When a rope is flexed the strength will be reduced over time. There are several causes of this including:
Fibre abrasion; where the rope fibres rub on each other as the rope bends.
Compression fatigue (aramids); where the fibres on the inside of a bend go into compression and form kinks, Aramids are particularly susceptible to this.
Differential creep; where the fibres on the outside of a bend are under higher load and creep more than the fibres on the inside.
Thermal degradation; in extreme cases rope will heat up when repeatedly bent, this can cause damage to the fibres, UHMPE in large sizes is particularly susceptible to this.
The life of a rope when repeatedly bent is exceptionally difficult to predict because of the interaction of a large number of variables that can all have a significant affect on the fatigue. These variables include:
Rope material: different fibres and even fibre grades have different resistance to fatigue and are affected by different mechanisms.
Rope construction: Some constructions are more resistant to fatigue then others, for example 3 strand ropes have less fibre crossings and so are resistant to fibre abrasion while short rope pitches are more resistant to differential creep and compression at the expense of strength.
Degree of bending: Sheave diameter and amount of wrap has a significant affect on life.
Number of cycles
Cycle speed: the speed of cycles affects the heat build up/loss and so can have significant affect.
Rope size: The thermal properties of a rope do not scale linearly; large ropes are more affected by these fatigue mechanisms.
Fibre coatings: there are many coatings that can enhance (or reduce!) the fatigue life.
Temperature: the temperature of the environment the rope is in can affect fatigue.
Water: whether the rope is cycled is a wet or dry condition can affect the life.
Contamination: Dirt and other materials that get into the rope can affect the life.
Some of these factors can have a huge affect; for instance in one test performed by DSM a change in period (cycle speed) from 10 sec to 12 sec doubled the life of the rope when all others conditions remained the same. Some coatings can increase the fibre abrasion resistance by a factor of 10. Similarly contaminants, including salt crystals, can rapidly abrade fibres reducing the life by orders of magnitude. For these reasons it is normally impractical to attempt to predict the fatigue life of a rope in a specific application as even apparently insignificant details can hugely affect the results. If fatigue is a concern in an application then replacing the ropes early and testing for residual strength is the best way to build a picture of the life in the specific conditions this rope sees.
Read our recent article on flex fatigue or contact the technical team for more information