Magnetic nondestructive evaluation of corrosion in wire ropes

摘要

Wire ropes are used as members to bare very heavy load. The integrity of wire ropes deteriorates with time owing to corrosion, fatigue and abrasion and they finally may break the ropes. As breakage of ropes, especially large diameter ropes, will cause a miserable disaster, preventing from such a disaster we must inspect wire ropes periodically by a proper NDT method evaluating the degree of deterioration. Flaws caused by fatigue are localized, and are detected easily by a conventional magnetic NDT method. On the other hand, corrosion and abrasion cause loss of metallic cross-sectional area, which usually are not localized but spread, therefore they are difficult to be detected by the conventional method. So far corrosion and abrasion have been evaluated mainly by visual inspection. The authors have developed a new magnetic NDT method, which is a modification of main flux method (MMFM). The system of the developed method consist of magnetizing units, sensor units and a personal computer and can evaluate the loss and residue of metallic cross-sectional area of wire ropes quantitatively. As some of the wire ropes used for mooring and structures are getting old nowadays, we must make haste to inspect them by a proper NDT method to keep them safe. The results of a series of experiment to certify the accuracy of MMFM are described. In the experiment 330 mm diameter ropes, which are bundles of 31 ropes of 52 mm diameter and 6 ropes of 34.5 mm diameter, 54 mm diameter ropes and 60 mm diameter ropes were inspected.