Yarn strain

Stress relaxation studies were conducted on samples of nylon yarn at a constant strain of 2% and the following results were obtained ... 

As expected, the residual extensibility of the fiber decreases at higher draw ratios. What is not so predictable is that the true stress at failure increases as the draw ratio increases fiber failure strength is improved by drawing the yarn. If a curve is drawn to connect the end points of the stress-strain curves, it is seen that there is an inverse relationship between tenacity and elongation to break (eb). The form of this relationship is as follows ... 

Figure 13.6 (a) Elongation as a function of wind-up speed for partially oriented yarn, (b-d) Stress-strain curves of fibers of PET blends with 3% copolyester of 1,4-phenyleneterephthalate and p-oxybenzoate (CLOTH) and 3% copolymer of 6-oxy-2-naphthalene and p-oxybenzoate (CO), spun at 3500, 4000 and 4500 m/min (1) PET control (2) 3 % CLOTH (3) 3 % CO the loci of the theoretical extensions of the PET control are shown as dashed curves [17]. From Orientation suppression in fibers spun from melt blends, Brody, H., J. Appl. Polym. Sci., 31, 2753 (1986), copyright (1986 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc. 

In addition, the drawability of a spun yam can be measured in a similar way by increasing the draw ratio up to yarn break at an experimental drawing position. The optimal draw ratio can also be deduced from the stress-strain curve of the spun yam. 

The mechanical response of composites, as shown in these exploratory studies, indicates dependence on the ease with which fracture can occur between fibers, yarns, and plies. Poorly crystallized matrices result in composites that are strong and stiff but with little yield so that failure occurs catastrophically. In contrast, more crystalline matrices seem to be not quite as strong and to have a lower effective modulus, but their increased strain capability ensures that failure is not catastrophic the composited strength decays gradually as further strain is applied. Thus, the energy required for total failure is increased, and the composite with more crystalline matrix is more tolerant of defects or stress risers. 

FIG. 13.88 Diagram of the specific tenacity (ffb/p) versus the initial specific modulus (Ea/p) for conventional man made fibres. 0 is the limiting tangential slope in the stress-strain diagram for strain tending to zero. The diagonal lines show the indicated ffbr/ 0-ratio this varies from = 1 for elastomeric filaments and 0.2 for tyre yarns (ty) to 0.03 for yarns such as polyacrylonitrile. 

FIG. 13.94 Generalised stress-strain diagram for nylon 6.The tex is a standardised symbol for the linear density of a yarn 1 tex = 1 g per 1000 m. Divided by the density it is proportional to the cross-section of the yarn (after Herwig, 1970). 

FIG. 13.102 PpPTA yarn data showing the relation between tensile strength and initial modulus together with the curves calculated with Eq. (13.171) for five different values of critical shear strain, P from left to right P = 0.12, 0.115, 0.11, 0.105 and 0.10 rad used values are gd = 1.8 GPa and ecu = 240 GPa (see Table 13.18). Constructed from Figs. 22 and 23 in Northolt et al. (2005). The points above ( ) and below (O) the line for P — 0.11 are from different sets of PpPTA fibres. 

Figure 2. IR and tensile changes PmPiPA photodegradation. Xenon arc Weather-Ometer stress-strain data 44 tex, 2 ply yarn Nomex. A A, vacuum irradiated 0, air irradiated. Initial elongation at break, 30%. Initial tensile strength, 9.0 X 107 g cm"2. IR data 10p films. Trans, + IRS, x(dp = 0.55fi).

Figure 12.10 displays tire stress-strain behavior of PET fibers that were prepared from tire same spun yarn, but drawn to different ratios. The curves represent tire elongation and stress in terms of initial fiber area (decitex ). The open circles represent true stress values, where stress values at break are corrected for the decreased area of tire fiber after extension on tire testing device. 

Figure 26 Stress strain curve for 33 tex viscose yarn.

Figure 11.6 shows the typical stress-strain curve for either cellulose acetate or triacetate yarn [35]. The linear part of the curve under low stresses is not as steep as that for cotton or rayon. At about 0.9 g/den stress, the curve starts to show large increments of strain for very small increments of stress. This continues until the fiber breaks at about 1.36 g/den and 26% strain or elongation. Tenacities and elongations for various acetate and triacetate fibers may be slightly different, depending on the particular manufacturing conditions. 

Single filaments are merged into multifilaments and wound on a spool. These filaments may then be twisted to protect them from the successive textile processes or to give the multifilaments special properties, e.g. a desired stress/strain behaviour. The twist may vary between one and several hundred turns per metre. To compensate for weak spots in the yarns, two multifilaments may be twisted together to form a ply yarn. 

Smith JC, McCrackin EL, Schniefer HP. Stress-strain relationships in yarns subjected to rapid impact loading. Part 5 Wave propagation in long textile yams impacted transversely. Textil Res J 1956 28(4) 288-302. 

Yarn Setting temperature, °C held for 2.5 s Stretch, % Tenacity, Ntex Breaking strain, % 2% modulus, Ntex Shrinkage at 150 °C, %... 

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