T.O. 33B-1-14-55shown in Figure 4-44. The amount of lift-off obtained because of changes in probe angle depends on the diameter andshape of the probe tip. Rounded tips of small diameter cause less lift-off than flat tipped probes with larger diametersor impedance display instruments, lift-off effect can be lessened by changing the vertical to horizontal sensitivity ratio.Figure 4-44. Lift-Off Resulting from Probe Wobble.4.5.5 EffectsOfCrackLocationOnDetectability.4.5.5.1 CrackLocationAndOrientation.To provide the optimum sensitivity and reliability for crack detection, the maximum information should be availableregarding the expected crack. Often, this information is provided from previous history of cracks in the designatedlocations. In other cases, such information may be determined from knowledge of stress distribution during service.Increasing definition of crack location and orientation permits the inspector to reduce his inspection time. For manualeddy current inspection, reduction in scanning time provides less operator fatigue and consequent improvement ininspection reliability.4.5.5.2 CracksAtPartEdges.The edgeof a part can be represented as an infinitely large crack and, consequently, produces a strong signal duringeddy current inspection. The problem in inspecting part edges for cracks is separation of crack response from thestrong edge response (edge effect).4.5.5.3 InspectionAtPartEdges.Two approaches can be used to inspect for cracks at part edges. The first method is to null the instrument with theprobe at the edge of the part. Then, usually with a non-conductive fixture or some other method, the probe ismaintained at the edge as it is scanned along the edge. If this position can be maintained, the inspection can be donewith greater sensitivity than is possible with the same instrument and probe away from the edge. The second approachis to use a shielded probe, thus minimizing response from edges.4.5.5.4 FixturesAndHoldersForEdgeInspection.One of the simplest methods for eddy current inspection adjacent to a linear edge of a part is to tape or hold a straightedge at a predetermined distance from the edge. Nonmetallic straight edges should be used for this purpose. A simplefixture which can assist in positioning the probe adjacent to an edge is shown in Figure 4-45. This fixture maintainsthe probe center 1/8 inch from the edge, but closer edge inspection can be obtained by varying the position of the drilledhole.
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