremely high speeds. The upper limits of scanning speed are based on the operating frequency and the
sampling rates of the recorder or readout. The principal use for automated eddy current equipment by the
military is for the inspection of bolt holes. In this application, rotational speeds of 100 rpm can be obtained by
the inspection system.
Use of Recorders or Oscilloscopes.
The use of recorders or oscilloscope (CRT) type eddy current instruments permits increasing the speed of
manual scanning to the limits imposed by the reaction time of these instruments. Generally, other
restrictions related to guiding the probe in the prescribed scanning pattern become the controlling factor
when recorders or oscilloscopes are employed.
The scanning pattern required for eddy current inspection is based upon the possible initiation site of the
crack, the orientation of the cracks, and the size of the cracks which must be detected. If cracks initiate from
an edge in thin material (0.050 inch or so), eddy current inspection is usually limited to a single scan of the
edge. For thicker materials, scans might be required on both surfaces adjacent to the edge and one or more
scans of the material between the edges. When cracks initiate beneath the heads of nonremovable fasteners,
the pattern usually consists of a single scan around the protruding head of the fastener to detect cracks
growing outward from the hole. If cracks can occur at a variety of positions and orientations, as is possible on
f lat surfaces, in radii, and on cylindrical surfaces, scanning must be performed in a manner which will assure
detection of the smallest cracks required to be found. For these types of inspection areas, the direction of
scanning, the number of scans, and the distance between scans should be specified.