This stretch occurs in three phases:
1) Initial or constructional stretch during the early period (Run-In) of rope service, caused by the rope adjusting to the operating conditions.
2) Following the run-in period there there is a extended period – the longest part of the ropes’s service life – during which a slight increase in stretch takes place over an extended time. This results from normal wear, fatigue etc. On a graph this portion would almost be a horizontal straight line inclined slightly upward from its initial level.
3) Thereafter, the stretch occurs at a quicker rate. This means that the rope has reached the point of rapid degradation; a result of prolonged subjection to abrasive wear, fatigue, and inner undetected wire breaks, etc. This second upturn of the curve is a warning indicating that the rope should be removed to avoid sudden catastrophic rope failures.
Elastic Stretch / Elastic Limit
In many instances it is not easy to clearly distinguish between (the remains of) constructional stretch and elastic stretch as they may occur together especially when the rope is new. The values for Elastic Stretch are dependent on rope construction, lay length and type, steel material, tensile strength of wires etc. An approximation is 0.25% to 0.6% at WLL (or lifting capacity). The E-module varies similarily from about 11 Million to 16 Million lbs/inch2. For exact values please contact us for further information.
Elastic stretch turns into a ‘permanent’ stretch when the rope is loaded beyond 55%-60% of its breaking strength (or beyond 2-1/2 to 3 times its WLL). At that point the steel material elongates and deforms permanently and renders the rope inoperable as the individual wires will have lost much of their mechanical properties to withstand material fatigue.