Applications of A.C. Field Measurement Techniques for the Aircraft Industry

THE a.c potential drop (acpd) technique has been used for many years for non‐destructive evaluation in a wide range of applications. The technique relies on the generation of a unidirectional incident current flow in the region under inspection, either by direct injection or by induction. This then allows the current perturbations caused by a defect to be derived analytically for a wide variety of geometries. These theoretical predictions are then inverted so that crack sizes can be obtained from surface potential difference measurements without the need for prior calibration. The skin depth in a metal is given by δ = (πμσƒ)−½ where μ is the magnetic permeability, σ the electrical conductivity of the metal, and f is the a.c. frequency used for the current input. The analytical solutions are considerably easier at the two extremes of the ratio of skin depth to crack depth, δ/d. At operating frequencies of a few kilohertz, the thin‐skin extreme, δ/d«1, typically occurs in magnetic materials or in metals of high conductivity such as aluminium, while the thick‐skin extreme, δ/d»1, occurs for short cracks in most other metals including stainless steel, nickel alloys and titanium. Other situations can be made to satisfy one extreme or the other by a suitable change in input frequency.