Bhuvenesh C. Goswami

Tensile Fatigue Behavior of Staple Yarns

The failure of warp yarns on a loom is often caused by repeated cyclical elongation at small stresses well below the breaking point applied under static load. The phenom enon, commonly known as fatigue, is caused by the gradually diminishing resistance of the material, attributable to cumulative damage. Generally, there is no prior indi cation of impending failure due to fatigue. In this work, we have studied the fatigue behavior of warp yarn under cyclical elongation accompanied by abrasion action as measured on a Sulzer-Ruti Webtester on the basis of three criteria—failure, damage rate, and visual appearance. Fatigue behavior expressed in terms of cycles at failure exhibits a wide scatter, displaying a pattern that deviates from normal distribution. A three-parameter Weibull distribution fitted to experimental results consistently displays a unimodal pattern. The rate of fatigue damage expressed in terms of loss in tensile property indicators has proven useful in assessing the fatigue-sustaining capacity of yarn and thereby predicting impending failure. SEM photographs display the abrasion fatigue damage inflicted on the fine structure of yarns and fibers. We have systematically studied, the effect of fatigue parameters that characterize the intensity of fatigue action, such as base tension, speed of fatiguing, abrasion pin position, and strain amplitude. Increasing intensity of fatigue action results in rapid yarn deterioration. Base tension and abrasion pin deflection have profound effects on fatigue. Fatiguing speed and strain amplitude tend to accelerate yarn deterioration, though the experimental results show a slower rate but greater variability. Representing fatigue data based on failure criterion in terms only of characteristic lifetime, which is one of the three parameters of the Weibull distribution, is not enough. A representation in terms of complete distribution yields more useful information about the extent of dispersion and the extreme value in the experimental results.

STEADY-STATE UNWINDING OF YARN FROM CYLINDRICAL PACKAGES

Over-end unwinding involves pulling yam from a fixed package to be used in textile processes such as knitting, sewing, twisting, weft insertion in weaving, and doubling. During unwinding, the yam slides on the package surface and lifts off into the balloon formed between the package and the guide eyelet. Variations in yam tension and bal loon shape can adversely affect process efficiency and final product quality. This paper investigates the steady-state motion of inextensible yam during unwinding. Previously developed models that predict the yam tension and motion in the unwinding balloon and on the package surface are modified to include the effects of nonzero wind angle packages and to allow comparison with experimental results. In the experiment, PC controlled video, tension, speed, and yam rotation sensors measure nine dynamic un winding process variables. Synchronized tension measurements and video images allow simultaneous measurement of eyelet and lift-off tension, balloon height, and lift-off and...Over-end unwinding involves pulling yam from a fixed package to be used in textile processes such as knitting, sewing, twisting, weft insertion in weaving, and doubling. During unwinding, the yam slides on the package surface and lifts off into the balloon formed between the package and the guide eyelet. Variations in yam tension and bal loon shape can adversely affect process efficiency and final product quality. This paper investigates the steady-state motion of inextensible yam during unwinding. Previously developed models that predict the yam tension and motion in the unwinding balloon and on the package surface are modified to include the effects of nonzero wind angle packages and to allow comparison with experimental results. In the experiment, PC controlled video, tension, speed, and yam rotation sensors measure nine dynamic un winding process variables. Synchronized tension measurements and video images allow simultaneous measurement of eyelet and lift-off tension, balloon height, and lift-off and unwind height and angles. The optical balloon rotation sensor measures balloon rotation rate and balloon shape. In agreement with the theory, increased balloon height and decreased air drag lead to higher tension. Multiple loop balloons have lower tension and occur at larger balloon heights. Yam tension during forward unwinding (i.e., lift off point moving toward the guide eye) is significantly smaller than in backward un winding due to increased balloon rotation rate. A higher unwinding tension and liftoff angle result in a longer yam sliding path on the package surface.

The Hairiness of Cotton Yarns An Improvement over the Existing Microscopic Technique1

The hairiness of cotton yarns made from three different cottons with varying staple length and fineness and spun under varying spinning conditions has been studied. The parameter adopted to characterize the hairiness of yarns was N1L1—the cumulative protruding length in 1-mm length of yarn. A new technique—a yarn-clamping and -rotating device—was used to count the number of protruding ends N1, and measure the corresponding protruding lengths L1. This technique helps to count the fiber ends more accurately than other methods. The technique involves rotating the yarn and thus bringing the fiber end projecting into a plane perpendicular to the screen and also exposing the actual protruding length of the fiber in the plane of the screen. This device thus overcomes the possibility of a fiber being excluded from the count, which would otherwise be hidden by the yarn shadow and by the tangled appearance of the yarn edges. The results obtained have been found to be fairly accurate and reproducible and agree with the subjective judgment of hairiness of yarns.

The Concept and Measurement of Extinguishability as a Flammability Criterion

Burning experiments in enclosed spaces have shown that oxygen depletion leading to flame extinction occurs even in a relatively large volume of air, because convection is only partially effective in supplying air to a burning flame and removing products of combustion and pyrolysis. Quantitative evaluation of extinguishability through oxygen depletion determined under realistic burning conditions would, therefore, be a valuable indication of the hazard potential of a material. Accordingly, a method has been developed for establishing a characteristiol minimum-burning condition in terms of oxygen concentration which results in a more stringent criterion for the innate flammability of a material than the conventional oxygen index (O.I.) value. The TRI Flammability Analyzer is used to measure steady-state flame propagation rates at several oxygen concentrations high enough to support steady burning. Extrapolating the resultant linear burning rate oxygen concentration relation to zero burning rate-yields an intrinsie (O.I.)0. With the analyzer. it is possible to determine intrinsic indices for the more common and vigorous upward directions of burning, and the upward indices obtained for a group of natural and synthetic fabrics, woven and knitted, are apprediably lower than the indices for the downward direcion. Indeed, all fabrics studied. except Nomex®. are found to be inherently capable of upward burning in air.

Failure Mechanism in Staple Yarns

The failure mechanism in staple yarns is strongly influenced by yarn structure. Manufacturing methods impose certain constraints on the disposition and distribution of fibers in the yarn cross section. This paper investigates the failure mechanism in ring, rotor, air-jet, and vortex yarns. The yarns are subjected to uniaxial loading on a tensile tester, and images of the yarns before and after breaking are recorded. Image analysis of the failure regions yields some interesting features and reveals typical mechanisms occurring in different yarn structures. The failure mechanism in each yarn type is discussed in terms of some basic parameters characteristic of the structure.

Characterizing Lateral Contraction Behavior of Spunbonded Nonwovens During Uniaxial Tensile Deformation

An experimental method based on video recording and image analysis is presented to characterize the lateral contraction of spunbonded nonwoven fabrics during uniaxial tensile deformation. A systematic study reports the effect of specimen dimensions on the nature of lateral contraction of a nonwoven fabric. Lateral contraction measure ments are made over the entire profile of the specimen, thus examining the validity of Saint Venants principle to justify the assumption of homogeneous strain in the region of measurement. Some important observations are reported, which in future will influence the methodology of measuring transverse strains corresponding to applied longitudinal strains in a uniaxial test.

Structure-Property Relationship of Blended Cotton Yarns Made from Low and High Tenacity Fibers

The tensile response of staple yams is usually determined by their structure and the mechanical properties of their constituent fibers. Yarn structure is influenced by the radial disposition of fibers along the yam length, commonly known as migration, and the packing density of the fibers in the yarn cross section. These, in turn, are affected by fiber properties, yarn factors, and the dynamics of preparatory and spinning pro cesses. Yarn structure development is further complicated when the yarn is spun from blends of different fiber types because the blending method usually profoundly influ ences the position and orientation of the fibers in the yam matrix. This investigation is concerned with the effect of fiber properties and yarn structure on the tensile properties of ring spun yarns made from a 50:50 blend of high (Pima) and low (Upland) tenacity cotton fibers spun using intimate and drawframe blending techniques. The drawframe blended yarn in this work is produced by arranging all high tenacity fiber slivers in the center of the drawframe creel to determine if preferential positioning in the core of the yam helps to improve the strength of drawframe blended yarn compared with intimate blended yarns. The findings reveal that intimate blended yam has better tensile strength than drawframe blended yarn, even though more high strength Pima cotton fibers are positioned in the core of the yam. This is because non-uniform fiber distribution re sulting from blending of different fibers affects the migratory behavior of fibers. The tensile failure of such yams is governed by the mode of fiber breakage and fiber slippage as determined by the yarn structure—fiber distribution and migration—rather than predominantly by fiber breakage compared with strength.

Predicting the Biaxial Tensile Deformation Behavior of Spunbonded Nonwovens

An analysis is presented to predict the biaxial tensile behavior of spun-bonded non woven fabrics using knowledge of the stress-strain behavior of their constituent fibers and their orientation angle distribution. Nonlinearity in fiber stress-strain behavior is considered in order to predict fabric stresses in the longitudinal and transverse direc tions under biaxial loading conditions. Only a special case of biaxial loading is con sidered in which the transverse strain is forced to be zero, i.e., strip biaxial loading conditions. Experimental results are obtained by evaluating four spunbonded fabrics on a Kawabata strip biaxial tester KES-G2 and are compared with the theoretical results.

Tensile Properties and Static Fatigue Behavior of Cotton Warp Yams

Warp yams under a state of dynamic loading on a loom have to withstand cumulative damage caused by tensile fatigue at relatively low loads coupled with the abrasive action of the machine parts. To evaluate the fatigue phenomenon of sized cotton yams and its correlation with performance on a loom, one must first understand basic mechanical properties of yams in terms of fiber properties and fiber length distribution. The first section of this paper reports on the effect of fiber length distribution on tensile properties of yams at various tensile strain rates, and the second section presents the . fatigue behavior of 100% cotton yams. Fiber length and its distribution in cotton yams seem to influence the tensile behavior of both sized and unsized yams at all strain rates, but strain rates do not play a significant role in determining the properties of sized yams. Adding size tends to mask the effect of fiber length variation, particularly at higher size loading. The tensile properties of sized cotton yams are also influenced by the size type, such as polyester, carboxymethyl cellulose, and polyvinyl alcohol, at all add-on levels. The fatigue behavior of 100% cotton unsized and sized yam is eval uated using a Sulzer-Ruti Webtester. The mechanism of yam failure due to fatigue/ abrasion for unsized yam is affected by short fiber content. The base tension has the largest overall effect. Unsized yam containing 10% comber noil has an optimum bal ance of short and long fibers, making the yam structure more resistant to fatigue / abrasion. In sized yam, the effect of short fibers is partly masked by the properties and compatibility of size materials with the substrate. Polyester size has poor com patibility with cotton fibers; carboxymethyl cellulose and partially hydrolyzed polyvinyl alcohol tend to produce very highly increased resistance to fatigue and abrasion. Rep resenting fatigue /abrasion data in terms of mean or average lifetimes is questionable due to contradictory evidence. Representing fatigue data in terms of a three-parameter Weibull distribution leads to realistic interpretations.

The Effects of Moisture on the Flammability Characteristics of Textile Materials

The TRI Flammability Analyzer has been used to determine the effects of moisture on the flammability characteristics of textile materials. Both moisture in the fabric and in the test environment have been considered for a number of un treated and flame-retardant-treated fabrics. The results are reported and interpreted in terms of several measurable quantities: mass transformation rate (MTR), oxygen sensitivity (ΔMTR/Δ% O2), and extrapolated oxygen index (O. I.)0. Variations in fabric moisture content appear to have a greater effect on the behavior of relatively hydrophobic fibers (such as polyester), as compared to cotton or nylon.