T.O. 33B-1-1
4-56
Figure 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 the
minimal lift-off effect of the small size of the probe tip. For most applications involving inspection of curved surfaces
with small diameter pencil probes, flat standards can be satisfactorily used for curved surfaces in establishing sensitivity
requirements.
4.5.5.6
Subsurface Flaw Detection.
Increasingly, applications arise where it is desired to inspect for cracks initiating beneath an accessible surface. This
could be a crack initiating on the opposite side of the accessible surface, in the structure contacting the opposite surface
of a accessible surface, or beneath a conductive coating or plating. Eddy current inspection can be a powerful tool for
the detection of subsurface flaws.
4.5.5.7
Impedance Plane Analysis Of Subsurface Flaws.
If the required frequency is used with impedance plane analysis instrumentation, eddy current penetration to the flaw
area can be obtained. The phase and amplitude information received from the flaw can be directly related to the flaw
depth.
4.5.5.8
Meters For Subsurface Flaw Detection.
In most instances, eddy current inspection employs meter type instruments to detect cracks initiating at an accessible
surface. Meter type instruments are very successfully used for surface eddy current inspection. They can be very useful
for measuring depths of indications which are open to the surface being tested. Due to the inability of most meter type
instruments to be calibrated or metered for phase information, it is almost impossible to obtain accurate information
which can be related to buried flaw depth and size. With some meter instruments, the amplitude response with flaw
depth is non-linear. That is, as the depth of the flaw increases, the meter response can increase or decrease
unpredictably. The limited frequency range of most meter type instruments prevents them from being utilized over a
wide range of material thickness and conductivity ranges.
