Fatigue in Textile Fibers

Abstract

A single-station machine on which a fiber can be fatigued in a localized region, by being axially rotated in a bent configuration, has been built. The test fiber, on either side of the bend, is mounted on shafts at 90° to each other, driven at the same rotary speed. Speeds up to 550 rpm are attainable. Nylon 66 and nylon 6 monofils in a number of diameters from 8 to 28 mils have been fatigued in biaxial rotation on this machine, in a fourfold range of rotary velocities and for periods of 15, 30, and 45 min. They were then tested for residual breaking tenacity and extension. A few fibers have been fatigued for other periods of time, or to rupture, for microscopic examination of the fatigued region. In most samples the rupture properties decrease with increasing rotary velocity, for any given fatiguing period. The rate of this decrease tends to be lower in the range of higher velocities. The shapes of the breaking-extension curves are largely determined by those of the breaking-tenacity curves, reflecting the near-linearity of the tenacity- extension relationship. The monofils of smaller diameter, especially those of nylon 6, are less affected by the increase in rotary velocity. In most of the samples, the residual breaking tenacity appears to be largely determined by the total number of revolutions sustained by the specimen, with no consistent influence of rotary velocity on the results. Monofils of the higher draw ratios and those that had been heat-set show a much sharper decline in residual properties with increasing rotary velocity than do the non- heat-set samples of lower draw ratio. However, the breaking tenacities of the heat-set samples retain their relatively higher positions after fatiguing under most conditions. Representative photomicrographs of monofils fatigued at various velocities and for different periods short of rupture show progressive development of diagonal fissures in the fatigued regions, especially in the nylon 66, and of a bulbous expansion in the nylon 6 specimens. Photomicrographs of the broken ends of monofils fatigued to rupture indi cate that the breakage in nylon 6 occurs across planes coinciding with the diagonal fissures. In nylon 66 the breakage appears to occur in a roughly transverse plane.