Indian Journal of Textile ResearchVol. 13, December 1988, Pp. 198-203

Use of Tensile Strength in Characterizing Degradative Behaviour of Grey andSized Yarn on Repetitive Stresses in Weaving

SKAGGARWALAhmedabad Textile Industry's Research Association, Ahmedabad 380015, India

andP K HARI

Department of Textile Technology. Indian Institute of Technology, New Delhi 110 016, India

Received 23 November 1987; accepted 23 June 1988

On cyclic extension and abrasive stresses a low CSP yam deteriorates in terms of its tensile and abrasion propertiesat a faster rate than a high CSP yam. The degradative stresses inherent in the weaving process lead to the creation of newweak places in the yarn and thereby increase the variability in tensile strength, elongation and modulus. The differencesin the performance of yarns have been explained on the basis of their structural features. These reasons explain as to whya low CSP yam is more susceptible to break in weaving than a high CSP yarn.

Keywords: Abrasion resistance, Inter-fibre slippage, Weaving stresses, Tensile strength, Yarn degradation, Weaving

1 IntroductionThe weavability of a yarn is generally assessed in

terms of warp breakage rate in weaving. Aggarwaland Subramanian I have shown recently that there isa very close correlation between warp breakage rateand grey yarn quality characteristics, such as CSPand the frequency of thin and thick places in yarn.This relationship was explained on the fact that theCSP is a measure of the strength of low-extensionthreads in a warp yarn and that the capacity of yarnto withstand the weaving stresses is well correlatedwith CSp2. The work reported in the present paperfurther substantiates the merit of CSP as a weavabil-ity index.

In the process of weaving, on account of repetitivestresses, any yarn degrades continuously in tensile,abrasion and hairiness properties in its passage fromwarp beam to cloth fell. Data.are presented to showthat CSP is a good index of this degradation of greyand sized yarns subjected to simulated weavingstresses on the Reutlinger web tester.

2 Materials and MethodsThree samples of cotton ring-spun yarn having

same linear density (15 tex) and twist factor (41) butwidely ranging tensile strength (CSP) were preparedby using suitable fibre mixings in ATIRA pilot mill.These yarns were sized to 12% add-on with thinboiling starch on a laboratory sizing machine. Theyarn properties are given in Table 1.

198

Table I-Properties of Grey and SizedYarn

Property Yarn

A B C

Grey yarnActual count (Ne) 40.2 39.6 41.0Lea-count strength product (CSP) 1788 2181 2568CSP CV'/o 9.6 8.0 6.4Single yarn strength (g) 173 208 229Singh yarn strength CV'/o 13.3 13.3 11.5Single yam elongation (%) 5.2 6.1 6.8Single yam elongation CV% 15.8 12.9 11.4Uster% 17.3 15.9 12.0Imperfections/WOO m

Thin places 830 150 20Thick places 1790 850 630Neps 1640 980 180

Sized yamSingle yarn strength (g) 221 260 279Single yarn elongation (0;'») 4.3 4.9 5.5

The yarns were subjected to various levels (num-ber) of weaving cycles on Reutlinger web tester:'.This instrument simulates most of the weavingstresses, such as basic tension, cyclic extension, flexabrasion and bending, except beat-up forces.

The normal and abraded portions of yarn weretested for their tensile properties on lnstron.

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AGGARWAL and HARI: DEGRADATIVE BEHAVIOUR OF YARNS ON STRESSES IN WEAVING

3 Results and Discussion3.1 Mechanism of Yarn Breakage on Reutlinger Web Tester

When the Reutlinger web tester is in action, cyclicextension and axial abrasion act simultaneously onthe yarn which is maintained under constant tension.As a result of cyclic stretching and abrading actionsthe yarn structure loosens and this, in turn, promotesinter-fibre slippage. As the loosening of yarn pro-gresses the abrading element rubs off fibres from theyarn structure, causing fibre loss. This fibre loss alsoenhances inter-fibre slippage. During the process,therefore, on account of these forces a relaxation ofyarn takes place and the tension tends to decrease.At this stage, the mechanism provided to maintainthe tension level constant pulls the yarns subjectingthem to elongation. This additional elongation (E )is mainly due to inter-fibre slippage on account 6floosening of yarn structure. The. abrading actioncontinues till sufficient fibres have been rubbed offthe yarn structure, leaving a few fibres which can nomore withstand the stresses, and, therefore, the yarndisintegrates. Fig. 1 shows the progressive degrada-tion of a sized yarn on account of weaving stresses si-mulated on Reutlinger web tester.

3.2 Correlation between Grey Yarn CSP andDegradative Behaviour of Grey Yarns

As mentioned in 3.1, during weaving the degrada-tion of yarn takes place due to loosening of yarnstructure, causing fibre loss. A quantitative estima-tion of the extent of degradation of yarn, at any stageduring weaving (abrasion), can be made by calculat-ing the amount of fibre loss. The relative perform-ance of different yarns can also be compared bycomparing the values of fibre loss at any given num-ber of weaving cycles.

To examine the strength of correlation betweenyarn esp and rate of yarn degradation, three sam-ples of yarn of the same linear density and twist butof different esp levels were subjected to increasingdegrees of stress on Reutlinger web tester. The de-graded portion of the yarn was then carefully cut offafter removing the protruding fibres and the weightper unit length of each of the samples was calculated.From these data the % fibre loss on abrasion was cal-culated as follows:

% Fibre loss (L)=b-- LAX 100Lp

where 4 is the wt/ em of unabraded portion of yarn;and LA' the wt/cm of abraded portion of yarn.

Table 2 shows that after any given number of cy-cles, the value of L decreases with increase in esp. Infact, there exists a relationship between fibre loss (L)

... (1)

Table 2-Fibre Loss (%) on Increasing WeavingCyclesNo. of CSPcycles

1788 2181 2568

W L W L W L0 14.70 14.90 14.40

50 13.34 9.3 14.15 5.1 14.08 2.2100 10.15 30.9 12.81 14.0 13.02 9.5150 5.91 59.8 11.30 24.2 12.33 15.1200 1.85 87.4 8.53 42.8 10.91 24.3250 4.83 67.6 9.18 36.2300 2.53 83.1 7.63 47.0450 1.15 89.5

W-Weight of yarn in glcm (x 10-5)

L-% Fibre loss

Table 3- Values of Regression Constants a and b forYarns

aX(1O-3)31.712.26.7

Yarn CSP178821812568

b1.481.551.54

ABC

and number of weaving cycles (C) to which the yarnhas been subjected.L=axC' ... (2)where a and b are the regression constants. The va-lues of a and b for the three yarns are given in Table3. The table shows that irrespective of the yarn espthe value of b is nearly constant (1.52). The constantof proportionality a, however, decreases with in-crease in esp. A higher value of a for the low espyarn shows that as compared to the high esp yarn, alow esp yarn degrades at a faster rate. The reasonfor this is explained in 3.4.

3.3 Correlation between Grey Yarn CSP andDegradative Behaviour of Sized Yarns

Of the three yarns, yarns Band e (of esP's 2181and 2568 respectively) were sized to 12% add-on.The sized yarns were subjected to different levels ofweaving cycles-75, 150, 225 and 300-on theReutlinger web tester. The normal and abraded por-tions of the sized yarn were tested for their tensileproperties on Instron. The test length was kept 50mm. Because of the presence of size component, theloss in weight (fibres) of yarn on abrasion could notbe reliably measured.

The average values of tensile strength, elongation-at-break and modulus (Tan e) are given in Table 4.The load-elongation curves for the two yarns at dif-ferent levels of weaving cycles are shown in Figs 2and 3.

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Table 4 shows that the tensile strength, elongationand modulus of sized yarn decrease progressivelywith increase in the number of weaving cycles. Fur-ther, the variability (CV>fo) in individual values ofstrength, elongation and modulus also increases withincrease in the number of weaving cycles. This indi-

cates that the process of abrasion creates new weakplaces in the yarn and, therefore, introduces into it ahigh frequency of low strength/elongation threads.This observation is similar to that of Radhakrish-nan". The effect is more pronounced in the case of alow CSP yarn compared to high CSP yarn.

Fig. l=-Degradativc behaviour of yarn on increasing weaving stresses: (a) normal. (b) af-ter 75 cycles, (c) after 150 cycles. (d) after 225 cycles, (e) after 300 cycles. and (0 broken

yarn

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AGGARWAL and HARI: DEGRADATIVE BEHAVIOUR OF YARNS ON STRESSES IN WEAVING

Table 4-Change in Tensile Properties of Sized Yam on Increasing Weaving Stresses

Yarn No. of Tensile properties CV'/oweavingcycles Strength Elongation Modulus Strength Elongation Modulus

g % Tan 8B 0 260 4.9 2.8 18.5 10.9 4.3

75 218 5.0 2.0 15.0 19.9 8.2150 132 3.2 1.9 46.6 32.5 8.3225 87 2.8 1.4 49.5 21.1 24.8300 57 2.3 1.0 58.6 52.9 26.8

C 0 279 5.5 2.9 9.7 15.3 4.175 275 5.3 2.8 10.2 13.4 3.2

ISO 272 4.7 2.7 14.1 9.9 3.2225 177 3.6 2.5 41.1 22.2 6.9300 II7 3.6 1.5 42.2 24.5 21.0

300 -~---~- ----llOa HI

LI Yarn B250

length = 50 mmlest 250 Y~rn C

L2No. 01 Ie st I~ngth = 50mm

200rn cycles 200 No. of~ L 1 '" Cyclesu 0'" L 2 HI - 0.'3 150 75

L 3 150 H4 -225L 4 225 HS-lOO

too L 5 300

a o

Elongation, m m Elongation, mm

Fig. 2-Load-elongation curves oflow CSP yam Fig. 3-Load-elongation curves of high CSP yarn

To have a more meaningful comparison and tonormalize the initial differences in the tensile pro-perties of the two yarns, the fractional-retention va-lues of strength, elongation and modulus of abradedyarn were calculated as

.H = 2568 CSP" l = 2181 CSP

1·0

o-ss::;. osc

0·.~ H

Vi )·2. a1·0

c 0·8.s 0.6;; Ha- o.zcgw 0·2

a1·0

0·8.•,H"3

" l0l:

Fractional-retention value

= Value of the abraded yarnValue of the unabraded yarn

Fig.4 shows the plot of the fractional-retention va-lues against the number of weaving cycles to whichthe yarn has been subjected during testing. The plotshows that the rate of decrease in the fractional-ret-ention values of strength, elongation and moduluswith increase in number of weaving cycles is higherfor a low CSP yarn than for a high CSP yarn.

Further, for the high CSP yarn, the decreasingtrend in strength and modulus is feeble up to 150 cy-cles as compared to that for the low CSP yarn. In

10 '00 150 20C 250 300

Fig. 4-Fractional-retention values of low and high CSP yarnson increasing weaving stresses

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INDIAN J. TEXT. RES., VOL. 13, DfCEMBER 1988

other words, a high CSP yarn requires a very highnumber of abrasion cycles to abrade off the size filmand to loosen/weaken the yarn structure. Converse-ly, in the case of low CSP yarn, right from the start,the strength and modulus decrease sharply with in-crease in weaving cycles. The observed differencesin the performance of two yarns are explained in 3.4.

The plot of fractional-retention values of yarnelongation against the number of weaving cyclesshows that the loss in yarn elongation for both theyarns is almost same up to the initial 75 cycles, andthereafter the low CSP yarn loses its elongation at afaster rate than a high CSP yarn.

This experiment brings out a point of practical sig-nificance: under identical conditions of weaving alow CSP yarn reaches its endurance limit at a muchfaster rate than a high CSP yarn. Therefore, by thetime an element of weaker yarn leaves the healds andreed zone it becomes more susceptible to break thanan element of stronger yarn.

3.4 CSP as Characterizing Yarn for Its Degradation Behav-iour

The differences in the performance of low andhigh CSP yam, when subjected to degradation cy-cles, can be explained on the basis of differences instructural features. The structural features that arethought to have major influence on the rate of yarndegradation are: (i) packing density of yarn, and (ii)number of points and extent of contact between fi-bres. These structural features are not easy to mea-sure or even to observe. However, it is necessary toquantify these features to understand the basis fordifferences in the performance of yarns.

Table 5-Properties of Fibres

Yarn CSP Fibre Properties

A high CSP yarn spun out of long and fine fibres(Table 5) is generally expected to have a higher num-ber of fibres in yarn cross-section and hetter fibre-to-fibre interlocking, or in other words more pointsand extent of contact between fibres or fihre bun-dles, than a low CSP yarn spun out of short andcoarser fibres. This leads to a higher degree of inter-fibre binding (packing density) and friction in thecase of stronger yarns. Therefore, during abrasion,rubbing/plucking fihres off the yarn structure as wellas inter-fibre slippage would be rather delayed in thecase of more compact high CSP yarn compared tothat in a less compact low CSP yarn.

We considered two different yarns to illustrate itfurther. In the first yarn, the inter-fibre locking andfriction were such that cohesive forces holding the fi-bres together in the yarn were very strong, whereasin the second yarn, these forces were relatively weak.When subjected to weaving stresses, the forces deve-loped in the first yarn dissipated largely in the fibresby development of tensile and other stresses. In theinitial stages, the inter-fibre slippage was very less.However, repetition of weaving stresses resulted infibre fatigue, leading to rupture and to some extentfibre slippage in the yarn. In the second yarn, wherethe fibre cohesion was relatively poor, the stressesdeveloped could easily displace or remove fibresfrom their normal position in the yarn. Table 6 sup-ports the above explanation. The data show that: (i)the packing density of the fibres in the yarn increaseswith increase in yam CSP; (ii) the total number of cy-cles required to break the yarn increases with in-crease in CSP; and (iii) the rate in inter-fibre slippage(Ep per cycle) decreases with increase in esp.

The rate of inter-fibre slippage during testing wascalculated as

Effective Mean Micronaire Bundle E; per cycle = EIO- Eo... (3)

length length value tenacity It;omm mm (1/8 in)

g/tex where Eo is the initial yarn extension due to basicA 1788 25.0 19.2 4.2 17.0 tension; EJO, the extension at tenth break; and It;o,B 2181 32.4 22.8 4.0 22.6 the number of cycles required to break the tenthC 2568 34.4 24.6 3.4 26.2 thread.

Table 6-Effect of Structural Parameters on Yarn PerformanceYarn CSP Yarn degradation Mean cycle to Packing density Inter-fibre slippage

rate break the yarn factor of yarnaX 10-' (k, Ell EIt/ Ep e, per

cycleXlii>'

A 178R 31.7 IRS 0.553 0.70 3.52 2.82 15.2B 21MI 12.2 324 0.571 0.66 .l34 2.6M M.3C 256M 6.7 477 0.583 0.56 4.33 3.77 t»

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4 ConclusionsOn repetitive weaving stresses a low CSP yam de-

teriorates in terms of its tensile and abrasion pro-perties at a faster rate than a high CSP yarn. This isbecause a high CSP yarn, spun out of longer and fin-er fibres, is more compact (fibres are denselypacked) and has a lower rate of inter-fibre slippageduring testing than a low CSP yam spun out of shor-ter and coarser fibres. The process of weaving alsocreates new weak places in the yam and hence in-creases the variability in tensile properties. This ef-

feet is more pronounced in low CSP yam. Becauseof these reasons a low CSP yam is more susceptibleto breakage during its passage to cloth feUin weavingthan a high CSP yam.

References1 Aggarwal SKand SubramanianT A, Text ResJ, 58 (1988) 11.2 Hari P K, Subramanian T A and Aggarwal S K, Text Asia,

58(2)(1987) 16.3 Trauter J and Weisenberger W, Text Prax Int. 34 (1979) 1134.4 Radhakrishnan T, Mehta P C and Shelat B R, Text Res J, 27

(1957)439.

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