The Tensile Fatigue Behavior of a Warp Yarn and its Influence on Weaving Performance

Abstract

The tensile fatigue of a 50-50 viscose-polyester warp yam subjected to cyclic loading has been studied. Yarn loading conditions were selected to closely simulate those occurring on the warp during weaving. By examining the lifetime statistics as a function of gauge length, the existence of a "weakest-link" effect was confirmed, since both the lifetime level and standard deviation decreased with about the 1.2 power of the specimen length. From these data, it was also concluded that the "independence assumption" is valid under certain conditions. The three-parameter Weibull dis tribution provides an excellent fit of the observations, while the normal, logarithmic-normal, and first asymptotic dis tributions fit only within the central region of the distributions. The parameters of the Weibull distribution for the fatigue data obtained at varying strain levels were determined using five different estimation techniques. Only one of the methods produced results which were physically plausible in every case. When using this method, the estimated minimum lifetime for samples strained 2.0% or higher was approximately zero, while a non-zero minimum lifetime was inferred for all cyclic strains below 2.0%. Empirical models relating each of the three parameters of the Weibull dis tribution to the cyclic strain level were developed. From these models, curves were obtained showing the relationships between probability of survival, strain amplitude, and cycles to failure. Over the range of conditions studied, it was not possible to observe an endurance limit (a strain below which a given fraction of the specimens can be expected to survive an indefinitely large number of cycles). An endurance limit was apparently being approached, however, when the cyclic strain amplitude was reduced below the elastic limit of the yarn. The results indicate that, in the case ex amined here, tensile fatigue is not an important cause of yarn failure during weaving.