Modulus of elasticity The modulus of elasticity of a material is defined as the proportional factor between load and elongation.
The modulus of elasticity is a material property. Besides the elastic properties of the wire material used, the modulus of elasticity of wire ropes is dependent on the rope geometry and the load history of the rope. Since this is not a material property, ISO 12076 recommends calling this factor the “rope modulus”.
Figure 25 shows a load-elongation diagram of a wire. Here the modulus of elasticity can be determined as the gradient of the curve in the linear area.
Figure 26 shows a load-elongation diagram of a strand. As the strand consists of several wires of different lengths and different lay lengths or different lay angels, here the shorter and less elastic elements get loaded first. For this reason the curve is not linear in the lower area. The strand only gets linear, when all the wires in the strand bear the load together.
Figure 27 shows the load-elongation diagram of a rope. Here you will also find a non-linear correlation in the lower area between load and elongation. Here again the nonlinearity explains itself by the overload of the shorter and the less elastic rope elements. The load-elongation diagram is linear in the area in which all elements share the load and do not yet flow. As a consequence of settling effects, the modulus of elasticity of wire ropes increases over the lifetime. The biggest part of this change happens with the first loading of the rope. Later the modulus of elasticity varies only very slightly. For this reason a new wire rope should always be loaded and relived multiple times before measuring the modulus of elasticity. The determination of the modulus of elasticity is described in ISO 12076.
Figure 27: Load-elongation diagram of a rope
Radial stability
The radial stability of a rope is a function of the rope geometry and the line pull. The radial stability of a steel wire rope will typically reduce with an increasing number of rope elements. It will also increase with increasing line pull. Ropes with insufficient radial stability are not suitable for multi-layer spooling.
Structural stability
It is essential that a rope maintains its structure during its working life. Adding a plastic layer between the IWRC and the outer strands can increase the stability of the rope structure. The plastic will fix the position of the rope elements relative to each other.
Diameter reduction of a special wire rope
With increasing line pull, a special wire rope will not only get longer, it will also reduce in diameter. A great part of that diameter reduction is reversible which means that the rope diameter will increase again after unloading. Part of the diameter reduction, however, is permanent. If the diameter reduction of a steel wire rope under load is too high, in multi-layer spooling the rope might pull into deeper layers of the drum. Therefore the diameter reduction of steel wire ropes must be considered when designing ropes for multilayer applications (figure 28).
Figure 28: Diameter reduction under load