The prediction of cyclic load behaviour and modulus modulation for polyester and other large synthetic fiber ropes

Fibre Rope Modeller (FRM) is a computer program for the analysis and prediction of synthetic fiber rope behaviour. It calculates the static tension-extension and torque-twist behaviour of fibre ropes using the basic fibre or yarn properties and the rope constructional parameters. It accounts for internal friction, internal polymer heating and material abrasion and fatigue to predict rope performance and life. This paper outlines the modulus modeling modifications recently made to FRM and describes how cyclic loading and its consequences are implemented into FRM. This paper also describes how the program has been validated using data from cyclic loading tests. A full assessment of rope axial stiffness must include the effects of cyclic loading. Cyclic tension generates loops in the rope tension-extension plot. The area enclosed in these loops represents work done on the rope, and results in damping and hysteresis. For polyester ropes these loops do not follow the static tension-extension curve but a steeper curve; the analogy for this steeper curve is the unloading curve for post yield loading of steel. This steeper curve results in axial stiffening or modulus modulation, which produces a higher rope modulus than predicted by the static model. FRM has been enhanced to use these material properties and to predict their effect on rope behaviour. These improvements now enable FRM to predict hysteretic damping, modulus stiffening and rope set caused by polymer residual strain. Further recent enhancements include the modelling of plaited and braided ropes, distortion and dilation of rope diameter due to bedding-in and creep strain.