The Detection and Measurement of Overlapping Fatigue Cracks at Welded Joints by Thin-Skin Electromagnetic Fields

Fatigue cracks occur at different locations at the toes of welded steel joints. With continued load cycling it is often found that the cracks grow towards each other and overlap in adjacent parallel planes. It is useful to be able to detect and measure overlapping cracks and in this paper the characteristic responses to be expected from these features are calculated for the alternating-current field-measurement method of non-destructive evaluation by thin-skin theory. Typical signal profiles are calculated for probe traverses across the cracks and parallel to the edges. These are compared with experimental data obtained by using three pairs of overlapping notches with different proportions, cut in a steel block. The notches were interrogated by a thin-skin field at a frequency of 6 kHz. The calculations show that overlapping cracks give rise to asymmetric cross-crack signal profiles, which do not occur for single cracks, and to characteristic changes in curvature in the probe response profiles. These features are well confirmed by the experiments. The results also show that traverses along the crack edges can be used to find the positions of the crack ends.