Wire strain variations in normal and overloaded ropes in tension-tension fatigue and their effect on endurance

This paper investigates wire strain variations in tension-tension fatigue for two six-strand rope constructions under normal and overloaded conditions. It has been found that for ropes in tension there is a considerable variation in wire strains both on different wires at the same cross-section and to a marginally lesser extent along the length of the same wire. It has also been found that a Lang's lay rope has a wider variation of strains than the identical ordinary lay rope. Load cycling has been found to reduce the distribution slightly for an initial period, around 8 per cent of the rope's life (for the load range used), after which the variations in wire strains do not change significantly for the rest of the life of the rope. Tests involving initial overload have shown a considerable reduction in wire strain variation. By way of an example for an ordinary lay rope an initial overload caused the wire strain standard deviation to decrease from 22 per cent of mean to 11 per cent of mean and was accompanied by an increase in rope endurance by a factor of 2.4. A statistical model has been used to illustrate the influence of strain range distribution on the fatigue endurance of a rope using a Gaussian probability distribution. It is suggested that wire strain distribution (measured by standard deviation as a percentage of mean) provides a good indicator of rope manufacturing quality.