tensile properties of fibres 2

8/17/2019 Tensile Properties of Fibres 2

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TENSILE PROPERTIES OF FIBRES

INTRODUCTION

The tensile properties of textile fibres (fibre strength and elongation) are very important from the

point of view of their behavior during processing and the properties of the final product.Generally, the fibre strength is the maximum load / force / tension that it can sustain before it

breaks. The simplest way to study the fibre strength is to apply a gradually increasing load to a

fibre and measure the load and elongation at the time the fibre breaks. ncreasing loads atvarious stages and corresponding elongation can be recorded! a load elongation graph can be

constructed as shown below"

Terms related to Tensile Properties:

Load" #oad is the force applied either by dead weight or by any other means to a specimen in

the direction of its axis. The load causes tension to be developed into the material (fibre!arn"# The load is generally expressed in terms of gm weight (gravitational unit of force) in case of a

dead weight or in terms of $ewton ($) or %enti&$ewton (c$) in case other types of load

applications.

'. reaking load" The load at which the specimen breaks is called the breaking load.. *tress" To compare the tensile properties of different types of fibres independently or the

direct effect of their dimensions, in place of load, +stress is used. *tress is defined as theload or force acting per unit cross section area of the material.

Stress= Load /( Force)

Areaof Cross−section


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The units of stress may be g/cm or $ewton/m (-ascal) or dynes/cm

. ass *tress" The cross sections of many fibres and fibre structures are irregular in shapeand difficult to measure. To simplify the matter a dimension related to cross&section is

used. The linear density is such a dimension. The linear density may be expressed in

denier or tex count and the +mass stress then becomes the ratio of the force applied to thelinear density (mass per unit length).

Mass Stress= Force Applied

Linear density

The unit of mass stress therefore becomes grams weight per denier or grams weight per tex. 0ere again abbreviations are used (g/denier or g/tex).

1. Tenacity or the *pecific *trength" The tenacity of a material is defined as the mass stressat break. The units are same as those of mass stress i.e. g/tex, g/den. 2n alternative term

for tenacity is specific strength.

3. reaking #ength" The breaking length is the length of the specimen which will 4ust break under its own weight when hung vertically. $aturally we do not build tall towers in order

to measure this breaking length but calculate it from the results of tests or short lengths.

The expression of strength in terms of breaking length is useful for comparing the

strength of different fibre structures e.g. for comparing single fibre strength with the yarn

strength. The unit of breaking length is kilometers. The other unit for breaking length is56. 56 stands for 75eiss 6ilometre8 in German and 75esistance 6ilometri9ue8 in

:rench. 56 means kilometers of yarn for break.

;. *train" xtension " >xtension is the strain expressed as a percentage

Extension= Elongation Initiallength

x 100

The extension is sometimes referred as +strain percent.

?. reaking extension" The +breaking extension is the extension of the specimens at the

breaking point.


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@. #oad >longation %urve" 2n extremely important curve is produced when the load on a

specimen is plotted against the elongation. This curve describes the behavior of aspecimen from Aero load and elongation up to breaking point.

:rom a close study of this curve very important information can be obtained such as

initial youngs modulus, work of rupture, yield point etc.

*tress strain curve of viscose derived from a load extension

curve of a BB denier yarn in approximate standard atmosphere

The textile fibres differ from metals in that, metals are crystalline materials while the

fibres are visco&elastic. The load elongation curve of fibres is therefore somewhat

different. The load elongation curve of purely elastic materials like metals would be astraight&line while the load&elongation of most of the fibres shows a straight line in the

initial part up to small stresses. 0owever, as the stress is increased further, the stress

strain curve bends sharply and large extensions are produced by small stresses. 2 sort of plastic flow of material occurs. *ince different fibre materials have different molecular

structures their stress strain curves will be different.

'B. nitial Coungs modulus" 2 particularly important part of the stress strain curve is the

initial portion starting at Aero stress and strain. :rom the :ig. it can be seen that this

initial part is fairly straight, indicating a linear relationship between stress and strain. nother words, material in this portion behaves like an elastic material and obeys 0ookes

law. This part of the curve is sometimes referred as 0ookean region. The significance of

this portion is that when the load is removed the material recovers its original length or

very nearly so.The tangent of the angle between the initial part of the curve and the horiAontal (x axis) is

the ratio stress/strain. This ratio is defined as initial Coungs modulus and it describes the

initial resistance to extension of the material.

f the stress unit is grams/denier, the initial Coungs modulus will also have the same unit

as the strain has no units.

''. Cield -oint" 2fter the initial straight path the curve bends towards right and beyond this

bending point, the material no longer behaves as elastic material. #arger extensions are


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produced by relatively smaller increase in stress and most of this stretch is irrecoverable.

This bending or yield region is located by the yield point! which is determined

geometrically. The point at which, the tangent to the curve is parallel to the line 4oiningthe origin and the breaking point is taken as the yield point.

The yield point can be defined in terms of yield strain or stress. 2lternative terms for

yield point are limit of proportionality and elastic limit.

'. The +


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0ence the total work done in breaking the fibre

D work of rupture D ∫O

break

F x dl

D area under the load elongation curve

*pecific work of rupture" The work of rupture is proportional to the cross&section of the fibre

and to its length. Therefore to compare different materials a term +specific work of rupture is

used.

Specific work of rupture =Work of rupture

( mass/unit length ) x initial length

The units of specific work of rupture can be derived as follows.

Specific work of rupture =Work (energ!

mass/length x length

=Force x length

mass/length x length

=Force

mass/length

D $m / kg (* system)

D $ / tex Jirect $ / denier system

'.


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have a value of elastic recovery between B to '.The elastic recovery can also be expressed

as percentage.

>lastic recovery values are affected by several factors such as I time allowed for recovery, the moisture in the specimen, and the total extension used in the test.

Therefore, in order to make comparison between the different materials it is necessary to

specify the conditions under which the elastic recovery is determined.

*tress&strain curves

%omparison of the two stress&strain curves in the figure below would result in conclusions

similar to the following"

:iber 2 is stronger than fiber and should be able to carry more load & the tenacity of fiber 2 is

greater than the tenacity of fiber .

:iber 2 is stiffer than fiber and should be more resistant to initial deformation & the modulus

(slope of the curve) of fiber 2 is greater than the modulus of fiber .

:iber is more extensible than fiber 2 and should absorb large deformations more readily & the

extension to break is greater for fiber than for fiber 2.

The fibers appear to be e9uivalent in energy&absorbing capacity, but fiber would probably be a

better candidate for seat belt webbing! the area under both curves appear to be e9ual, but fiber

is more extensible.

:iber 2 is more elastic than fiber & the yield point in curve 2 is more pronounced than the yield

point in curve .

:igure" *tress&*train %urves %omparison


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tensile properties of fibres 2

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