A series of low temperature destruction tests by Bridon1 on galvanised and bright, 6x36 IWRC, RHO, 1770 grade, 16mm ropes exhibited consistent increases in breaking strength, typically between 5.5% to 5.9% at -50°C. This testing data showed no evidence of unusual fracture characteristics. This finding is similar to tests on multi-strand ropes reported by Buschmann show increased breaking load when testing ropes at temperatures between -90° and -120°C.
There is only limited published physical property data to reliably quantify the behavior of individual carbon-steel wires at low temperatures. However, the data consistently shows the following differences at -60°C compared to properties measured at +20°C:
a. an increase breaking strength
b. a drop in relative ductility
c. the ductile-brittle transition temperature increases with carbon content
d. cold-drawn steel wires have a fine grain size which reduces the ductile-brittle transition at sub-zero temperatures.
Research conducted by Boleantu and Babeu on reverse bends and torsion tests on individual carbon-steel wires did not show a brittle transition at test temperatures of -60°C. However, it is reasonable to assume that when a wire is elastically or plastically deformed in any mechanical test the heat generated will increase the temperature of the test sample, as it would when working in a rope.
In plain carbon steels, the notch sensitivity increases at reduced temperatures4. While notch sensitivity and low impact resistance can be a problem in steel plates at low temperatures to date no correlation has been found with strands formed from steel wires in a rope.
Notch sensitivity is a characteristic that is not usually associated with wire and wire ropes for the following reasons:
• no sharp notches are present on the wire surface after the manufacturing process
• the rigidity of each steel unit is small
• because each wire is a single element the failure of a wire does not create a stress concentration in the adjacent wires giving no propagation of the fracture.
Consequently, it is important to prevent mechanical handling and service damage to the wire surface which can act as notches e.g. through de-icing.
Attempts at British Ropes (Bridon) to complete bend fatigue tests on wire ropes at temperatures below 0°C have been unsuccessful because it was not possible to maintain
testing temperatures. This was attributed to the friction between the rope and the capstans and internal friction in the wire rope combining to heat the test sample and raise its temperature. Another important observation from the dynamic tests was the behaviour of the lubricant.
It is essential to use suitable rope lubricant compatible with sub-zero applications. Wire rope lubricants can become ineffective at low temperatures due to flaking or cracking resulting in the rope operating similar to a dry rope. This can reduce the fatigue life significantly. Bridon can lubricate ropes with Brilube 8 or Brilube 16, which operate up to -50°C. Bridon also offers a novel Bristar polymer core, called Bristar Polar, which is designed to support the rope at -50°C.