MRT Devices: How does it work?

The MRT method is a non-destructive, magnetic, and contactless procedure that allows the detection of broken wires, distortions, and corrosion on steel wire ropes.

This magnetic method was introduced in cableways to identify external and internal defects of the wire for safety. In the 80s, the method expanded into offshore heavy lifting due to the increasing needs and efforts affecting efficiency and safety in the marine environment.

The most recent version of ISO4309 includes MRT as an assessment method for the inspection of heavy lifting wire ropes.

If the wire rope is magnetically saturated, and a local flaw appears, a part of the magnetic flux lines moves from the inner part of the rope to the surrounding air. The quantity of this flux and its path depend on the characteristics of the damage. Inside the device, special probes acquire the flux distortions and translate them into a measurable quantity (i.e. voltage or current). A diagram in the user interface will then show the effects of the flaw.

Each peak in the graph below corresponds to one or more broken wires in the inspected wire rope. The LF signal is fully regulated by the EN12927 standard, which has to be accomplished.

The LMA signal is based on the measurement of the main flux that moves inside the rope. Under certain hypotheses, this flux is proportional to the wire rope cross-section and this signal can give useful indications of phenomena such as extended corrosion, wear, and so on. It should be said that the LMA signal is aimed at ‘long’ flaws and is not suitable for localized damages such as broken wires.

In the 2nd picture above, it is possible to see a distinctive signal corresponding to a corroded zone in which the cross-section decreases by 10% approx. In our devices, the LMA signal is optional because it may provide the wrong result in case certain conditions are not fulfilled.

Training:

A perfect MRT inspection is the combination of good acquisition and good interpretation.

A fully automatic interpretation is rarely possible, especially for mobile applications. While they may be used to trigger alarms for further inspection and interpretation by the inspection teams, a fully automatic system is still fully possible without human intervention. This is why training is essential to the technique.

The user must know how the equipment works and looks for all the causes that could affect its performance. For this reason, signal interpretation is fundamental in order to define the rope status.