T.O. 33B-1-14-324.3.4.2.4This same effect continues as the crack is submerged deeper under the surface. The more subsurface the crack goes theless of the eddy current flow is interrupted, which decreases the amplitude of the crack response. And the farther awayfrom a surface effect the signal amplitude gets smaller and the phase angle rotates clockwise, away from lift-off (seeFigure 4-29).Figure 4-29. Deep Subsurface Crack4.3.4.2.5Another explanation for this phase angle shift is that changes in the eddy current field take time to travel any distance.Changes at the surface of the part are seen immediately by the coil, while disturbances to the field at some depth in thepart require some travel time to return to the surface where they are seen by the coil. Electrically, this is described asphase lag at depth, and the amount of phase lag is 1 radian (57O) per standard depth of penetration (see Figure 4-30).Figure 4-30. Depth in PartThis phase lag from the lift-off (surface) signal may be used to measure the depth of defects, and this effect is put togood use in tube testing. The phase angle of a defect signal correlates to defect depth.
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
