Thickness, alloy content and cracks in aluminum measured by an alternatingcurrent magnetic bridge

Examples of the use of complex reluctance planes for the measurement of thi ckness, alloy content and the detection of fatigue cracks in aluminum are d escribed. Complex reluctance planes are used to display results from the no ndestructive application of electromagnetic fields obtained by means of alt ernating current magnetic bridge technology. They are used much as the comp lex impedance planes of eddy current technology. The results may be more si mple to interpret because at the frequencies which Rue normally employed (s everal hundred hertz to several kilohertz) magnetic bridge technology is re markably insensitive to liftoff variations over liftoff distances from six to eight times those normally associated with eddy current technology.