Multi-Frequency Eddy Current Testing of Ferromagnetic Welds

Abstract

Single frequency phase analysis eddy current techniques have limited potential for inspections for surface cracking in inhomogeneous ferromagnetic welded surfaces [1). Signals from these cracks are masked in noise produced by lift-off, probe wobble and local changes of the permeability of the weld, the most troublesome noise source. Surface cracks and changes in magnetic permeability create signals with phase angles that are too close to be discriminated from each other. Another method, in addition to phase discrimination, must be used to distinguish crack signals from signals created by changes in magnetic permeability. This paper explores the use of the comparison of phase and amplitude data from two probe frequencies to distinguish cracks from changes in local magnetic permeability.