Elongation and elasticity

When a wire rope is subjected to a load it will stretch or elongate. This elongation is derived from two sources: constructional and elastic.

Constructional stretch:

When a load is applied, the helically laid wires in the strands and the strands around the core constrict slightly resulting in a slight reduction in diameter and some lengthening of the rope. The amount of constructional stretch is influenced by the following factors:

  • Type of core (WSC, IWRC, FC, etc.)
  • Rope construction (6x7, 6x19, 6x36, 8x25, Etc.)
  • Lay and direction (ordinary \ lang lay, lay length).
  • Material strength (grade).
  • The load applied.

Ropes with steel core (WSC, IWRC) have less constructional stretch than fiber core ropes (FC). Because the constructional stretch is influenced by many factors, it is not possible to determine a definite value but the following table gives an approximation:

 

 

Ropes with 6 strands and IWRC ¼% ÷ ½%
Ropes with 6 strands and FC & rotation resistant ropes ½% ÷ ¾%
Ropes with 8 strands and FC ¾% ÷ 1%

 

The constructional stretch is not recoverable. If the load is removed the rope will not "shrink" back to its former length. With steel core ropes the constructional stretch will stabilize after a relatively short time but with lightly loaded fiber core ropes this may take a considerable part of the rope's life.

 

Elastic stretch:

The elastic stretch is the amount of recoverable elongation of the rope due only to the load applied. When the load is increased it will stretch more and if the load is decreased it will shorten. A wire rope is not a homogenous material so it doesn't have a proper modulus of elasticity. The load\elongation curve of a wire rope is not linear up to an elastic limit.

After the constructional stretch has ceased, it is possible to calculate an approximation for the elastic stretch using a value of E≈60000 N/mm² (3,978,000 psi). The cross section should be taken as the area of a circle of the nominal rope diameter. If a more exact figure is required it is possible to test samples from the rope to obtain the actual load\elongation curve from which the actual modulus of elasticity is calculated.