Nowadays multi-wire steel ropes are widely used in engineering structures and play a critical role while carrying heavy weights in different sectors of industry. Due to environmental and operational conditions, also fatigue, multiple defects can develop in such ropes. Usually they are inspected using acoustic emission and magnetic flux techniques. However, magnetic flux method provides ability to test only relatively thin wire ropes and is very sensitive to the way in which the wire rope has been magnetised, while acoustic emission enables to inspect only short sections of the rope and requires a complicated analysis of acoustic signals. Ultrasonic Guided Waves (GW) are rather promising for detection of various structural non-homogeneities, because they may propagate long distances and are quite sensitive both to surface and internal defects. However, in the case of multi-wire ropes the key issue is whether GW will propagate only in a one particular wire, in which it was excited, or it will propagate in the whole multi-wire rope. In other words, the possibilities of inspection will depend on the character of GW propagation inside the rope which in turn is affected by the excitation method and a mechanical contact between neighbouring wires.
Therefore, the objective of this work is to investigate propagation of GW along multi-wire ropes and to find out how deep into the structure of the rope the inspection is efficient in the cases of various types of acoustic contacts between neighbouring wires and the internal core.
The propagating ultrasonic GW modes along the multi-wire ropes were determined by modelling using analytical and SAFE techniques. In order to investigate the effects of GW propagation, the two types of the acoustic contact between neighbouring wires were simulated using Finite Element method (FE) as well. The obtained results showed that in the case of the edge type excitation and a solid contact between the neighbouring wires, mainly the L(0,1) mode is generated. However, in the case of the solid contact the F(1,1) mode is also generated due to mode conversions at each junction between neighbouring wires. In the case of the excitation from the top, mainly the F(1,1) mode is generated for the both types of the contact: slip and solid. The key question for development of the inspection technique was estimation of the actual boundary conditions between neighbouring wires (solid or slip) and the real depth of penetration of GW into the internal structure of the rope. Therefore, in order to verify the results of FE modelling, the multi-wire rope possessing the overall diameter of 35 mm, made of six steel strands and with an internal polymer core, has been investigated experimentally. It was obtained that the best results of crack type defects detection are obtained when measurements of GW are performed on the same strand as excitation. For this reason a special testing technique using a pair of the contact-type transducers with a fixed measurement distance, the length of which is equal to full rotation of a single strand in the rope, was proposed.