Application of Alternating Current Field Measurement for Determination of Surface Cracks and Welds in Steel Structures at Lift-off

Alternating Current Field Measurement (ACFM) is a novel technique for Non-destructive inspection (NDI) and sizing of surface breaking cracks on conductive surfaces. This document examines the precision and accuracy of the ACFM technique for inspection of simulated surface breaking geometrical defects (ex., cracks) and electromagnetic discontinuities (i.e., welding, excessive grinding or corrosion) in high strength, low permeability steel plates at lift-offs ≤ 30 mm. An off-the-shelf standard array probe was shown to be effective at measuring crack lengths with an accuracy of ± 6% and a precision of ≤ 4 % at 0 and 4.4 mm lift-offs. The specialized high lift-off probe exhibited a precision of ≤ 5 % and an accuracy that decreased with increase in lift-off from ± 5% (at 0 lift-off) to ± 9 % (at 19.4 mm lift-off). The high lift-off probe was capable of detecting and sizing welding at 30 mm lift-off. Investigations are underway to improve the accuracy of the high lift-off probe to quantify depth.

References
  1. Topp, D. “Recent Developments and Applications of the ACFM Inspection Method and ACSM Stress Measurement Method”, Proceedings of the 9th European Conference on NDT, Berlin, 2006.
  2. Lewis, A.M., D.H. Michael, M.C. Lugg and R. Collins “Thin-skin electromagnetic fields around surface breaking cracks in metals” J. Appl.Phys. 64(8), pp 3777-3784, 1988.
  3. “Standard practice for examination of welds using the Alternating Current Field Measurement Technique”, American Society for Testing and Materials, ASTM E2261-07, 2007.
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