Content area
Full text
Although the materials of construction are typically carbon steel, the intricacy of rope construction and unusual service characteristics can result in very complex engineering considerations for the analyst.
Among load-bearing steel constructs, wire ropes and cables are possibly the most widely used and most highly stressed. Wire ropes are assemblages of intertwined steel wires and wire strands for pulling or lifting. They serve in critical applications and in severe environments, and failures are common. One particularly damaged rope that failed via fatigue is shown in Fig.1. From a metallurgical failure analysis perspective, wire rope failures can appear deceptively mundane. However, investigation of these failures involves many complicated and unique considerations that are worthy of review.
This article includes a description of the mechanical properties of wire ropes, their chemical composition, and a detailed discussion of the failure analysis process.
Mechanical properties
The mechanical properties of a wire rope result from the individual wires and their arrangement or construction. Unlike most other metallic components, rope wires are stressed alternately in tension, compression, torsion, and shear. The wires and strands are designed to slide in relation to each other, distributing the complicated applied stresses more effectively. For this reason, wire rope is often called a "machine." As machines, ropes require prudent inspection, maintenance, and periodic replacement. The inherent friction coefficient between bare steel strands mandates presence of adequate lubrication to allow the requisite wire movement.
Wire rope constructions are identified by the number of strands, the number of wires per strand, the type of core, the "lay" (length) direction, left or right handedness, and many other attributes. Rope grades are rated in tons of breaking strength. The individual wires provide strength, whereas the con struction dictates service characteristics. It is generally accepted that many smaller wires provide better fatigue resistance, while fewer, larger wires provide better abrasion resistance. Independent wire rope cores (IWRC) provide better crushing resistance than fiber cores.
Because of the complex geometry of the assembled wires, the ultimate tensile strength is not equivalent to a large wire of equal cross-sectional area. A certain percentage of the load on individual wires produces shear stresses, rather than axial tensile stresses, and steels exhibit lower (approximately 30%) ultimate strength in shear. Nevertheless, a synergy of...