Detection of broken wires using a high resolution magnetic test method

Single broken wires can be found very easily with electromagnetic test heads which are equipped with search coils encircling the rope and have been used successfully for many decades. Many experts are working with this method and therefore continuously evaluating their diagnostic skills (Mueller, 1952; Mueller, 1957; Rieger,1983 and Beck, 1998). Other forms of rope damage like corrosion, torsional defects or strand contact can be detected from the recorded charts by an experienced examiner. However, a reliable diagnosis of clusters of broken wires (i.e. single broken wires which are close to each other) is not possible. This is especially true for the LMA-method, which cannot be used particularly in this case for exact measurements of local reductions of metallic cross sections. An exact measurement of the cross section loss is only possible if it extends over a length of about 5 times the rope diameter. These restrictions are the reason for the (practically simultaneous) development of the "High Resolution Magnetic Induction Wire Rope Test Method" at two different research institutes (Haller, 1995 and Nussbaum, 1996). The "Imaging Method" or "High Resolution Wire Rope Test Method" is a significant improvement of the well-known electromagnetic wire rope inspection. The development of the new test method needs two major elements: first, development of the test equipment {hardware) and second, a knowledge of "how to read" the acquired data. These methods provide more information about the stray field, but up to now little information is provided about the correlation between broken wires and their magnetic stray fields. It is very difficult to analyze clusters of broken wires, because one wire break with a large gap causes a stray field similar to that caused by two wire breaks close to each other with small gaps. Haller developed the "Imaging Method" by testing samples of discarded ropes. Therefore radiographic testing (X-ray) was necessary. Haller found that 10% to 30% of the broken wires (in clusters) are not detectable but these "hidden" wire breaks very often generate magnetic stray fields. In practice it is almost impossible to maintain constant testing conditions in multiple tests, for example when working on a 65 m tower in windy, rainy weather. The number of samples is limited and all damage has occurred accidentally. To overcome these limitations Nussbaum (1999a and 1999b) investigated theoretically the correlation of broken wires (cause) and magnetic stray fields (consequence). The causes are well defined (air gap, position inside the rope, number of wire breaks in clusters, etc.), and contrary to the finite number of samples employed by the "Imaging Method", examination by the means of calculation offers the possibility of considering an unlimited number of samples.