T.O. 33B-1-14-56Figure 4-45. Edge Probe Guide.4.5.5.5 Curvature.When small diameter pencil probes are employed, curvature has minimal effect on crack response. This is due to theminimal lift-off effect of the small size of the probe tip. For most applications involving inspection of curved surfaceswith small diameter pencil probes, flat standards can be satisfactorily used for curved surfaces in establishing sensitivityrequirements.4.5.5.6 SubsurfaceFlawDetection.Increasingly, applications arise where it is desired to inspect for cracks initiating beneath an accessible surface. Thiscould be a crack initiating on the opposite side of the accessible surface, in the structure contacting the opposite surfaceof a accessible surface, or beneath a conductive coating or plating. Eddy current inspection can be a powerful tool forthe detection of subsurface flaws.4.5.5.7 ImpedancePlaneAnalysisOfSubsurfaceFlaws.If the required frequency is used with impedance plane analysis instrumentation, eddy current penetration to the flawarea can be obtained. The phase and amplitude information received from the flaw can be directly related to the flawdepth.4.5.5.8 MetersForSubsurfaceFlawDetection.In most instances, eddy current inspection employs meter type instruments to detect cracks initiating at an accessiblesurface. Meter type instruments are very successfully used for surface eddy current inspection. They can be very usefulfor measuring depths of indications which are open to the surface being tested. Due to the inability of most meter typeinstruments to be calibrated or metered for phase information, it is almost impossible to obtain accurate informationwhich can be related to buried flaw depth and size. With some meter instruments, the amplitude response with flawdepth is non-linear. That is, as the depth of the flaw increases, the meter response can increase or decreaseunpredictably. The limited frequency range of most meter type instruments prevents them from being utilized over awide range of material thickness and conductivity ranges.
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