Essential Variables in ET

T. Stepinski

15th World Conference on NDT, Rome, Italy, 15-21 October, 2000

Abstract:

This paper is concerned with the analysis of essential variables in eddy current NDT. Our attention is focussed on the essential variables (EV) that may effect the result of an ET inspection but because of various reasons are not considered as essential in common practice. We present a qualitative analysis of the influence of the above mentioned factors on the ability to detect and size flaws using mechanised ET. Eddy current (EC) inspection employs some type of coil or probe sensing magnetic flux generated by eddy currents induced in the tested specimen. An amplitude-phase modulated signal from the probe with test frequency f0 is sensed by the EC instrument. The amplitude-phase modulated signal is amplified and demodulated in phase-sensitive detectors removing carrier frequency f0 from the signal. The detectors produce an in-phase and a quadrature component of the signal defining it as a point in the impedance plane. Modern instruments are provided with a screen presenting the demodulated and filtered signal in complex plane. We focus on such issues, related to the EC equipment as, distortion introduced by phase discriminators and signal filters, and effect of probe resolution on sizing. The influence of different variables is investigated by means of physical reasoning employing theoretical models and demonstrated using simulated and real EC signals. In conclusion, we discuss how the demodulator and filter type and the presence of harmonic distortion in modulated signal may effect the result of EC test. We also present a number of practical recommendations for the users of ET and indicates the arias that are to be further analysed.