Fatigue Mechanisms and Residual Properties of Polyester Ropes
Stephen J Banfield, John F Flory, John W S Hearle, Martin S Overington
A first study has been conducted to compare modulus properties between textile yarn and rope. A good agreement has been found between yarn and rope for both loading to failure in a rested condition after cyclic loading and in cyclic loading.
There are large differences in modulus response between loading in a rested condition after cyclic loading and cyclic conditions. This is most likely due to changes in the polymer as a result of visco-elastic effects where the time dependent extension recovery does not have time to recover in cyclic loading.
In the 48 million cycle fatigue test, internal abrasion between strands was found to be significant. No other forms of fatigue mechanisms were found. This enables the designer to understand the potential mechanisms that may operate in long life applications around 20 to 30 years. It should be noted that this fatigue life is far greater than steel and well beyond that required for offshore moorings.
In contrast a lower helix angle rope showed no internal abrasion for the same test conditions, but 12 million cycles. Rope lay angle is a very important rope design parameter for long term applications and if correctly specified could design out or minimise effects of internal wear.