NDE of Multiple Cracks by Means of Measurement of Magnetic Field Produced by D.C. Current Flow.

Abstract

A method is developed for nondestructive evaluation of multiple cracks by means of measurement of magnetic field produced by d.c. current flow. Two-dimensional multiple cracks are treated. The distance between cracks is assumed to be known in advance and the depth of cracks is to be found. First the distribution of the change in magnetic flux density is measured over each crack by input and output of d.c. current on both sides of each crack. Next the distribution of the change in magnetic flux density is calculated numerically by assuming the depth of every crack. Then by comparing the measured and calculated changes in magnetic flux density, a correction factor is obtained for the assumed depth of the respective cracks. Modifying the crack depth by the correction factor is repeated until the difference between the measured and calculated distributions of the change in magnetic flux density is minimized. It is shown that multiple cracks are sized accurately by the present method.