Detection of Deep Flaws in Aluminum Structure with Magneto-Resistive Sensors

Abstract

It is well known that using progressively lower frequencies in eddy current detection permits deeper penetration into conductive samples since the skin depth increases as the inverse square root of the frequency. Since the signal of interest is produced by Faraday’s Law, the amplitude of the signal voltage is proportional to the excitation frequency. This fact limits the use of normal eddy current techniques as frequency decreases. This paper presents a method of detecting the field due to the induced eddy currents rather than its time derivative, so that at least the “return” portion of the signal is not proportional to frequency.