The riddle of free-bending fatigue at end terminations to spiral strands

• The role of interlayer fretting in free-bending fatigue of strands is emphasised.
• The limitations of traditional maximum stress models are highlighted.
• Extensive large scale free-bending fatigue test data validates Raoof's model.
• Using the test data, the question of potential size effects is addressed.
• Possible limitations of the new S–N curve are critically addressed.

The importance of design against free-bending fatigue at points of end fixity to multilayered spiral strands used in various onshore and offshore structural applications is emphasised. A fairly extensive set of large scale test data covering a wide range of spiral strand construction details with the cables subjected to widely varying mean axial loads and bending amplitudes is used to critically examine the generality of the contact stress–slip versus fatigue life (S–N) model of Raoof which, unlike the traditional maximum bending stress models, takes the crucial effects of interlayer fretting on the strand free-bending fatigue life into account. The possible practical limitations of the Contact Stress–Slip approach are also discussed in some detail with much emphasis placed on the question of potential size effects associated with the free-bending design S–N curves.