Bending-Tension Fatigue The Influence of Fluctuating Load on the Safety of Ropes in Bending

Technical Meeting SAFETY, ACCEPTANCE AND DISCARD OF WIRE ROPES – Nantes 1991

As the most significant and identifiable fatigue problem, the repeated bending of ropes running over sheaves at a constant tensile Ioad (bending-over-sheaves, or BOS, fatigue) has been the major preoccupation of OIPEEC. As such it has been the focus of the great majority of OIPEEC meetings and Bulletin articles. The fatigue of fixed ropes under fluctuating tensile Ioads (tension-tension, or T-T, fatigue), whilst always a consideration, has only received significant attention in recent years. The main reason for this change has been the use of very large diameter ropes for the critical application of mooring floating offshore structures. Particularly in connection with this last application, it has become apparent that these two basic modes of fatigue, BOS and T-T, can interact in a composite fatigue mode that has been called "bending-tension" (Chaplin (1986)). There are in fact many applications for rope in which there are appreciable dynamic fluctuations in tension which are directly associated with relatively small amplitude bending motions of the rope on and off a sheave, a combination that is the characteristic of bending-tension, or B-T, fatigue. Such circumstances may be found in the mooring ropes of offshore structures; diving bell hoist ropes; crane ropes when there are significant dynamic "snatch" components at the start of a Iift; boom stay ropes for cranes; at the compensating sheave on a mine shaft conveyance attached to twin hoist ropes; and numerous others.

Author(s): I.M.L. Ridge & C.R. Chaplin