Seems what old is new again... Some info about synthetic cabling. Dux and other plastic cabling will increasingly replace steel rigging into the future. This has been the case in other industries such as heavy lifts, cranes & logging. So no news there. A few things to clarify the above:
1) Creep is the condition in which a material elongations over time due to a constant stress. Its unlikely that rigging actually "creeps" as the termination distance does not change so the strain cannot increase to maintain the constant stress. If you hung a weight on a tree limb so the load (hence stress) was the same and the line could elongate over time then you would have a creep condition.
2) "Relaxation" however is the decrease in stress due to an applied strain and this is a condition our rig can experience. We strain a line to a set level and the line relaxes (gets longer) reducing its stress.
3) Mechanical setting is the process of the fibers bedding in and slightly reorganising themselves into the best geometry. Whether its metal or synthetic cables this occurs. Once set there is no more play in the system, this is primarily what preload does, it provides a load that allows the rig to "set" and once set it remains like that forever (technically). Its like old fashioned re-tensioning of head bolts, we are removing all the mechanical compliance in the assembly. Buy the way for those out there that do FEA work you do not need to pretension FEA rig models because this mechanical looseness does not exist in a CAD model, I digress. This is one reason that new splices in old rope fail early. We have upset the "set" of the rope while the old splices have had time to bed in, the new splice hasn't settled and its fibres are not load sharing as well as a splice that has small loads and some time to settle into its job.
4) Synthetic cabling - There are two types; crystalline fibres and none crystalline fibres. Dux & Kevlar are non crystalline whereas PBO, carbon and vectran are crystalline. Lets start with crystalline. When these fibres are made their crystal structure is fixed or "aligned" so does not mechanically set like the non crystalline fibres. Just like our rope, individual fibres have structure and things called orientation and entanglment. Crystalline fibres are highly oriented and entangled at birth and stay that way. Non crystalline fibres like DUX are disorientated and poorly entangled at "birth". To create DUX fibres they are heated and stretched to highly orient the fibre bundle, then they are held at temp to set the bundle. This process is not 100% efficient so there is scope for some of the fibres to relax when in use. Vecran/PBO/carbon do not relax or creep due to stress related to this internal reorganisation as there is none, whereas DUX types do. Crystalline fibres can relax or creep due to other effects but these are not usually present in sailing conditions. There is lots of data available to design these things out there. But a rule of thumb is that if you load Dux under 30% of its BS then relaxation and creep is minimised and you only have to deal with the mechanical set.
UV is high energy radiation and it attacks the bonds of plastics and breaks them down. PBO is particular UV vulnerable. DUX and Vectran take an initial degrade then stabilise. Only the outer fibres degrade so most of the fibres are fine.
A note about the loss of strength of your fibres in test. Its nearly impossible to estimate this accurately (as Franta says) the best way is to have keep a piece of pristine rope from the same roll and put it in the cupboard. Then when you test the old one you know the control rope and the test rope are identical as it can be. The 30% drop could be that that particular piece started life 30% below average.
Hope this helps and no one should be concerned using synthetic cabling on their boats, just read up on how to look after hemp ropes and its about the same.
Regards Peter S
