Flux Leakage from Various Metal Loss Defects
Analysis of flux leakage fields around metal-loss defects is complex because the leakage field does not have the same shape as the defect. The following figure shows the flux leakage field around a semi-spherical pit. The top half of the figure shows the axial component of the leakage field; the bottom half shows the size and shape of the pit. The axial direction, which is the direction of the applied flux lines, is represented by arrows. The wire mesh is used for data visualization purposes only and does not represent flux lines.
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Note that the flux leakage field spreads in the circumferential direction making the flux leakage field elliptical rather than circular. In addition, the leakage field dips before and after the defect. So, the shape of the leakage field is not a good indicator of the shape of the metal-loss region.
The follow links provide additional examples of this point:
- This defect is the reference: A single 1-inch long by 1.0-inch wide by 50% deep - Pull rig defect set number 38. Well-defined signal with single peak; signal is wider than it is long.
- Extending the length from 1 to 3 inches: 3-inch long by 1.0-inch wide by 50% deep - Pull rig defect set number 52. The amplitude is less than before and a second set of peaks is generated, before and after the initial peak.
- Extending the width from 1 to 3 inches: 3-inch long by 3.0-inch wide by 50% deep - Pull rig defect set number 55. The amplitude increases and the effect of the second set of peaks drops.
- New comparison - Two 1 inch defects on circumferential arc: each is 1-inch long x 1.0-inch wide x 50% deep - together they appear as 1-inch long x 3-inch wide x 50% deep - Pull rig defect set number C01. Signal is very similar to that of 1-inch long by 3-inch wide by 50% deep defect - difficult to result individual defects in circumferential direction.
- New comparison - Three 1-inch round defects on diagonal - 1-inch long by 1.0-inch wide by 50% deep - together they appear as 3-inch long by 3-inch wide by 50% defect - Pull rig defect set number C14. Signal is somewhat similar to that of 3-inchlong by 1-inch wide by 50% deep defect (#2 above).
- New comparison - 1-inch round defect at 50% deep in a 6-inch round defect at 20% - Pull rig defect set number C18. Nice superposition of pit signal on background signal from larger patch - this nice superposition is unusual.