Low cycle flex fatigue of nickel cobalt chromium molybdenum medical cables

Cables comprised of seven 0.001 inch diameter Ni-C-Cr-Mo /Ag DFT (Drawn Filled Tube) wires were subject to fully reversed bending fatigue using a bellmouth bend fixture with a bellmouth radius of 0.0625 inches and a range of ±60°. The cables were tensioned using weights ranging from 0.025 lbf to 0.150 lbf. Three sets of bellmouth fixturing were used, each with a different relative height between the axis of rotation and the position of the bellmouth. Our tests examined two questions. While less cable tensioning during fully reversed bend fatigue generally equates to less damage, a sufficiently small cable tension during fully reversed bending might reverse the trend and adversely affect fatigue endurance due to a phenomenon in which some of the cable strands undergo compression and consequently flare out. The test results suggest that the trend does not reverse at low tensions. Secondly, we examined whether small changes in the relative height of the bellmouth significantly impact fatigue life. The rotation of the bellmouth produces a small vertical translation of the wire, which in turn produces a vertical translation, and thus acceleration, of the tensioning weight. The inertia of the tensioning weight produces a cyclical stress on the cable. Changes in the relative height of the bellmouth may lead to changes in the amplitude of this cyclical stress. The test data showed no significant connection between the relative height and fatigue life. The test results show that the fatigue life of the cables is highly dependent on the mass of the tensioning weight, with larger weights leading to shorter fatigue life. An analysis of the cumulative stress, using Miner's Law for cumulative stress and data from the literature for Ni-C-Cr-Mo wire fatigue, revealed that the cyclical stress produced by the weights was a significant, and in some instances leading contributor to fatigue failure. The Von Mises stress was calculated on the inside and outside of the bend combining the bending, inertial, static, and contact stresses.