[Failure Mechanism in Spinning]

7/27/2019 [Failure Mechanism in Spinning]

1/9

http://trj.sagepub.com/Textile Research Journal

http://trj.sagepub.com/content/71/12/1087The online version of this article can be found at:

DOI: 10.1177/0040517501071012092001 71: 1087Textile Research Journal

Maria Cybulska, Bhuvenesh C. Goswami and David MacAlister IIIFailure Mechanism in Staple Yarns

Published by:

http://www.sagepublications.com

can be found at:Textile Research JournalAdditional services and information for

http://trj.sagepub.com/cgi/alertsEmail Alerts:

http://trj.sagepub.com/subscriptionsSubscriptions:

http://www.sagepub.com/journalsReprints.navReprints:

http://www.sagepub.com/journalsPermissions.navPermissions:

http://trj.sagepub.com/content/71/12/1087.refs.htmlCitations:

What is This?

- Dec 1, 2001Version of Record>>

at Ministry of Higher Education on May 9, 2012trj.sagepub.comDownloaded from
http://trj.sagepub.com/http://trj.sagepub.com/http://trj.sagepub.com/http://trj.sagepub.com/content/71/12/1087http://trj.sagepub.com/content/71/12/1087http://www.sagepublications.com/http://trj.sagepub.com/cgi/alertshttp://trj.sagepub.com/cgi/alertshttp://trj.sagepub.com/subscriptionshttp://trj.sagepub.com/subscriptionshttp://trj.sagepub.com/subscriptionshttp://www.sagepub.com/journalsReprints.navhttp://www.sagepub.com/journalsReprints.navhttp://www.sagepub.com/journalsPermissions.navhttp://trj.sagepub.com/content/71/12/1087.refs.htmlhttp://trj.sagepub.com/content/71/12/1087.refs.htmlhttp://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://trj.sagepub.com/content/71/12/1087.full.pdfhttp://trj.sagepub.com/http://trj.sagepub.com/http://trj.sagepub.com/http://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://trj.sagepub.com/content/71/12/1087.full.pdfhttp://trj.sagepub.com/content/71/12/1087.refs.htmlhttp://www.sagepub.com/journalsPermissions.navhttp://www.sagepub.com/journalsReprints.navhttp://trj.sagepub.com/subscriptionshttp://trj.sagepub.com/cgi/alertshttp://www.sagepublications.com/http://trj.sagepub.com/content/71/12/1087http://trj.sagepub.com/


2/9

DECEhfBER 2001 1087Failure Mechanism in Staple Yarns

MARIACYBULSKA'ND BHUVENESH. GOSWAMISchool of Textiles, Fiber & Polynier Science, Clemson University, Clemson, Soiith Caroliua 29634

DAVIDMACALISTER11USD A, ARS, Cottoii Qirality Resea rch StclrionABSTRACT

The failure mechanism in staple yams is strongly influenced by yam structure. Man-ufacturing methods impose certain constraints on the disposition and distribution of fibersin the yam cross section. This paper investigates the failure mechanism in ring, rotor,air-jet, and vortex yams. T he yam s are subjected to uniaxial loading on a tensile tester, andimages of the yam s before and after breaking are recorded. Ima ge analysis of the failureregions yields some interesting features and reveals typical mechanisms occurring indifferent yarn structures. The failure mechanism in each yam type is discussed in termsof some basic parameters characteristic of the structure.

Th e mechanism of yarn failure is usually explained onthe basis of stress-strain characteristics of yam s. F igure 1reveals the nonlinear mechanical behavior of a yam withlinearity restricted fo r very sma ll stress only (region I),where slippage is prevented by friction. In region 11,fibers start to slip, and for higher stress (region III) , bothslippage and breakage of fibers until yam breakage canbe observed.

EFIGW 1. Stress-strain curve for a staple yam.

A staple yam m ay fail either because of fiber slippage(e .g. , in low twist ring or rotor and air-jet yams) orslippage and/or breakage in medium and highly twisted

' Current address: Technical University of L6di. Faculty ofTextileEngineering, L6di. Poland.

yams. Most publications on the mechanics of stapleyams are primarily concerned with ring spun yams.Gregory [3 , 41 studied the strength of twisted yams inrelation to fiber properties and yam structure. In order toincrease staple yam strength, the individual fibers mustgrip each other when stress is applied, mainly from twistpressing fibers together and developing friction betweenthem. On the other hand, s ince the fibers are inclined tothe yarn a xis, only the co mponents of fiber stress in th eaxis direction balance the applied load, and the fullcontribution of fiber strength is not realized. Th is effectof obliquity increases as the twist increases. The twistrequired to generate maxim um strength in a yarn elementcan be determined based on a V-shaped twist curve fortwisted yam elements. Gregory reported that the twistfactor for maximum strength can be presented as a func-tion of the mean length of fibers in the yam cross section,the coefficient of fiber-upon-fiber friction, and fiber sur-face per unit mass.

The work reported by Hearle [5]on the tensile behav-ior of staple yams mainly conc erns ring spun yams. Th eidealized yam is assumed to be uniform along its length,with a circular cross section and uniform specific vol-ume. The yam consists of perfectly elastic fibers with thesame dimensions and properties. Fiber ends are distrib-uted randomly through the yam. Tensile properties of astaple yam are explained in terms of the combined ef-fects of obliquity and fiber slippage, which cause yamstrength losses. Taking into account the effect of twist,migration, and discontinuities at the fiber ends, Hearledeveloped an expression for the yam modulus.

Texfile Rex J. 71(12), 1057-1094 (2001) WO-5175/$15.00

http://trj.sagepub.com/http://trj.sagepub.com/http://trj.sagepub.com/


3/9

1088 TEXTILE ESEARCHOURNALTh e structures of rotor spun and ring spun y am s differsignificantly from each other. Rotor spun yarn has atwo-phase structure rather than a helical one, with morefolded fibers. The presence of wrapper fibers additionallyaffects yam strength. Xiubao and Reiyao [9] studied thetensile properties of rotor spun yams by taking into

account Hearle's models for the strength of staple yam.Determining the proportion of wrapper fibers in the yamand assuming that total yam extension can be expressedas the sum of extensions caused by the strain of fibers,stretching of curly fibers, and fiber slippage, the authorsdeveloped a formula for rotor spun yam strength.Krause and Soliman [ 6 ] analyzed the tensile behaviorof wrapped yarn. To predict yam strength, their modelwas based on the ideal yam structure. They assumedconstant fiber length, uniform distribution of the wrap-ping fibers on the yam length and cross section, constantwrapping angle, and uniform wrapping pressure alongthe yam length. They described the mechanism of yamfailure as a simultaneous breaking of wrapping fiberswhile core fibers partially slip and break. T hey assumedthat the total strength of a yarn is the sum of th e strengthsof the core fibers, the wrapping fibers, and the tensiondeveloped by friction of slipping c ore fibers. The ir modelwas based on an idealized yarn structure and didn'taccount for parameters describing yam structure irregu-larity, which may cause losses in yam strength.Th e irregularity of the'y arn structure and its effect onthe mechanical properties of air-jet spun ya ms has beenwidely studied in recent years. The structure of an air-jetya m is usually divided into three classes according to theproperties o f wrapper fibers [7]-class I characterized byuniform wrapping angle, class I1 with wrapper fibers atdifferent wrapping angles, and clas s I11 with no w rapperfibers. Chasm awala et al. [ I ] divided the wrapping fibersinto five classes-core, wrapp er, wild, core-wild, andwrapper wild. They showed that yam strength dependson the proportion of each class of fibers in the yamstructure, and that yam strength decreases with an in -creasing number of wild and wrapper wild fibers andincreases with an incrcasing number of wrapper fibers.Rajamanickam, Hansen, and Jayaraman [ S ] analyzedthree kinds of tensile fracture behavior in air-jet yams-catastrophic when all fibers in the failure region slip andbreak at the same load, noncatastrophic if fibers do notbreak or s lip completely at the sam e load, and failure bytotal fiber slippage. They divided the structur e of the yarninto three classes and accounted for the proportion ofeach class. They showed that yam strength increases

tensile behavior of air-jet yams, although explaining therelationship between yarn structure and properties, didnot allow for good prediction of yam strength. No sig-nificant study of the mechanical behavior of vortex yam shas appeared in the literature.Yam failure, although widely described in the litera-ture, is not yet fully understood, New testing methods-amo ng others, imag e analysis-can help us observ e theyam when applying the load and determine the yamstructure before and after failure. We can also identifythe failure region and deter mine the yam characteristicsin terms of structural parameters. This method shouldhelp better our understanding of the failure phenomenonand formulate assumptions for models to be developedfor predicting y am tensile behavior.Ou r main objectives in this study are to determine thetypical failure mechanisms that occur in different yamstructures and to characterize the failure mechanism ofyarns in terms of their structural parameters.

Materials and MethodsW e chose four kinds o f staple yams for our study: ( 1 )air-jet spun yam, 5060% cottordpolyester, 29 tex lineardensity, (2) rotor yam, 50/50% cottordpolyester, 30 texlinear density, 5.11 tumdcm twist , (3 ) ring spun yarn,

50/50% cottordpolyester, 24 tex linear density, 4.86tum sk m twist, and (4) vortex y'am, 100 % cotto n, 22 texlinear density, 6.45 tums/cm twist.It is well known that the tensile behavior of yarnsstrongly depends on their fiber properties, the most im-portant of which are fiber fineness, fiber length and itsdistribution, fiber strength, and th e coefficient of friction.To avoid the effect of fiber parameters on the failuremechanism, we have chosen yams spun from fibers ofsimilar mechanical and geometric properties.The yams were subjected to uniaxial loading on atensile tester. Before and after testing, an image of theentire yam sample was registered, and the structuralparameters of the yam s were determined. After breaking,the failure region and the values of the structural param-eters were identified.Tensile tests involved an Instron tensile tester forsample length equal to 50 mm and a cross-head speedequal to 2 mrdmin. An image of the structure of thewhole yam sample was obtained by registering the im-ages of successive 2-mm long sections. On the basis ofeach image, the yams' structural parameters were mea-sured and are presented in the form of diagrams.

Structure of Staple YarnsImages of typical structures of different staple yarns

are shown in Figure 2, A-D. The structures of the yam s

with a high frequency of the class I structure and de-creases with a high frequency of the class I11 structure,especially if these sections are agglomerated in someparticular regions of the yam length. T heir models for the



4/9


5/9

1090 TEXTILEESEARCHOURNALof a ring spun yam. The outer zone consists of fiberswrapped around the yam core. The fibers are generallyless oriented w hen compared to the ring spun yam, andthey form loops or folds that may lower ya m strength.Th e structure of a vortex yam is very similar to a rotoropen-end yam, but with more poorly oriented fibers.When analyzing the image of a vortex yam, we can seefolded fibers even on the yarn surface and hairy fibers inthe form of loops.From the point of view of the external structure, wecan characterize rotor an d vortex yarn s by the sa me set ofparameters, e.g., the diameter of the yarn core d , param-eters A d , A d d , an d CVd defined as for the ring spunyam, and the wrapper angle a measured on the basis offibers seen on the yam surface.AIR-JETPUN Y A R N

Th e structure of an air-jet spu n ya m is highly irregular.Usually it has been assumed that the air-jet spun yamconsists of a core created by untwisted fibers, which arewrapped together by wrapper fibers. The core axis isassumed to be a straight line, with the diameter of thewrapper fiber helix equaling the diameter of the yamcore. However, when analyzing the yam image in Figure2D, we notice that generally these assumptions are notsatisfied. We assume the core axis forms a helix ofrandom diameter d&. In the particular case when d,,= 0, the axis of the core forms the straight line. Let d,denote the yarn core diameter and d, , the diam eter of thewrapper helix. We can characterize the structure of anair-jet spun yam by the disposition of the w rapper fibersin relation to the yam core using the coefficient ofrelative disposition RD given by the following expres-sion:

successive wraps 1 ~ ~ .ll these parameters can fullycharacterize the structure of an air-jet spun yam.Characterizing Failure in Staple Yams

Ou r characterizations of the failure regions in terms ofthe structural parameters of the different yams on thebasis of al l the yam samples we have analyzed arepresented in the form of diagrams of structural yamparameters. For each kind of yam analyzed, we show atypical failure region and s om e characteristic features ofthe yams structure.RINGSPUN YARN

For all samples of ring spun yarns, we did not obs erveyam rupture during extension. Instead, yam failure oc-curred due to fiber slippage. Th e fibers slipped out firston the yam surface, and then when a higher load wasapplied, failure extended to the inner layers of the yam.The failure region was relatively long compared to theother kinds of yam s we investigated. When analyzing theimage and characteristics of a typical failure region in thering spun yarn presented in Figures 3 an d 4, it is apparen tthat the break occurred in the yam section with thelowest diameter and the highest value of parameter A d .For al l samples of ring sp un yarn, failure occurred in theregion of minimum yam diameter and maximum A dvalues for the whole yam sample. The twist angle in thefailure region for most cases had relatively low or min-imum values.

Using the coefficient of relative disposition, we cancharacterize three different structures of air-jet spunyarns: first, RD > 1, where the core axis is helical andthe yarn has a screw-like form; second, RD = 1, wherethe core axis is a straight line, and the diameter of thewrapper helix equals the diameter of the yam core; andthird, RD < 1, where the core axis forms a straight line,the wrapper fibers are loose, and the diameter of thewrapper helix is higher than the diameter of the yamcore.Wrapper fibers are randomly distributed along theyarn length, and they create wra pping ribb ons consistingof random n umb ers of fibers. W e can characterize awrapping ribbon by the value of the wrapping angle a,the width of the ribbon w r . and the distance between

FIGURE . Image of a section of ring spun yarn specimen rile beforeand after breaking.

VORTEX Y A R NFigures 5 an d 6 show a typical failure region in avortex yam and its characteristics in terms of the yarnsstructural parameters. Th e wrapper fibers in the failure



6/9

DECEhlBER 2001 1091region were loose and folded in the form of loops, sothey could not prevent the core fibers from slipping. Inthe failure region, the yarn diameter significantlychanged its value, perhaps indicating more fiber ends inthe region, thus additionally facilitating fiber slippage.For all but one sample of vortex yam, the yam diam-

eter had a minimum value and the parameter A d th emaximum in the failure region for the whole samplelength. The wrapper angle didnt show any typical fea-tures.1 -i lure rcsion I

50)am sarnplc length [mrn]

FIGURE. Characteristics of the structure of ring spun yam specimenril0 in terms of structural parameters.

ROTORYARNFailure in open-end yams was d ue to fiber slippage for allsamples we investigated. When analyzing the typical failureregion of a yarn and its characteristics, shown in Figures 7

and 8, it is obvious that failure occurred in the rcgion ofminimum yarn core diameter and at relatively high wrap-ping angle.This resulted from the presenceof wild wrapper

FIGURE. Image of a section of vortex yam specimen v07 beforeand a fter breaking. FIGURE. Image of a section of rotor yam specimen ro5 beforeand after breaking.

failure region0 50yarn sample lenzth [mm]

FIGURE. Characteristics of the structure of vortex y a m specimen

5 00 yarn sample length [rnm]

FIGURE. Characteristics of the stmcture of rotor yarn specimen ro5- .v07 in terms of structural parameters. in terms of structural parameters.



7/9

1092 TEXTILEESEARCHOURNALfibers on the yam surface, which did not add any strength tothe yam and facilitated fiber slippage from the yam's innerlayers.For all but one sample of the open-end spun yarns, thefailure region could be characterized by the lowest valueof yam diameter. The value of parameter A d , describing

yam diameter unevenness, was at a maximum or muchhigher than the average. As for the vortex yam, thewrapper angle did not show any typical features.AIR-JETYARN

Although for air-jet spun yams, the failure region didnot show any typical characteristics such as those ob-served in other kinds of yams, for most samples weanalyzed, the yam diameter in the breaking region wasminimal or lower than the mean value for the whole yamsample. The failure region and the tensile behavior of anair-jet yarn were only partially determined by the features ofthe yarn core, and were highly influenced by the propertiesof wrapper fibers. The failure region of the air-jet yamshown in Figures 9 and 10can be characterized by the verylow value of the RD coefficient, which means that thewrapper fibers were loose and did not prevent the corefibers from slipping. In m ost cases, yam failure was causedby slipp age of core fibers resulting from the very low valueof coefficient RL m or e ' th an 50%, or large lengths be-tween successive waps-70% of the sampIes we analyzed.The other typical feature of the failure region was he smallnumber of fibers creating the w rapping ribbons, accountingfor 70%of the air-jet yam samples.Relationships Between Structural Parametersand Mechanical Properties of Staple YarnsThere are certain significant relationships between thetensile behavior of a yam and som e of its structural param-eters, although parameters characterizing yam failure arenot necessarily those that are related to the ya m's mechan-ical properties, such as breaking load and elongation.All our yam samples were divided into four classesaccording to the value of breaking load: class 1-breakingload lower than 0.3 kgf, classes 2 and 3-breaking loadfrom intervals (0.3, 0.4,) and (0.4, .5>) accordingly,

FIGURE. Image of a section of air-jet spun yarn specimen b5Mbefore and after breclking.

- ab501

I I---,\ I

I failure region50 'yarn sample lcngh [mm]0

FIGURE0. Characteristicsof the structure of air-jet spun yamspecimen b5W in terms of structural parameters.

and class 4-breaking load higher than 0.5 kgf. For yamsfrom each c lass separately, we calculated the mean val-ues of diameter d, parameters A d d and C V d , as well asthe mean values of breaking load, elongation at break,and energy to break. To determine the significance ofdifferences between mean values of parameters for yamsfrom different classes, we used a one-way ANOVA. Whenanalyzing the results presented in Table I , it is apparent

TABLE. hlem values of mechanical and structural parameters of staple yams from classes 1-4.C l s s of breaking Breakin g load, Elongat ion Energy to break. Diame terload, kgf kgf at break, W kgf-mm d , mm A dld CVd

1. B L 5 0.3 0.2 19 6.56 0.458 0.216 0.121 0.1322. 0.3 < B L 5 0.4 0.358 9.62 1 . o n 0.229 0.061 0.113. 0.4 < B L 5 0. 5 0.441 10.25 1.441 0.230 0.018 0.0774. 0. 5 < B L 0.551 10.79 1 874 0.258 0.013 0.066



8/9

DECEMBER001 1093that the mean diameter value for yam s from classes 2 and3 does not differ, while it is lower for class 1 yam s andhigher fo r class 4 ams. At the sam e time, the yam s aresignificantly different according to parameters CVd andA d / d , with the exception of classes 3 an d 4. This meansthat the diameters of the yams with the lowest breakingloads are lower and much less uniform than those withthe higher breaking loads. Analysis of T able I shows thatdespi te yam technology resulting in different migrationand relative disposition of fibers, yams with higher andmore uniform diameters can be characterized by higherbreaking load, elongation at break, and energy to break(Figures 1 1 and 12).

I

clJ5 0 . 5 -

- *co.x.E 0.4 -2 -

0.3-0

0.2-4 . 1 . 1 . 1 . 1 . 1 0.18 0.20 0.22 0.24 0.26 0.28

yarn diameter d [mm]

FIGW 11. Relationship betaeen breaking load and diameterof staple yams.

For all the yarns we investigated, there was a rela-tionship between the yarns structural parameters andits mechanical parameters. We determined these rela-tionships using a correlation analysis. For each pair ofparameters, we calculated the coefficient of correla-tion R and corresponding significance level p . Theresults of our statistical analysis , presented in Tab le 11,show that fo r all kinds of yarns, a significant correla-t ion exists between yarn diameter and breaking load,which is typical for each yarn type. The higher theyarn diameter, the higher the breaking load. There isalso a correlation between yarn diameter and elonga-tion at break and energy to break. The lack of suchrelationships for vortex yarns may have been due toproperties that were similar for all the samples weanalyzed. For air-jet, rotor, and ring spun yarns, forwhich the sam ples were highly diversif ied, the corre-lation coefficients were higher.There was descending relationship between the twistangle and the energy to break for ring spun yam. There

TABLE1. Relationships between structural parameters and mechan-ical properties of staple yams ( R = coefficient of correlation, p= significance level).Energy to Elongation atYam Parame ter Breaking load break break

Air jet d , R = 0.7554 R = 0.8057 R = 0.7119p < 0.0002 p < 0.0001 p = 0.0005- - -l\+ -

I&, - -id R = 0.7525 R = 0.6038 R = 0.5227p < 0.0002 p = 0.0017 p = 0.0181

- --

- -d -Ring d R = 0.6801 R = 0.9112 -R = 0.7462 -

CV d - - -A d -Rotor d R = 0,7895 R = 0.6181 R = 0.5856p = 0.0022 p = 0.0322 p = 0.0354

p = 0.0149 p < 0.0001p = 0.0053

-

- -

aCV d - - -A d -R = 0.6475 - -

p = 0.0329- -

Vortex daCV d - - -Ad - - -

All d R = 0.6114 R = 0.5727 R = 0.6044p < 0.0001 p < 0.0001 p < 0.0001R = -0.6441 R = -0.6133 R = -0.4535p < 0.0001 p < 0.0001 p = 0.0006Adld R = -0.7878 R = -0.7581 R = -0.7353p c 0.0001 p < 0.0001 p < 0.0001

yams CV d

was no relationship between the wrapping angle andmechanical parameters for other kinds of yams. Forair-jet spun yams, the higher the R D coefficient, thehigher the breaking load, elongation at break, and energyto break.

The relationships between yam structural parametersand mechanical properties are even more clearly re-vealed w hen w e take into account all the yam s together.Despite yam technology, the higher and more uniformthe yam diameter, the higher the breaking load andelongation and energy to break. When comparing therelationships between yam mechanical properties anddiameter unevenness, shown in Figures 12 and 13, wesee that the parameter Ad gives m ore useful informationfor predicting the tensile behavior of yarns than thecoefficient of variation of y am diameter. Thi s is becausethis parameter can reflect the way fiber ends are distrib-uted along the yam axis better than the CV of diameter.Yam regions with relatively high Ad values can becharacterized by higher than average numbers of fiberends, which c an result in lowe r frictional resistance andeasier fiber slippage in those regions.



9/9

1093 TEXTILEESEARCHOURNAL

SOS-c- 2 .00.4 ---Y .B

0.3-

0.2-

0.61 . R=-0.611p

Documents

[Failure Mechanism in Spinning]