While other approaches are discussed briefly, devices using electromagnetic (e.m.) techniques, the only ones practically applied so far, are dealt with in detail, covering operating principles, capabilities, accuracy, and use of AC and DC instruments.
Current e.m. NDT experience derives mainly from work with tramway and mine hoisting ropes, but the author concludes that much of it should be applicable to offshore ropes, although some equipment modifications and trials may be needed for large ropes.
Since wire ropes degrade with use and exposure to the elements, periodic inspection to determine their suitability for continued use is a very common, inescapable task. Most ropes used as major elements in moving people and in moving, supporting, or positioning loads and structures must be replaced before they fail if serious, perhaps even catastrophic, consequences are to be avoided.
This paper deals briefly with the problems of visually inspecting rope and with NDT approaches investigated, and in some detail with current electromagnetic (e. m.) rope NDT devices. While much of the material derives from work with mine hoisting ropes, much of it relates to rope in general, since rope degradation varies more often in degree than in kind from one application to another.
Inspecting a wire rope to determine its flexibility for continued use requires (1) discovering, then (2) measuring degradation. These two tasks are the essence and goal of inspection. (3) Evaluating the inspection data - the goal is to estimate the strength of the rope and its rate of change. (4) Deciding whether or not to replace the rope this task involves comparing the results of evaluation with removal criteria.
Two categories of degradation mechanisms can be considered:
short-term mechanisms; accidents, mishandling, and operational problems; and
long term mechanisms; wear of all kinds, corrosion, and fatigue, alone and in synergistic combinations.
The short-term mechanisms lead to conditions that usually severely degrade the strength of the rope, particularly if wear, corrosion, and fatigue are well established. These conditions can develop in a minute, as it were, which calls for frequent inspection if the application is prone to such damage. Discovering this sort of damage is usually easy, for it takes highly visible forms such as kinks, bends, bird cages, crushing, cork screwing, or broken strands (strands are distinct groups of wires). Evaluation and decision making is usually no problem either; ropes are commonly replaced if the affected area cannot be removed by an end cut.
The long-term mechanisms are at work to some degree in all wire ropes, regardless of application, and are the ones that cause the greatest problems for inspection and evaluation.
Symptoms of wear, corrosion, and fatigue are unmistakable--worn, corroded, and broken wires.