Free‐Bending Fatigue Life Estimation of Cables at Points of Fixity

Abstract

The shortcomings of the traditional design approaches based on the extreme fiber (in terms of simple beam theory) direct wire stresses as the factor controlling strand restrained bending fatigue performance, have been resolved. A new parameter has been proposed, which largely overcomes these limitations and is capable of predicting free‐bending fatigue life of axially preloaded spiral strands clamped at the end. The model explains some experimental observations, which have found the initial wire fractures at the fixed ends being invariably located at the so‐called neutral axis rather than (as might be expected), at the extreme fiber position. The theory attributes the occurrence of interwire‐interlayer fractures to the fretting fatigue phenomenon, and takes interwire friction fully into account. The new model is found to give very encouraging correlations with available experimental fatigue data on some substantial spiral strands. The theory also demonstrates the possibility of substantial reductions in sheathed spiral strand free‐bending fatigue life in deep‐water applications. In the past, such effects were invariably ignored in the design of mooring systems in, say, deep‐water guyed‐tower platform applications. By employing practical simplifying assumptions, a similar parameter for estimating the free‐bending fatigue life of multistrand ropes, in the vicinity of end terminations, has also been proposed.