Sheave-bending and tensile fatigue of aramid-fiber strength members for communications cables

The influence of cyclic loading upon the residual strength of aramid-fiber strength members has been examined experimentally. Cyclic bending over sheaves, while under low tension, and tensile fatigue with low cyclic amplitude were studied. Residual strengths of the test specimens were measured as a function of number of cycles, and the tension and temperature during fatigue. It was found that the parallel-lay internal strength-member design suffered strength losses of up to 50% due to just 2000 bending cycles, despite using a very high sheave-to-strength member diameter ratio (D/d) of 120. Other internal strength-member designs shown to be more tolerant of bending fatigue were of braided construction and/or used low-modulus aramid fibers. The strength degradation of aramid-fiber external braids was demonstrated to be quite sensitive to braid angle (in the range of 12–15°), and was relatively unaffected by both temperature and tension during cyclic bending fatigue, under low tension (1–5% of break strength) with D/d=40. The tests conducted here established that aramid-fiber strength members are insensitive to tensile fatigue when the cyclic amplitude is a small fraction (2%) of the break strength.