The effect of wire break distribution on the breaking strength of a wire rope
The work presented in this paper was conducted with a view to assess the effect of the distribution of wire breaks on the breaking strength of a spiral strand rope. It has been previously noted that the loss in breaking strength of a rope with wire breaks can be more than the effective loss in metallic area and this is especially true if the breaks are asymmetric. A ratio "k" has been used to characterise this where a value of 1 means that the strength loss is exactly equivalent to metallic area loss and higher values of k mean that the strength loss is (proportionally) higher than the metallic area loss. A review has been made of previous work undertaken by Oplatka  and SIMRAC (the South African “Safety in Mines Research Advisory Committee”)  for various constructions of six strand rope. The results appear to show a value of k varying from 2 - 3 for increasingly severe levels of asymmetric damage, with a value closer to 1 for symmetric damage. In addition to the earlier studies, series of tests on artificially damaged six strand ropes have been undertaken and the results of these tests appear to be broadly in agreement with the previous work. A series of breaking tests have been made on two different constructions of wire strand with various levels of damage. An FE simulation has been developed for one construction and comparative measurements made on a strain gauged sample, undertaking load-unload tests for various levels of damage. The experimental results indicate that the k factor does not deviate significantly from 1 for all of the asymmetric configurations considered which was not as expected or predicted by the initial FE model. The strain gauge experiment provided some insight as to the reason for this behaviour. Tests on a single wire show a large level of plastic deformation before the wire fails (at 4%) and although significant asymmetric load sharing of wires is measured for lower loads, as the loads moves towards the rope break there is a convergence of wire load values which become much more evenly distributed at the point of ultimate failure. By re-running the FE model with the non-linear stress strain relationship it is possibly to partially simulate this behaviour although there is still a discrepancy, possibly caused by physical wire realignments taking place which have not been considered in the model. It is concluded that although k is a useful parameter for six strand rope, it is not an appropriate measure for determining strength loss in an asymmetrically damaged strand. However, there will be much more significant endurance loss caused by variations in wire load in standard operation load range of the rope. The use of an amplification factor to quantify this is proposed, with the suggestion that it could be incorporated into the rope discard criteria.
Author(s): J.M. Teissier, I.M.L. Ridge, J.J. Evans and M. Fournier