The Torsional Behaviour of Singles Yarns. Part II: Evaluation

Abstract

A theoretical analysis has been developed to gain an insight into the torsional behaviour of singles yams, which may be bulky and which may have non-uniform fibre-packlng-deasity distribution. In this paper, a detailed experimental evaluation of the theory for two different cases of yam torsion is reported whereby its predictions of a torque-twist relationship are compared with results obtained for woollen-spun carpet yarns spun at various twist levels. Also presented are typical theoretical results ofthe relative contributions to yarn torque due to fibre tension, fibre-bending, and fibre torsion and of the variations of both fibre-tonsile-strain distribution in the yam and yam axial and lateral dimensions wbich lake place as tbe yam is further twisted.The evaluation shows that, as the yam is twisted at fixed length, it jams progressively from the inside to the outside. Initially the outer fibres remain unstrained. In the theory of Postle et aL, these outer fibres are assumed to carry a real tensile strain. They have a large helix angle and thus a large component of tension-creating torque. This is the main reason why earlier theories of yarn torque have given predictions much higher than the values observed experimentally. In general terms, the new theory gives reasonable predictions of the observed torque behaviour of woollen carpet yarns, given the precision limitations with such a material.Most interestingly, the theory clearly predicts that yams statically twisted at constant load will become hollow in the centre -a feature which has often been reported in yam-crosssectional experiments.

Mechanical Modeling of Singles Yarn

“…Figure 10 illustrates that for a comparatively small given pre-tension fiber torsion contributed the most to the total yarn torque and the fiber bending played an important role as well. This observation is different from that of Tandon et al [18] in which the author stated that the contributions to yarn torque due to fiber bending and torsion are negligible. With the increasing of the yarn twist the contributions to yarn torque due to fiber bending and fiber tension become more and more important.…”

“…In contrast, all three contributions were significant and important in the torsional model when the pre-tension was comparatively small. This finding is different from that of Tandon et al [18] in which it was concluded that the contributions of both fiber bending and torsion to yarn torque were negligible.…”

“…The energy method which has a much simpler form was first proposed by Treloar and Riding [14]. Together with the shortest-path hypothesis and discrete-fiber-modeling principles [15], it was used to investigate the tensile behavior and torsional properties of staplefiber yarns [16,17,18]. However, most of the predictions were unsatisfactory in terms of accuracy.…”

On the theory of localised snarling instabilities in false-twist yarn processes

“…These formulae are also used below to estimate this order of magnitude. Data given in [13], and more recently in [14], show that the ratio K /B given above is in approximate agreement with their data for textile yarns.…”

Application of micro-computed tomography (micro-CT) to study unevenness of the structure of yarns

“…However, the verification of the models of the density distribution of fibers in the yarn cross-section proved the existence of significant differences between the experimental results and the implementation of the model. Tandon et al 16 described a torsion model for yarn, applying the so-called ''energetic method'' to characterize the phenomenon of migration of staple fibers in the yarn (calculation of the work required to stretch the yarn, bend it, and twist) under conditions of uneven distribution of fibers in the yarn's cross-section.…”