Technologies : Yarn Failure in Woven Fabric Mechanical Properties of Fabric Woven from Yarns Produced by Different Spinning

A study has been conducted on the mechanisms of in-situ tensile failure of staple yams during uniaxial tensioning, as in a conventional ravel strip test. The yarns were PET/cotton blends processed on ring, rotor, and airjet spinning systems, and then woven into plain or twill weave fabrics. Load-extension behaviors of the yarns were recorded for the in-fabric state as well as for the free state (out-of-fabric), and SEM comparisons were made of the fractured yam ends obtained in the two states. When the tensioned yarns became jammed between cross yarns before straightening, the fracture ends were abrupt, similar to those observed in near zero gauge length tests of free-state yarns. However, when fabric structure was such that tensioned yams could straighten without cross yam jamming, the resulting failure zones were considerably longer, with a mixture of fiber fracture and slippage similar to that observed in long gauge length tests of free-state yams. The interaction between yarn properties and weave geometry had a strong influence on the local disturbance of cloth structure resulting from isolated yam failure during fabric tensioning. The extent of such disturbance permitted estimates of the stress recovery length of the failed yam and showed its dependence on cloth tightness and yarn type. In many cases, a woven fabric exhibits a much higher strength than that predicted from the strengths of its constituent yarns tested at the same gauge length [ 5, 6, 12 ] . Such discrepancies have been termed fabric assistance. Lord and Radhakrishnaiah [ 5, 6 ] discussed fabric assistance for different yarn systems, showing relatively higher assistance for friction and rotor spun yarns than for ring spun yarns of comparable size. They associated this with contact pressures at yarn crossovers, explaining that yarns in denser fabrics experience closer contact pressure zones along their length, and so there is less chance for in-fabric yarn failure than in less dense fabrics. In providing further explanations of such fabric assistance, Shahpurwala and Schwartz [ 12 have defined the term &dquo;overload&dquo; or &dquo;subbundle&dquo; length to represent a short segment of yarn possessing the appropriate mean strength and strength distribution to permit valid prediction of fabric strength. In practice, they first determined the mean strength and strength distribution of yarns removed from the fabric and tested at a 152.4 mm gauge length. Then, using weakest liak theory with equal load sharing and with local load sharing n~les, they scaled down to the appropriate subbundle length to match fabric strength, also measured at a 152.4 mm gauge length. Then they used the subbundle length in a statistical model of fabric strength in a manner analogous to that employed in composite materials models. The subbundle lengths for the cotton yarn fabrics in equal-load and local-load sharing cases were thus estimated to be less than 17 mm, shorter than the average cotton staple length of the yarn used in the test fabrics. These quantities, it should be noted, were based solely on statistical considerations, i. e. , weakest link theory, not micromechanical analysis. Shahyww+ and Schwartz [ 12 ] further illustrated fabric assistance by defining a friction factor, which combined yarn size, 1 This paper was presented at the December 10, 1991 Technical Conference of The Fiber Society in New Orleans, LA. 2 Present address: Konkuk University, Seoul, Korea. 3 Present address: Georgia Institute of Technology, Atlanta, GA. 4 Present address: University of California, Davis, CA. 5 Present address: Cornell University, Ithaca, NY. © 1993 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution. at CALIFORNIA DIGITAL LIBRARY on November 12, 2007 http://trj.sagepub.com Downloaded from