fabric structure, properties and testing

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After covering the design and manufacturing of wovenfabrics in the previous chapters, it would be properto analyze the fundamentals of woven structure inthis chapter. The most important fabric properties areexplained. Testing principles, methods and equipmentfor fabric testing are summarized. The purpose of thischapter is to relate the fabric properties to themanufacturing process and end use performance.

Fabrics are flexible yet strong. Flexibility is one ofthe most important characteristics of woven fabrics.The fabric flexibility is mostly due to flexible fibersand yarns in the fabric. Due to their polymeric natureand fine diameters, fibers are quite flexible. A stapleyarn is a lot more flexible than a monofilament yarnof the same count. A multifilament or staple yarn mayconsist of several hundred or thousand fibers in itscross section. Although the fibers are twisted togetherin a staple yarn, there is still room for the fibers tomove relative to each other (called fiber migration)under different types of loading, including bending,which results in a flexible structure of yarn. Increasingtwist increases the stiffness of the yarn and thereforeof fabric. Restricting fiber movement or slippage inthe yarn, which is the case in sizing of warp yarns,increases the stiffness of the yarn. Fabric weavestructure also affects flexibility to a certain extent.

The major factors that contribute to fabricstrength are fiber inherent strength and yarn strength.

Weave design also affects fabric strength, especiallyfabric modulus.

13.1 WOVEN FABRIC STRUCTURE

Woven fabric technology is deeply rooted ingeometry. A fabric consists of millions of fibersassembled together in a particular geometry. Theproperties of a fabric depend on material properties,fiber and yarn structure and properties and fabricstructure and geometry as shown in Table 12.1.

Peirce [1] specified eleven structural parametersthat represent fabric construction as shown inFigure 13.1:

L : length of yarn between yarn intersectionsP : projected length of yarn between the

intersectionsC : yarn crimpH : distance between the center of yarn and fabric

planeα : angle between horizontal direction and the yarn

axisD : sum of the warp and filling yarn diameters

The first five parameters are for both warp andfilling. These eleven interdependent parameters canbe used to impart the major properties to the fabric.Although fabrics are considered to be two-

13

Fabric Structure, Properties and Testing

© 2001 By Sulzer Textil Limited Switzerland

362 FABRIC STRUCTURE, PROPERTIES AND TESTING

dimensional for practical purposes, they are indeedthree-dimensional.

Warp count is the number of warp yarns per unitwidth of fabric and filling count is the number offilling yarns per unit length of the fabric. Today, usingoptical/electronic technology, fabric yarns per unitlength can be measured rapidly with extremeaccuracy and repeatability.

13.2 WOVEN FABRIC PROPERTIES

Fabric Weight and Thickness

Fabric weight can be expressed in two ways: directand indirect system. In the direct system, the fabricweight per area is given, e.g., g/m2 or oz/yd2. In theindirect system, which is used less in practice, usuallythe running length per weight is given. However, inthis case the fabric width should also be specified.Fabric weight is affected by the following:

• fiber density• yarn size• contractions• fabric construction• weave pattern• tensions during weaving• finishing

Fabric thickness is important since it affectspermeability and insulation characteristics of fabric.The standard test method for measuring thicknessof textile materials is given in ASTM D1777 [2].The thickness measurement is done at a specifiedpressure of the thickness gauge. The gauge pressureand area are usually reported along with themeasured thickness.

Fabric Cover Factor and Density

One of the most important functions of fabrics is thecovering function. There are two types of coverfunctions of fabrics: optical cover and geometricalcover.

The reflection and scattering of the incident lightby the fabric surface is called optical cover function.Optical cover characteristics of fabric depend on thefiber material and fabric surface. Dyeing andfinishing can change the optical cover properties.Geometrical cover is the area of fabric covered byfibers and yarns and is characterized by fabric coverfactor.

Fabric cover factor (CF) is defined as the ratio ofprojected fabric surface area covered by yarns to thetotal fabric surface area and given by the followingequation (Figure 13.2):

(13.1)

where Cw is the warp cover factor and Cf is the fillingcover factor.

where nw : warp countnf : filling countdw : diameter of warp yarndf : diameter of filling yarn

FIGURE 13.1 Geometry of the unit cell for a plainweave.

FIGURE 13.2 Cover factor diagram of a plain weave.


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The maximum cover factor is 1 in which the yarnstouch each other. This situation is called the jammingstate. Theoretically, the cover factor can be largerthan 1 in which the yarns pile up on each other givingmultilayers of yarns. The property of a fabric that isaffected most by the cover factor is the permeability.Liquid and gas (air) permeability depend on the coverfactor to a great extent.

The density of a fabric is the weight per unitvolume of the fabric. The unit volume is calculatedby multiplying the unit area with fabric thickness.Theoretically, the fabric density can be very close tofiber density.

Crimp

Due to the necessity of interlacing in a woven fabricstructure, at least one of the two yarn sets must havecrimp. In most of the cases, both warp and fillinghave crimp. However, theoretically it is possible tohave straight filling and crimped warp and vice versaas shown in Figure 13.3. The amount of crimp ineach yarn can be largely controlled by controllingthe yarn tensions during weaving. Crimp ratiobetween warp and filling can also be changed to acertain extent for some yarns such as monofilamentswith heatsetting or finishing processes. Tensioning ayarn causes its crimp to be reduced. This will resultin tension increase in the other yarn set. Anothermethod to change crimp ratio is to relax and shrink

the fabric with water or heat. Changing the ratio ofcrimp between the warp and filling yarn is calledcrimp transfer or crimp interchange.

Yarn crimp is also affected by the pattern of yarninterlacing in a fabric; high frequency of interlacingincreases yarn crimp. For example, the plain weavehas the highest frequency of interlacing andtherefore the highest yarn crimp level in both warpand filling yarn. Satin (or sateen) weaves have thelowest frequency of interlacing and hence lowestdegree of yarn crimp. Increasing yarn crimp in aparticular direction decreases the fabric modulusand increases the elongation in that direction. Thisis because the tensile load is initially used to decrimpthe yarn which is relatively easier than extendingthe yarn.

It should be noted that the fibers in a staple yarnalso have some crimp. Moreover, in addition to thecrimp due to interlacing, the yarn itself can also bewavy which could be considered another type ofcrimp. Crimp affects the weight, thickness, cover,flexibility and hand of fabric.

The American Society for Testing Materials(ASTM) makes two types of definitions related tocrimp: percent crimp and percent take-up as shownin Figure 13.4 [2].

(13.2)

(13.3)

In practice, another related term, yield, is used quiteoften during manufacturing. Yield is defined as theratio of woven fabric length to the warp length. Yieldaffects the fabric modulus. The yield, along withfabric design, material type, weave, thickness, warpdiameter and tension, determines how much a fabricwill stretch in a heatsetting operation. This will affectthe fabric modulus and the fabric final sizing toFIGURE 13.3 Possible yarn crimp variations in fabrics.

FIGURE 13.4 Crimped structure of a plain fabric.

13.2 Woven Fabric Properties



achieve the proper length. This, in turn, determinesfabric stability.

The uniformity of the yield is important becauseof the interaction among fabric properties.Changes in yield can cause differences in fabricthickness, air permeability, filling count, fabricwidth and appearance, as well as in fabricmodulus.

Yield can be measured with a simple method:make a mark on the warp yarns and on the fabric atthe same time; after, say, 10 cm of warp has comeoff the warp beam then make a mark again on thefabric. Measure the amount of fabric which wasmade in 10 cm of warp and divide this by 10 whichgives the yield in the fabric.

Tensile Strength

Tensile strength is the most important property of afabric. In almost every fabric development andmanufacturing, tensile properties are reported.Modulus, breaking strength and elongation at breakare widely used for quality control.

There are different types of fabric tensile teststhat are used depending on the fabric and purpose:strip tensile test, grab tensile test and wide widthtensile test. In the strip tensile test, a narrow stripof fabric sample is used (ASTM D5035 BreakingForce and Elongation of Textile Fabrics). The jawsof the tensile testing machine, which are wider thanthe fabric sample, clamp the sample on both endsand a tensile load is applied until fabric breaks. Inthe grab tensile test, the jaws are narrower thanthe fabric width to reduce the effect of Poisson’sratio (ASTM D5034 Breaking Force and Elongationof Textile Fabrics). Grab tensile test is more widelyused for heavy industrial fabrics such as geotextiles.Wide width tensile tests are also used mostly forindustrial textiles (e.g. ASTM D4595 forgeotextiles). Narrow fabrics such as webbings,ribbons, etc., are usually tested at full width. Fabricmodulus is measured using ASTM Test Method D885 [2]. Specifications of textile machines for tensiletesting are described in ASTM D 76. Theterminology of tensile properties of textiles is givenin ASTM D4848. Other ASTM test methods relatedto tensile testing include:

• ASTM D 1775 Standard Test Method forTension and Elongation of Wide ElasticFabrics (Constant Rate-of-Load TypeTensile Testing Machine)

• ASTM D 4964 Tension/Elongation ofWide/Narrow Elastic Fabrics by ConstantRate of Elongation Type Tensile TestingMachine

When fabric is extended in one direction (uniaxialload), first, crimp in that direction decreases. Fabricis relatively easy to extend during crimp decrease.After that, the yarn material starts bearing the loadwhich would reduce the extension rate of the fabric.While crimp is decreasing in one direction (loaddirection), it increases in the opposite direction.Crimp interchange continues until a forceequilibrium is attained. In biaxial loading, force isapplied in two directions. In this case, the crimpinterchange depends on the magnitude of the forces.

Warp and filling yarns exert forces onto each otherat the crossover points. Since yarns are compressible,these forces cause the yarns to deform and take anelliptic shape in the fabric structure rather than anear round shape. The height and width of the yarn’selliptic shape depend on the twist level. The ratio ofheight/ width is called the aspect ratio. Fabric coverfactor is affected by the width and the crimp isaffected by the height.

Tear Resistance

In a relatively dense fabric, individual yarns opposeto the tearing load one by one, that is whypropagation of tear is relatively easy. If the numberof yarns per unit length is low, then the yarns areallowed to displace themselves and form groups toresist the tear in groups rather than individually. Thisincreases tear resistance of the fabric, that is whyloosely woven fabrics have better tear resistance thandense or coated fabrics. For example, a gauze fabricis difficult to tear because of low number of yarns.Fabric weave also has an effect on tear resistance. A2×2 basket weave has higher tear resistance than aplain weave since two yarns act together against tear.

The tests that are used to measure tear resistanceof fabrics are tongue (ASTM D 2261), and


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Elmendorf test (ASTM D 1424). The first methoduses a tensile testing machine to measure the tearingforce. Elmendorf uses a pendulum and measures thetear energy. The trapezoid tear test, which usesanother method, is not used anymore.

Fabric Bow and Skew (Figure 13.5)

The condition in which the filling yarns lie in thefabric in the shape of an arc is called fabric bow.Bow is expressed as:

(13.4)

Fabric bow can be symmetrical or non-symmetrical.The condition in which the filling yarns in a fabric

do not lie perpendicular to the warp yarns is calledskew. Skew is expressed as:

(13.5)

Fabrics with bow or skew are not acceptable in today’squality conscious business environment. If notcorrected, bowed or skewed materials become secondsor reruns. Bow and skew detection and correctionmachines are used for this purpose. Figure 13.6 showsa single width terry towel bow and skew controlmachine. This particular machine detects and correctsbow and skew error by monitoring the cut bandsoriented in the filling direction of the single widthweb. Using an optical detection system, the exposedportion of the cut band’s filling yarn is electronicallymonitored. Correction signals are sent to the machine

where straightening of bow and skew distortionsoccurs automatically [3].

A bow and skew optical sensing system consistsof a sensing unit through which the fabric is threaded.The sensing unit consists of light sources that projectlight beams through the fabric and optical receiversthat read the filling angle at each sensing point. Thelight sources and receivers are mounted at a fixedangle to the fabric to optimize the signals throughdense plain and twill fabrics such as denims. Lightintensity can be automatically adjusted based on thedensity of the fabric for proper signal generation.The receiver heads on the opposite side of the fabrichave lenses to focus the image of the fabric on asemiconductor silicon chip. The chip has an arrayof radially oriented photosensitive lines that generateelectrical signals proportional to the light receivedthrough the fabric. These signals are amplified andsent to the angle computer card for calculation ofthe filling angle. Figure 13.7 shows a high intensityoptical sensing system for denim and other densefabrics.

Air Permeability

Air flow through a fabric is very complex due to thecomplicated structure of the fabric. The air flowthrough a fabric at a pressure difference betweenFIGURE 13.5 Fabric bow and skew.

FIGURE 13.6 Terry cloth bow and skew control machine(courtesy of Mount Hope).




the two surfaces is directly proportional to the openarea of the fabric and square root of the differentialpressure. High differential pressure furthercomplicates the air flow since the viscoelastic fibersmay deform under pressure.

Fabric air permeability is a measure of air flowthrough the fabric at the standard pressure drop. Theair permeability is measured as the air flow in cubicfeet of air per square foot of fabric per minute in thestandard system and in cubic meter of air per squaremeter of fabric per minute in the metric system.

Air permeability is of considerable value inpredicting insulation characteristics of a fabric.Increasing twist in the yarn increases the airpermeability of the fabric. Air permeability test oftextile fabrics is described in ASTM D737.

Void Volume

Void volume is the amount of space in a volume offabric that is not occupied by solid material (Figure13.8). Both fabric thickness and internal structureaffect absolute void volume.

Void volume can be calculated as follows:

FIGURE 13.7 High intensity optical bow and skew sensing system for denims and other dense fabrics (courtesy ofMount Hope).

FIGURE 13.8 Void volume.

(13.5)


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Abrasion Resistance

Both the fiber material and fabric geometry affectthe abrasion resistance of a fabric. Some polymersare intrinsically better abrasion resistant than others.The twist level, yarn crimp and weave design affectthe abrasion resistance of the fabric. The amount offiber and yarn surface that is in contact with theabradant is important. Increasing surface contactincreases the abrasion resistance of the fabric. Lowtwist yarns may present greater surface to theabradant; however, too little twist may leave loosefibers protruding from the yarn body which may besnagged or broken during abrasion. High twistreduces the abrasion resistance of the yarn. Withtoday’s technology, it is possible to arrange abrasionresistant fibers on the sheath while having fibers withhigh tensile strength in the core.

Abrasion resistance of fabrics is measured in termsof visual appearance, number of cycles to open ahole in the fabric and residual strength of the fabric.There are several tests for abrasion resistance:

• Inflated Diaphragm Test (ASTM D3886)• Flexing and Abrasion Method (ASTM

D3885)• Oscillatory Cylinder Method (ASTM

D4157)• Rotary Platform Double Head Method

(ASTM D3884)• Uniform Abrasion Method (ASTM D4158)• The Accelerotor (AATCC 93)• Martindale Abrasion Tester (ASTM D4966)• Special Webbing Abrader

Burst and Impact Resistance

Some applications require resistance of fabricsagainst pressure forces which are perpendicular tothe fabric plane. Filter fabrics, geotextiles,parachutes, transportation bags, air and tensionstructure fabrics must often withstand considerablebursting pressure. Bursting loading is similar tobiaxial tensile loading in which fiber and yarn moduliplay an important role. For better burst resistance,fabrics are designed to have equal properties in warpand filling directions. There are two types of tests tomeasure burst resistance of fabrics: the diaphragm

test (ASTM D3786 Mullen burst test) and the ballburst test (ASTM D3787).

Some fabrics are designed to withstand impactloading. Ballistic protective fabrics, airbags and seatbelts are examples of these types of fabrics. The keyto high impact resistance of a fabric is good energyabsorption in a short time. The energy absorbingcapability of a fabric is indicated by the area underthe load-elongation curve. There are different testmethods to measure impact resistance of fabricsincluding free falling weights, dropping pendulumsand shooting devices [4].

Flexibility and Stiffness

Strength and flexibility are the two properties thatmake textiles unique. Fabric flexibility is affectedby the flexibility properties of the constituent fibersand by the yarn and fabric structure.

To measure the stiffness of fabrics, two methodsare used: either the fabric is bent under its own weightor an external load is applied to the fabric. In thecantilever test (ASTM D1388), a strip of fabric isbent under its own weight. Bending length is one-half of the resulting overhanging length. The stiffness(flexural rigidity) is obtained by multiplying the cubeof the bending length by the fabric weight per unitarea. The Heart Loop Test method is described inASTM D1388 which requires no commercial tester.Another fabric stiffness test, the Circular Bend test,is done according to ASTM D 4032.

Drape and Hand

Drape and hand are extremely important for apparelfabrics. Drape can be defined as the ability of a fabricto bend under its own weight to form folds. Handor handle is a subjective property that can be relatedto the comfort perception of the fabric.

An analysis of fabric hand has been described bythe ASTM as being composed of eight components:compressibility, flexibility, extensibility, density,resilience, surface contour, surface friction andthermal properties. The measurement of theseproperties does not give one an evaluation of hand.Sueo Kawabata of Japan approached the task ofproviding a single value for hand by starting with




the development of instruments that would becapable of evaluating the desired fabric propertiesunder low load conditions. He believed that thiswould more closely relate to the human concept ofhand. His instruments were designed to measure thehand related properties: tensile and shear behavior,bending behavior, compressive behavior, and surfaceroughness and friction. These properties are similarto those listed by the ASTM with the exception ofthermal characteristics. Kawabata developed anequation that gives a weighing to each of themeasured properties and called the resultantsummation Total Hand Value. The weighing factorswere developed through extensive human subjectiveevaluations of a range of fabric types and the rankingof characteristics. The weighing factors are believedto be appropriate for the population within whichthe data were taken but there is some question as tothe application of the same weighing factors in adifferent culture. The instruments of the KawabataEvaluation System (KES) can be used for determiningthe listed fabric properties and are useful in providingrelative data for fabric comparisons.

Compared to other types of fabrics such as knit,braided and nonwoven, woven fabrics wrinkle andretain crease more. This is a good property whencrease is wanted, e.g., ironing. However, for the mostof the time besides ironing, wrinkle resistant fabricsare desired. Fabrics with high extensible fibers thathave good elastic recovery usually have good wrinkleresistance. Fibers with high secondary creep havelow wrinkle resistance. Fabric wrinkle resistance isalso affected by temperature and relative humidity.Densely woven fabrics have less wrinkle resistancedue to low freedom for fiber movement.

Flame Resistance

Flame resistance can be obtained in two ways:

a) by using inherently flame resistant fibers suchas Nomex® aramid

b) by treating (coating) the fiber or fabric withflame resistant chemicals

The disadvantage of using fiber/fabric coating isthe decrease in flame resistance with repeatedwashings. However, this method may be less

expensive than using inherently flame resistantfibers.

There are numerous standard test methods thatdeal with fire and flammability. A compilation ofover 100 ASTM standards is given in the book FireTest Standards published by the ASTM.

One of the beauties of textile technology is that itallows mixing of materials in almost every stage offabric production. In staple yarn manufacturing,different fibers can be intimately blended together toimprove certain properties of yarns. For example acotton, nylon and carbon fiber blend yarn can havecomfort properties due to cotton, good abrasionresistance due to nylon and flame resistance due tocarbon fiber. Another example is Nomex® III byDuPont, which is made of 95% Nomex® aramid forflame resistance and 5% Kevlar® aramid for strength.Morever, during yarn manufacturing, different singleyarns can be plied together to alter properties. In fabricmanufacturing, different warp and filling yarns canbe chosen. Moreover, filling mixing of several fillingyarns is possible with today’s technology.

13.3 WOVEN FABRIC IDENTIFICATION

Table 13.1 lists the more commonly produced fabrictypes by name. Woven fabric structures can beidentified with the naked eye or with microscope.The following guidelines are generally applicable inidentification of various characteristics of wovenfabrics.

Determination of Warp Direction

If one set of yarns have ply in the fabric, it is usuallythe warp yarns. Warp yarn needs to be stronger thanthe filling yarn due to heavy forces acting upon them.In general, the warp density (ends/unit length) is morethan the filling density (fillings/unit length). In thefabric, warp yarns are usually straighter than fillingyarns since filling yarns may have more tendency forbow and skewness. The selvage of the fabric runsparallel to the warp direction. In greige fabrics, thewarps may still have the size material on them whichmakes the yarns stiffer. Prominent stripes or marksare usually in the warp direction. Reed marks alsorun in the warp direction. If the crimp levels are


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different, it is a high probability that there is morecrimp in warp yarns.

Determination of Face and Back ofWoven Fabrics

In general, the fabric design is more visible on theface. For example, in a twill fabric, twill lines aremore prominent on the face. Ribs are more visibleon the face in a ribbed fabric. Satins are smootheron the face than the back. Slub yarn fabrics are moredistinct on the face. The face of the napped fabricsis fuzzier and softer. The face is usually finer andmore lustrous on double fabrics. The face would haveless reed marks than the back. In finished fabrics,the face has better finish quality. In printed fabrics,the prints on the face are more clear and the colorspredominate.

Determination of the Order of Interlacing(Weave)

Order of interlacing can be determined with thenaked eye for coarse fabrics or using a magnifyingglass or a microscope for fine fabrics. It is importantthat an undistorted sample that is larger than therepeat unit (by estimation) is examined from the

main body of the fabric for this purpose. Starting ata randomly selected point on the lower left side ofthe fabric, the interlacing pattern of the warp andfilling yarns is determined until a repeat is found inboth directions. Warp yarns are numbered from leftto right and filling yarns are counted from bottomto top. The selvage design is determined in a similarway. However, it is usually drastically different thanthe rest of the fabric.

Determination of the Presence of Sizeand Finish

Sometimes observation by the naked eye is enoughto detect the size or finish on the fabric. The nextstep would be to determine the hand properties offabrics such as stiffness, smoothness, etc. Ifnecessary, the sample can be observed under amicroscope.

Standard Test Methods

Tables 13.2 and 13.3 list the ASTM and AATCC(American Association for Textile Chemists andColorists) standard fabric test methods. Companytest methods are not included in these tables.

TABLE 13.1 Common fabric type names.

13.3 Woven Fabric Identification


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TABLE 13.2 Standard test methods used for fabric testing (copyright ASTM).


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TABLE 13.3 Standard AATCC test methods used for fabric testing [5] (copyright AATCC).


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TABLE 13.3 (continued).


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REFERENCES

1. Peirce, F.T., “The Geometry of Cloth Structure”, JTI,Vol. 28, T45, 1937.

2. American Society for Testing Materials, Annual Bookof ASTM Standards (Volumes 7.01 and 7.02),published yearly by the ASTM, Philadelphia, PA.

3. “Application of Weftrol™ for Single Width TerryTowel Bow and Skew Control”, Mount Hope,1996.

4. Adanur, S., Wellington Sears Handbook ofIndustrial Textiles, Technomic Publishing Co., Inc.,1995.

5. American Association of Textile Chemists andColorists, Technical Manual of the AmericanAssociation of Textile Chemists and Colorists,published yearly by the AATCC, Research TrianglePark, NC.

SUGGESTED READING

• Ganssauge, D. et al, “How Do Fabric AttributesInfluence the Handle Characteristics of a Fabric?”,Melliand International (2), 1998.

REVIEW QUESTIONS

1. Derive Equation (13.1).2. What fabric properties are affected by the crimp?

Explain.3. Which fabric design would have the highest modulus?

Why? Assume that all the other fabric properties arethe same.

4. How can you increase the tensile strength of a fabric?5. What is rip stop fabric? Explain.6. What equipment is used to correct fabric bow and

skew?7. How is air permeability of fabrics measured? What

is the significance of air permeability?8. What are the factors affecting abrasion resistance?9. For what kind of application of fabrics are burst and

impact resistance important? Why?10. How can you measure the hand of a fabric?

TABLE 13.3 (continued).

Review Questions


fabric structure, properties and testing

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