Bending and Torsional Fatigue of Nylon 66 Monofilaments

Abstract

AbstractUsing newly developed test equipment, the fatigue behavior of nylon 66 monofilaments was studied under two loading conditions, pure bending or simple torsion. In each case, the deformation was applied in balanced cycles of positive and negative strains. For each mode, results are expressed in terms of the measured decay in stiffness with numbers of cycles over a range of maximum applied strain levels. Fatigue lifetimes are presented in S-N format where the log number of cycles of fatigue for a 40% decay in stiffness (N) is plotted as a function of applied strain (S). The failure mechanism for these fibers in each fatigue mode reflects the morphology of semicrystalline oriented synthetic fibers. In torsion, many longitudinal cracks formed around the perimeter of the fiber as the result of cleavage of the relatively weak interfibrillar bonds in the nylon 66 fibers. In bending, cracks grew across the fiber at an oblique angle to the fiber axis along kink band boundaries.