Indian Journal of Textile ResearchVol. 4, December 1979, pp. 133-139

Tensile Behaviour of Core-Spun Yarns *K L LOKANATHA. B G SRINIVASALU & B BASAVARAJ

Govt. S.K.S.1.T. Institute, Bangalore 560001

Received 7 November 1978; accepted 15 February 1979

Core-spun yarns incorporating a textured nylon multifilament core were prepared by introducing the multifilament corealong with a drafted ribbon of fibres as the latter entered the nip of the front rollers of a ring frame. The geometrical dispositionof the core was altered by changing the twist multiplier, pre-tension and the type offeeding of the sheath material, and the effectsof these changes on the tensile properties were evaluated. Comparison of the stress-strain curves for the core-spun yarns withthose for the continuous filament and cotton yarns of the same counts spun under identical conditions highlighted thebeha viour of the components in a core-spun yarn undergoing strain. The contribution of the cotton components to core-spunyarn strength was assessed by the study of cotton fibre breakage at various levels of yarn strain. The results from the Instrontensile tester reveal the mode of strain distribution along the length of the yarn. The process of rupture of the core-spun yarnshas also been studied in detail.

The development of the spinning processes to producestronger yarns at a faster rate resulted in theintroduction of continuous filamental yarns. But staplefibre structures predominate over the continuousfilamental yarns for their improved use characteristicssuch as easy care, comfort, bulk, etc. To produce a yarnwith,tfntptove4~e chaJ:llderistics,"ilh idea wasconceived which would embody rational utilization ofthe properties of both the continuous filamental yarnand the staple fibre yarn. This resulted in theintroduction of core-spun yarns, which consist of ablend of a number of components wherein one of thecomponents, usually a continuous filamental yarn, isconstrained to lie permanently at the core, while theremaining components act as the covering fibresaround the central core.

A special feature of the core-spun yarn is the differentproperties in its inner and outer regions. The choiceand exploitation of the various properties in the innerand outer regions of the yarn commend their usage forspecific applications, namely tarpaulins, tentages, hosepipes, belting ducks, sewing threads and otherindustrial applications. The continuous filamentincorporated at the central axis of the yarn contributesmainly towards the strength, while the surfacecharacteristics of the composite yarn provideproperties of adhesion, heat insulation, bulk, etc. Theseproperties are exploited in the manufacture of belting,tarpaulins, hoses, etc., which demand an after-treatment with chemicals. Further, the production oflight-weight apparel fabrics, hitherto not possible byordinary methods, can be done using a soluble core.Much of the bagginess in wear can be eliminated by

• Part of the M.Sc. (Tech.) thesis submitted by one of the authors(K.L.L.) to Bangalore University, 1975.

incorporating elastomeric core-spun yarns into cuffs,neck bands of sweaters, collars, etc., and such types ofyarns find extensive applications in apparel fabrics,slacks, creepers, leisure wear, underwear, knittedgarments, etc. Also, by using a continuous filamentcore, breakages in spinning and cloth manufacture canbe reduced substantially.

Several methods for the manufacture of core-spunyarns, their merits and demerits, along with the enduses, have been reported+F ", A critical assessment ofthe merits of core-spinning through a comparison ofthe strength and extension of core-spun yarns withthose of all-staple yarns was made byBalasubramanian and Bhatnagar", Bhatia 7 in-vestigated the influence of some of the processingparameters on the properties of core-spun yarns. Newmethods for the production of core-spun yarns morerapidly and without the limitation of the staple lengthhave been reported'':",

Mechanical properties are of prime importance foryarns meant for industrial applications. Themechanical properties of core-spun yarns depend uponthe load-elongation characteristics of the components.So a study of the tensile behaviour of such yarns wouldcontribute to a better understanding of the mode ofstrain distribution along the length of the yarn and thecontribution of the components to the core-spun yarnstrength.

The production and properties of core-spun yarnscomposed of a 40 den textured nylon multifilamentcore are reported in this paper. Detailed studies weremade only on one count, namely 26 tex, where thefilament formed 17% of the yarn. Pre-tension and typeof feeding were suitably selected to alter the geometricdisposition of the filament in the composite yarn. The

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INDIAN 1. TEXT. RES., VOL. 4, DECEMBER 1979

effect of these changes on the tensile properties wasevaluated over a range of twists.

Materials aod MethodsProduction of core-spun yarns-The method for

producing core-spun yarns consists in introducing thefila mental yarn along with the drafted ribbon of fibresat the nip of the front rollers in a ring frame. If thefilament is delivered at the same speed as the staplefibre material, the filamental yarn always exceeds inlength that of the composite yarn and is not likely tostay at the core only, but is found to migrate, from timeto time, to the surface of the core-spun yarn. The twodifferent methods for obtaining a reduced delivery rateto the core are differential twist retraction and pre-tensioning, the latter being the more commonly usedmethod.

The core material, namely textured nylonmultifilament, was taken from the supply packagearound suitable guides to facilitate smooth unwinding.It was then drawn through a suitable tensioning devicecapable of providing a wide range of tensions. A disctype of tensioning device mounted on the top of thedrafting zone was used. The suitably tensionedfilament was then drawn over a porcelain guide rollerto enable the filament to be positioned accurately at thecentre of the drafted ribbon of fibres. The filament wasfinally drawn through the eye of a sewing machineneedle, mounted just above the drafted ribbon of fibresand just behind the nip of the front rollers to ensurethat the position of the filament remains unalteredthroughout the production of the core-spun yarn. Thetraverse mechanism for the roving guides was alsodisengaged to ensure the same.

The cotton sheath was processed through the sameset and sequence of machines under similar conditionsto reduce the variations as far as possible to aminimum. The roving from the roving bobbins waspassed over suitable guides and through a specialarrangement made at the cradles for double endfeeding. The drafted ribbon of fibres entered the nip ofthe front rollers along with the suitably tensioned andpositioned core. To take advantage of the natural foldand hence to obtain a better cover, the filament was fedat the left side of the drafted ribbon of fibres in the caseof single end feeding arrangement, while in the case ofdouble end feeding rovings were fed separately and thefilament was fed at the centre to ensure better cover andgood strength",

The present investigation was carried out on similarlines as that by Balasubramanian and Bhatnagar" witha few changes. A textured nylon multifilament 40 den(4.4 tex) was used as the core and cotton suitable forspinning 60's Ne (9.84 tex) was used as the sheath. Core-spun yams of 26 lex were produced whereby the

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filament formed 17% of the yarn. Two extreme tensionlevels, namely 4 g and 20 g pre-tensions, were selectedto vary the geometrical disposition of the filament andhence to study the effect of the same on the tensilebehaviour. It was found that an initial tension of 4 gwas necessary to remove all the kinks in the texturedcore and an optimum tension of 20 g was needed toensure that no buckling of the staple fibres takes placeduring spinning. Four different twist multipliers wereselected to investigate the influence of twist on thetensile behaviour so as to cover the usual range of twistmultipliers employed in normal practice. Further, theeffect of the feeding type was studied by making use ofmultipliers employed in normal practice. Further, theeffect of the feeding type was studied by making use ofboth single and double end feeding arrangements. Thesame rovings were employed to spin 100% cotton yarnsto be used for comparison purposes, by correspondingmanipulation of the spinning draft. The spindle speed,traveller size and other particulars were kept the samefor either method of spinning. The twist was variedover a selected range, and core-spun and cotton yarnswere spun under similar conditions.

Testing procedure-Good brand, Scott and Instrontensile testers were used for tensile testing of yarns. Theconditions of testing were as follows:

Goodbrand tensile tester: Test length, 48.5 em;and rate of tra verse,30 ern/min.Test length, 50 em;and rate of loadingfor most of the tests,1000 gf/rnin.Gauge length, 8 emfor most of the tests;and rate of exten-sion 60%/min.

Scott tensile tester:

Instron tensile tester:

From the load-elongation curves obtained, thetensile behaviour of the core-spun yarns was assessedby studying the (a) stress-strain relations, (b) cottonfibre breakage, and (c) the process of rupture.

Results aod DiscussionStress-strain relations-Core-spun yarn is a

composite yarn composed of a continuous filamentalcore and a staple fibre covering, the former providingsupport to the latter. Since the filament and the staplefibre material, namely cotton, are of different materials,the tensile properties of the composite yarn depend, asin the case of blends, on the load-elongationcharacteristics of the components. Comparison of thestress-strain curves of the core-spun and cotton yarns

LOKANATHA et al.: TENSILE BEHAVIOUR OF CORE-SPUN YARNS

of the same count, spun under identical conditions,highlights the behaviour of the components in a core-spun yarn undergoing strain. The load-elongationcurves for core-spun, all-cotton and nylon multifila-ment were obtained by a method similar to Kemp andOwen method 10. The stress-strain curves for the core-spun, all-cotton and nylon multifilament were derivedfrom the load-elongation curves. On the assumptionthat cotton and nylon behave in a similar manner in thecase of both core-spun and conventional yarns, thestress-strain curves for core-spun yarns were derived. Amethod similar to the one followed byBalasubramanian and Bhatnagar" was employed toobtain the predicted curves for the core-spun yarns.

The stress-strain curves (predicted and actual) forcore-spun yarns, prepared with cotton yarn and nylonmultifilament drawn up to the point of the firstbreakage, are included in Fig. 1. The core-spun yarnswere prepared with 20 g pre-tension using singlefeeding arrangement over a varied range of twists. It is

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Fig. 1-Predicted and actual stress-strain curves for core-spun yarnsprepared with 20 g pre-tension colton yarn using single feed andtextured nylon multifilament [(I) Textured nylon multifilament, 4.4tex, 10filaments; (2) core-spun yarn, predicted curve; (3) cotton yarn;and (4) core-spun yarn, actual curve. (A) TM =3.0; (B)TM =4.1;

(e) TM = 5.4; and (D) TM = 6,5]

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observed that at any given extension level. the stressborne by the core-spun yarn is less than the predictedstress. This indicates a small reduction 10 therealization of strength from the components. Thisfeature was noticed in the case of core-spun yarnsprepared with both 4 g and 20 g pre-tension usingsingle and double feeding arrangements at all twistlevels. This slight fall in strength realization may beattributed to the differences in geometric dispositionand hence the cohesion between the components of thecomposite yarn. At any given extension level below thepoint of rupture, the stress borne by the core-spun: arn(actual) is less than that of the components, Th ma ,be attributed to the serigraphic effect by virtue ofwhich in a composite yarn comprising cotton andnylon, which vary widely in their stress-straincharacteristics, the maximum load at any givenextension below the point of rupture of cottoncomponent is shared by the cotton component, whilethe incorporation of the nylon multifilament in thecomposite yarn does not contribute in any way tohigher strength realization. This also accounts for thelower tenacity of the core-spun yarns prepared usingdouble feed, than that of the equivalent cotton yarns, atall twist levels. The same feature was noticed for core-spun yarns prepared using single feeding arrangement,except at low twists. The presence of the continuousfilament in the latter case reduces the slippage of thecotton fibres and hence accounts for higher strengthrealization, The breaking extension of the core-spunyarns was found to be slightly higher than that of theequivalent cotton structures. The effect was found to bemore pronounced in the case of core-spun yarnsprepared using single feed than double feed.

The stress-strain curves for the core-spun yarnsprepared with 4 g and 20 g pre-tension over a variedrange of twists are shown in Fig. 2. The slope of thestress-strain curves was found to reduce progressivelywith the twist multiplier. The stress borne by the core-spun yarns at any given extension level, in general,increases progressively with reduction in the twistmultipliers. In addition, core-spun yarns prepared withhigher twist multipliers have higher breaking extensionvalues. This may be attributed to the fact that theincreased obliquity in the lie of fibres with the increasedtwist multiplier allows the highly extensible core totake the load and extend.

The stress-strain curves for core-spun yarnsprepared with 4 g and 20 g pre-tension using a singlefeed are shown in Fig. 3. It is seen that the breakingstress values for core-spun yarns prepared with 20 gpre-tension are greater than those prepared with 4 gpre-tension at all twist levels. The same feature wasnoticed in the case of core-spun yarns prepared using adouble feeding arrangement. These results show that

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INDIAN J. TEXT. RES., VOL. 4, DECEMBER 1979

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Fig. 2-Stress-strain curves for core-spun yarns [(I) Core-spunyarns prepared with TM = 3.0; (2) core-spun yarns prepared withTM =4.1; (3) core-spun yarns prepared with TM = 5.4; and (4)core-spun yarns prepared with TM =6.5. (A) Core-spun yarns preparedwith 20 g pre-tension; (8) core-spun yarns prepared with 4 g pre-tension using double feed; (C) core-spun yarns prepared with 20 gpre-tension; and (D) core-spun yarns prepared with 4 g pre-tension

using single feed]

the pre-t.ension certainly influences the geometricdisposition and hence the strength of the yarn. Thismay be attributed to the fact that the filament lies closeto the core and is in contact with the sheath materialover a greater surface area as a result of higher pre-tensioning, thereby improving the resistance toslippage. This improved resistance to slippage makesthe yarns spun with 20 g pre-tension even with thelowest twist multiplier to exhibit a single sharp break incontrast to core-spun yarns prepared with 4 g pre-tension.

The stress borne by the core-spun yarns, at any givenextension level, is more for 4 g pre-tension at 3.0and 4.1twist multipliers. But the core-spun yarns preparedwith 20 g pre-tension are found to have a higher stressat any given extension level at 5.4 and 6.5 twistmultipliers. A similar feature was observed in the caseof core-spun yarns prepared with a double feedingarrangement.

The stress-strain curves for core-spun yarnsprepared with a pre-tension of 20 g using both singleand double feeding arrangements over a varied range

136

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(B)TM=4.1; (C)TM=5.4; and (D)TM=6.5]

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Fig. 4-Stress-strain curves for core-spun yarns prepared with 20gpre-tension [(I) Core-spun yarns prepared with single feed; and(2) core-spun yarns prepared with double feed. (A] TM = 3.0;(B) TM

=4.1; (C) TM = 5.4. and (D) TM =6.5J

LOKANATHA et al.: TENSILE BEHAVIOUR OF CORE-SPUN YARNS

of twists are shown in Fig. 4. The stress-strain curvesfor the core-spun yarns prepared using a double feedhave greater slope than those prepared using singlefeeding arrangement. Further, the stress borne by thecore-spun yarns prepared by a double end feedingan;.angement is more than that in the case of yarnprepared by the single end feed at any given extensionlevel below the point of rupture. This feature, ingeneral, was also observed in the case of core-spunyarns prepared with 4 g pre-tension over a varied rangeof twists. This may be attributed to the better cover andhence cohesion developed as a consequence of thedouble end feeding employed. The improved bindingbetween the cotton fibres and nylon filament in thecore-spun yarns prepared with double feed does notallow relative movement and slippage of the cottoncomponent and hence a higher stress is developed at agiven strain with such yarns than with single feed core-spun yarns.

The breaking stress values for core-spun yarnsprepared using double feed were found to be higherthan those for the core-spun yarns prepared usingsingle feed for twist multipliers 3.0 and 4.1.The breakingstress values, for single end feeding arrangement,further increased as the twist multiplier was increasedto 5.4 and 6.5 compared to the corresponding yamsobtained using double end feeding arrangement. Thebetter cover of the core-spun yarns as a result of doubleend feeding results in improved resistance to slippageand consequently higher strengths were realized insuch yarns at low twists. The benefits from this sourceseemed to be outweighed by other considerations andhence, instead of improvement, drop in strength wasfound at higher twists in the case of double end feed.This is due to increased obliquity and non-simultaneityin the occurrence of breaks in the cotton component,which is the direct outcome of higher inter-fibrecohesion. This non-simultaneity in the occurrence ofbreaks does not permit the equalization of strains alongthe length of the migrating fibres. This effect is likely tobe more pronounced in the case of double feed as aconsequence of greater cohesion and hence wouldresult in strength deficiencies at higher twistmultipliers.

Cottonfibre breakage-The beha viour of the cottoncomponent in the core-spun yarn during straining ofthe yarn to extensions below and beyond the breakingextension of all-cotton yams highlights the contri-bution of the sheath component to the core-spun yarnstrength. An insight into the contribution of thecomponents to the core-spun yarn strength could beobtained by comparing the length-frequency distri-butions of cotton fibres removed from core-spun yamsstrained to different extensions below and beyond thebreaking extension of all-cotton yarns. This would give

a clear understanding of the process of cotton fibrebreakage during straining of the composite yam.Detailed investigations were made on core-spun yarnsprepared with a pre-tension of 4 g using double feedwith a twist multiplier of 3.0. A method similar to thatfollowed by Kemp and Owen"? was adopted. Thematerial was strained to the desired extent on aGoodbrands single thread strength tester. Two inkmarks spaced at a distance of 8 em were marked at arandomly chosen place on the strained yarn. The inkmarks on the strained yam were cut and the specimenwas slightly untwisted and all the cut (inked) fibreswere removed. To assist this operation, the specimenwas held firmly between two glass plates with a smalllength protruding beyond the glass plates with a weighton the top of them. After the removal of cut fibres,length measurements were made on 400 fibres pooledfrom several specimens selected along the length of8 em, and length-frequency distributions of cottonfibres obtained from core-spun yarns strained to 0, 10,20 and 30% extensions were drawn. The mean length,coefficient of variation of length and the % of fibresbroken deduced from the mean length are given inTable 1.

The length-frequency distributions for cotton fibresin core-spun yarns extended to different extents aredepicted in Fig. 5. It is seen that the modal value shiftstowards the origin with increase in strain on the core-spun yarns. As expected, with increase in strain beyondthe breaking extension of all-cotton yarn, greaternumber of fibres continue to break, which is indicatedby the shift in the modal value towards the left. It isclear from Table 1 that the cotton fibres continue tobreak even at strains beyond that at which all-cottonyarn ruptures. A value above the maximum value of100for the percentage of cotton fibres broken indicatesthat a portion of the fibres break more than once at30% strain. The sufficiently higher cohesion betweenthe sheath and the core component makes the cottonfibres continue to contribute to the load in the yameven after the first break. This is further accentuated bythe higher surface area available on the corecomponent as a consequence of the presence ofmultifilaments in the core as well as the texturedcharacter of the core.

Table I-Percentage of Cotton Fibres Broken at DifferentLevelsof Yarn Extension

Material Mean C.V. (%) Fibresfibre of broken

length length %mm

22.86 38.4918,92 51.86 20.8013.73 58.07 66.5010.98 60.36 108.20

137

Core-spun yam, 0% extensionCore-spun yarn, 10~~extensionCore-spun yarn, 20% extensionCore-spun yam, 30~~extension

INDIAN 1. TEXT. RES., VOL. 4, DECEMBER 1979

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FIBRE LENGTH,mm

Fig. 5-Fibre! length distribution of cotton fibres in core-spun yarnsextended to different extents [(A) Core-spun yarn, 26 lex, O~{extension; (B) core-spun yarn, 26 lex, 10% extension; (C) core-spunyarn, 26 tex, 20% extension; and (D) core-spun yarn, 26 tex, 30%

extension]

The process of rupture-Most of the specimens ofcore-spun yarns prepared with a pre-tension of 4 g anda twist multiplier of 3.0 using both single and doublefeeds were found to have breaking extensionssomewhere between the breaking extension of all-cotton yarn and that of the nylon multifilament. Thesespecimens were found to take the load up to a certainextent, beyond which higher extensions were realizedfor a slight increase in the load and did not exhibit asingle sharp break. Further, from the cotton fibrebreakage study, the percentage of fibres brokenexceeded 100 for yarn strains beyond 30%, indicatingthe occurrence of multiple breaks. A study of themechanism of breakage of the oomponents wouldcontribute to a better understanding of the contri-bution of the components to the core-spun yarnstrength. The contribution of the components to thecore-spun yarn strength can be obtained by studyingthe load on the specimen at different levels of strain. Anelectronic tensile tester would measure the load on thespecimen at different levels of yarn strain. Any drop inthe load would also be recorded on such an instrument.Detailed tests were made on core-spun yarns spun with4 g pre-tension using double feed with a twist multiplierof 3.0. Initial tests on an Instron tensile tester at gaugelengths of 50 and 30 em gave load-extension curvessimilar to those obtained on Goodbrand and Scottseriplane testers, the record showing a single sharpbreak. However, when the specimen length wasreduced to 8 em, the break was of a different kind, thespecimen showing stepwise breaks. In such specimens,

138

the load-extension curve proceeded smoothly up to thefirst breakage at which a sharp fall in the load on thespecimen was noticed, this being followed by a series ofstepwise breaks continuing to much higher extensions.

Fig. 6 shows some typical load-extension curvesobtained from an Instron tensile tester. For core-spunyarns prepared with a pre-tension of 4 g using doublefeeding arrangement, the load dropped by 20-135 g atthe time of the first break, but the residual load on thespecimen was still much higher than that of nylon atthis extension level, from which it can be inferred thatthe cotton component contributes substantially to theload in the yarn. Further, with increase in the strain, theload on the specimen was found to increase to a certainextent until the point of second break, after which adrop in the load on the specimen was noticed. Thisstepwise break continued to the point of final rupturelying somewhere around the breaking extension of thenylon multifilament. In most cases, the loads on thespecimen at the subsequent breaks were less than theload at the time offirst break. The stepwise rise and fallof load on the specimen may be attributed to themultiple breaks in the cotton component and the stickand slip effects. This accounts for a value over andabove 100% for cotton fibres broken at 30% strainobserved under cotton fibre breakage. The finalrupture point was found to be highly variable, lyingsomewhere around the breaking extension of the nylonmultifilament. Even the breaking extension of thenylon multifilament was found to be highly variable,depending upon the variations between and within thecomponent filaments.

20

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Fig. 6-Typical load-extension curves obtained using the Instrontensile tester [(I) Textured nylon multifilament, 4.4 tex, 10filaments;(2) core-spun yarn prepared with 20 g pre-tension using double feed;(3) core-spun yarn prepared with 4 g pre-tension using double feed

and TM = 3.0; and (4) all-cotton yarn, 26 lex]

LOKANATHA et al.: TENSILE BEHAVIOUR OF CORE-SPUN YARNS

At the time ofthe first break, the inter-fibre frictionalforces between the cotton fibres and the nylonmultifilament were sufficiently high. If the specimenlength is more, these high inter-fibre frictional forces donot permit the high tension developed in the specimento-be dissipated in portions in the form of fibre slippage,as a result of which the break-spreads across the cross-section at the place of breaking. This is why at largerspecimen lengths, the first break could not bedistinguished from the rupture point. Contrary to this,at short specimen lengths and at lower twistmultipliers, the inter-fibre frictional forces areinsufficient, and this allows the tension on the specimento fall in portions as a result of fibre slippage in the caseof lower twist multiplier; the insufficient stored energyavailable in the case of short specimen lengths causesthe break to be completed instantaneously.

Core-spun yarns prepared with a pre-tension of 20 gshowed noticeable differences from yarns preparedwith 4 g pre-tension. With core-spun yarns preparedwith 20 g pre-tension, more specimens were found tohave a sharp drop in the load on the specimen at thetime of first break, though it was of less predominatingtype in the latter. The extent of drop at the time of firstbreak was remarkable, lying anywhere in the range 20-135 g as against 20-50 g for specimens prepared with apre-tension of 4 g. Differences in geometric dispositionand hence the cohesion between the components mayaccount for this disparity. The contribution of cottonfibres to the load on the specimen by stick-slip effectswas also reduced in the case of specimens preparedwith 20 g pre-tension.

Conclusions(I) Comparison of the stress-strain curves for the

core-spun yarns with those for the continuous filamentand cotton yarn of the same counts spun underidentical conditions highlights the behaviour of thecomponents in the core-spun yarns undergoing strain.At any given extension level, the stress borne by thecore-spun yarn is less than that predicted from thecomponents, indicating a small reduction in therealization of strength from the components.

(2) In general, the tenacity of core-spun yarns is lessthan that of equivalent cotton yarns and the nylonmultifilament.

(3) The breaking extension of the core-spun yarns isslightly higher than that of equivalent cotton structuresand is more pronounced in the case of core-spun yarnsprepared using single feed.

(4) The slope of the stress-strain curves decreasesprogressively with increase in the twist multiplier.Hence, the stress borne by the core-spun yarns at anyextension level, in general, increases progressively withreduced twist multipliers.

(5) Pre-tensioning certainly influences the geometricdisposition of the filament and hence improves thestrength of the composite yarn by constraining thefilament to lie close to the core.

(6) The stress borne by the core-spun yarn below thepoint of rupture is more for core-spun yarns preparedwith double feed than for yarns prepared using singlefeed. The effect is more pronounced at higher pre-tensions.

(7) The relative merits of single and double feeds withrespect to the tenacity of the yarn 'vary with the twistmultiplier employed. A double end feeding isadvocated for core-spinning at lower twist multipliers.

(8) Study of the length-frequency distributions ofcotton fibres obtained from core-spun yarns strainedbelow and beyond the breaking extension of all-cottonyarns shows that the modal value shifts from righttowards left, indicating increase in fibre breakages withincreased strain. Cotton fibres continue to break evenat strains beyond that at which all-cotton yarnruptures and a portion of fibres break more than once.

(9) At any given extension level, the load borne bythe core-spun yarn is greater than that of the nylonmultifilament, which indicates a substantial contri-bution of the sheath component.

(10) Core-spun yarns like cotton yarns show a singlesharp break, except at short specimen lengths and lowtwists. For short specimen lengths at low twists, a sharpdrop in the load followed by a series of stepwisemultiple breaks due to stick and slip effects continuingto much higher extensions can be observed.

References

1 Primentas N, Text Mfr, Manchr, 87 (1961) 397.2 Standring P T & Westrop K J, J Text lnst, 49 (1958) P453.3 Millar G G. J Text lnst, 56 (1965) T33.4 Ruzio G V, Textil Praxis, (1965) 339.5 Robinson ATe & Marks, Woven cloth construction (The Textile

Institute, Manchester) 1973, 7.6 Balasubramanian N & Bhatnagar V K,J Text lnst, 61 (1970) 534.7 Bhatia K K, Stress-strain characteristics of core-spun yarns, paper

presented at 29th All India Textile Conference, G.S.V.M.Medical College, Kanpur, 8-9 April 1972.

8 Grosicki Z J & Maji M R, Text Inst & lnd, 13 (1975) 36.9 Nield R & Ali A R A,J Text lnst, 68 (1977) 223.

10 Kemp A & Owen J D, J Text lnst, 46 (1955) T684.

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