T.O. 33B-1-1
4-70
4.6.2.2
Inspection Requirements.
If inspection is required only for large cracks (greater than approximately 1/4 inch in length) adequate inspection can
usually be performed without special equipment or fixturing. For such cracks, inspection can be performed sufficiently
far enough from the edge to avoid interference from edge effects. To detect small cracks, a relatively constant probe-to-
edge distance must be maintained. For maximum reliability, a fixture or probe guide is used to establish probe
positioning.
4.6.2.3
Probe To Edge Spacing.
When inspecting for small cracks initiating from edges, probe-to-edge spacing can become a concern. There are two
basic approaches for inspecting at part edges. In addition to these two approaches, selecting other parameters that
minimize the volume of material sensed by the probe will improve the inspection results. Increasing the frequency of
the eddy current generating source; reducing the physical size of the eddy current generating source; and reducing the
physical size of the coil allow inspection closer to the edge because of the reduced volume of material sensed and result
in greater sensitivity to small flaws. Probe-to-edge spacing becomes even more of a concern when the edge of the part
is in contact with a ferromagnetic part such as a bearing or bushing. Again, minimizing the volume of material sensed
by the probe will optimize the eddy current inspection of such a geometry.
4.6.2.4
Fixtures And Guides.
The simplest eddy current inspection scanning guide is a section of thin flexible plastic cut to conform to the inspection
area with allowance for probe positioning. Such a guide can be easily prepared from used X-ray film. The flexibility
permits fitting of the guide to compound curvatures. It is necessary that the edge used to guide the probe be smooth to
allow steady movement at a constant distance from the edge of the opening. The guide can either be held in place or
taped in the required position. Another type of probe guide which can be used for small openings, including holes with
bushings, consists of a circular insert which fits into the hole and has a larger diameter at one end to provide the
required offset distance from the edge of the hole. Probe guides should be constructed to provide the required offset
from the edge for a specified type of probe and should not interfere with movement of the probe.
4.6.3
Fastener Holes Nonremovable Fasteners.
4.6.3.1
Inspection Application.
If a fastener cannot be removed from a hole because of fastener type or location, inspection can be performed around
the fastener to detect cracks growing from beneath the fastener head or nut. The size of cracks detectable is dependent
upon the distance which must be maintained between the probe and the edge of the fastener. In many respects, this
application is similar to inspection for cracks at the edge of openings and cutouts. Large low frequency probes and
sliding reflectance probes can also be scanned over countersunk fasteners and identify cracks at the 1st, 2nd and 3rd
layer.
4.6.3.2
Probe To Fastener Spacing.
If only required to detect relatively large cracks, such a those extending between two fasteners, eddy current inspection
can usually be performed sufficiently distant from the fastener heads to eliminate their effect on eddy current response.
When small cracks must be detected, the probe must be positioned closer to the edge of the fastener, and the probe to
fastener distance must be held constant during scanning. When fasteners fabricated of magnetic materials such as steel
are used in nonmagnetic parts, a relatively large spacing, usually a minimum of 1/8-inch, must be employed. Also,
shielded probes can be used to minimize the distance between the probe and the fastener, allowing inspection near to
the fastener.
4.6.3.3
Scanning Guides For Nonremovable Fasteners.
For nonferrous (nonmagnetic) fasteners, the head of the fastener may be used as a probe guide. Only those fasteners
which protrude from the surface of the part and are concentric with the hole can be used as guides. For fasteners with
heads not concentric with the holes, such as hexagonal and serrated heads, a collar fitted to the fastener head can be
used as a scanning guide. Most shielded probes can be scanned around steel fasteners without requiring a collar.
Templates must be positioned concentric to the fastener head to assure relatively consistent meter response from defect-
free material as the probe is guided around the fastener.
