Test Method for Determining the 6 Change-In-Length Properties of Fibre Ropes

The change in length characteristics of polyester rope, and probably other synthetic fibre ropes, can be completely described by the following 6 properties.

• Original stiffness represents tension VS. strain during the first lime a rope is loaded to a particular tension.

• Static stiffness represents tension vs. strain during subsequenl loadings to that particular tension.

• Dynamic stiffness represents tension vs. strain as the rope is repeatedly cycled.

• Construction Strain is a function of the highest load that has been applied to the rope and is permanent unless the rope is completely relaxed and flexed.

• Polymer strain is related to the properties of the polymer, is permanent, and increases at a diminishing rate with lime (except for HMPE).

• Wo/1(ing strain takes place when the constant tension or mean cydic tension increases and contracts when the constant tension or mean cyclic tension decreases, and thus is temporary.

This paper will describe the test method for determining these 6 fibre rope change-in length properties and for calculating the properties from the test results. It will indude examples of the application of these properties in mooring system design. The 6 change-in-Iength test method was developed in the DNV industry-sponsored JIP "Improving Fiber Mooring Design Practices". Several important characteristics of fiber rope change-in-Iength properties were discovered or confirmed. Some important findings simplified the test procedures and reduced the necessary number of tests. The measured properties apply to the fibre rope material and construction and are generally independent of rope size and strength. The properties are not unique to a particular pattern of rope loading. Thus the tests need only be performed once for a particular rope design. They can be scaled to different rope sizes and can be used to predict rope performance under any tension condition and history. Thus the tests need only be performed once for a particular rope design.

Author:  J.F. Flory