Nondestructive eddy current testing for the measurement of conductivity and surface buckling of metallic sheets

A cylindrical excitation coil, fed with ac current, is put in front of a metallic or ferromagnetic sheet. Closed formulae for the vector potential in the space and the change of the impedance of the exciting coil are given. The results are developed to show the effect of the geometrical properties of the measuring system upon the test of the buckling of the sheet, its material purity, and its conductivity. The phase angle rather than the amplitude of the change in the coil impedance is taken as the measured parameter.