Impact of strain aging kinetics on the failure of thin steel wire ropes

Under quasi-static loading an irregular failure pattern of high strength thin carbon steel cords were observed after low temperature thermal aging. Character and kinetics of damage in such wire ropes highly depend on the plastic elongation of the steel wires, which is significantly modified by the strain aging effect. Therefore, static strain aging effect on heavily drawn high-carbon steel wires and their cords was experimentally studied in the 100 -200°C temperature range.

The investigated cords had been designed with precisely optimized material as well as stranding parameters in order to possess the highest breaking load with the maximum possible elongation. These are equally important properties in their specific field of application as main reinforcement of high pressure oil&gas rubber hoses.

In case of cords of more complex structures serious loss of balance was found under heavy loading revealed by an adverse failure mode in the inelastic deformation regime. Finally, this instability caused significant drop in effective strain capacity as well as in ultimate strength. This unfavourable effect takes place as the thread experiences heat aging under 200°C for time periods typically less than 1 hour.

The phenomenon was investigated and explained in detail based on the different post-drawn strain aging behaviour of the employed thin pearlitic steel wire filaments. In order to gain reliable information about the material performance in aged condition comprehensive kinetic characterisation was given. Constants were determined for both the relevant Avrami and diffusion models and applicability of them was discussed.

These results directly make aging induced failure mode prediction possible and can also give a basis for structural optimization of wire ropes to avoid irregular breaking. The investigated aging temperature range covers the typical vulcanization condition of the mentioned rubber hoses, so the industrial importance of the present study is also highlighted.