INVESTIGATION OF THE EFFECTS OF NOTCH WIDTH ON EDDY CURRENT RESPONSE AND COMPARISON OF SIGNALS FROM NOTCHES AND CRACKS

Abstract

This paper reports on work conducted to investigate the effect that electrical discharge machining (EDM) notch width has on the eddy current (EC) signal as a function of coil drive frequency. The notch results are also compared to EC signals from laboratory‐grown fatigue cracks. This study builds upon previous work with titanium, Inconel and aluminum materials where the signal amplitude was shown to decrease, as expected, as the notch width decreases. The trend was captured well by numerical results and this allowed estimates to be made about the signals from idealized “zero‐width” notches. The results indicated that the signal reduction factor from a 0.127 mm (0.005 inch) wide, rectangular notch to a theoretical zero‐width semi‐elliptical notch of the same size ranged from 25 to 42% for low conductivity materials when data was collected at 2 MHz. For aluminum, the difference between signals from 0.127 mm wide notches and estimated signals for zero‐width notches was approximately 50%. However, 2 MHz is an uncommonly high frequency for inspecting aluminum alloys so additional work was necessary to investigate the notch width effect at lower frequencies. This study sought to determine how the notch‐width effect changed as a function of frequency for high conductivity materials such as aluminum.