Nondestructive evaluation by permeance testing

An AC permeance tester for the evaluation of defects in ferritic materials is examined. The permeance tester, having similar characteristics to a linear variable differential transformer (LVDT), is sensitive to variations in permeance. An E-type magnetic transformer core is used to produce a varying flux on its central limb with two pickup coils on the outer limbs. The voltages appearing across the pickup coils are connected in opposition and the difference voltage measured by a voltmeter. An analytical model is developed showing that the normalized difference voltage is directly related to the volume of any small defect in a ferritic sample which intercepts the flux between the limbs. Calculated theoretical defect size compares favourably with that measured by a prototype permeance tester. Measurement by the permeance tester offers the following: (a) a relatively simple, though approximate analytical relationship between the defect size and the indication given by the tester; and (b) the sensitivity of a permeance tester is of the same order of magnitude as the core-less eddy current tester