FEM study on the influence of air gap and specimen thickness on the detectability of flaw in the yoke method

Abstract

The yoke method is usually used as a magnetic testing method of welds. In this paper, we study the influences of the air gap between the magnetic pole and the specimen surface on the average magnetic flux density passing through the specimen, and the specimen thickness on the leakage magnetic flux density from a flaw using finite element method (FEM). When the air gap increases the average magnetic flux density at the center of the specimen length decreases. We can estimate the intensity of the magnetic field on the specimen surface by extrapolating the magnetic flux density in space to that at lift-off being zero. Moreover, the maximum leakage magnetic flux density from a flaw decreases with increase in the specimen thickness even if the average magnetic flux density passing through the specimen is the same.