To summarize what we have observed so far: A dent or liftoff signal creates a straight line increasing in amplitude with increasing frequency. A through wall hole signal maintains a constant amplitude while first rotating clockwise with increasing frequency and then rotating counterclockwise. An O.D. defect has continuing decrease in amplitude as frequency increases and continually increasing angle. An I.D. defect signal has increasing amplitude with increasing frequency and an angle that initially rotates clockwise and then counterclockwise. Certainly none of this is a great surprise to an experienced Eddy Current Inspector.
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