Chapter 5 Tensile Strength Testing of textiles

Chapter 4
Tensile testing of Textiles

. Normally the strength of fibres or fibre structures is commonly
regarded as the criterion of quality.
. Although strength in testing is of great importance, but
properties such as flexibility, resilience, moisture absorption, dye
affinity etc are necessary to be considered in the assessment of
goodness or quality of a textile material.
. It is important to note that the Relative importance of one
particular property will depend mostly on the end use.
. Most of the properties are interrelated.
. Before understanding testing of materials, it is essential to
know the different units and terminologies employed.

Load:
This refers to the application of pressure to a specimen
in it’s axial direction.
It causes a tension to be developed in the specimen.
The load is usually expressed in grams weight or
pounds weight like gravitational units of force.
(Axial: Located on, around, or in the direction of an axis.)

Breaking Load:
This is the load at which the specimen break usually expressed in
grams weight or pounds weight.
Stress:
This is the Ratio between the force applied and the cross-section
of the specimen. That is an effort is being made to compare the
fine and coarse structure of the specimen.
Stress: Force Applied
Cross sectional Area.
The force is accurately expressed in dynes or poundals. Hence,
the stress on a fibre may be given in terms of Dynes per sq
centimetre ( dyn / cm2).

Mass-Stress:
The cross section of many fibres and fibre structures are
irregular in shape and difficult to measure. To simplify
matters, a dimension related to cross section is used,
known as the linear density ( the amount of mass per unit
length))of the specimen. The Linear density may be
expressed in denier or tex –count and the mass-stress
then becomes the ratio of the force applied to the linear
density ( Mass per unit length).
Mass Stress = Force Applied
Linear Density
The Units of the Mass stress therefore become grams
weight per denier or grams weight per tex. The
abbreviated for of “Mass Stress” is ‘Stress’ and quoted in
grams per denier or grams per Tex.

Tenacity or Specific Strength:
The Tenacity of a material is the mass stress at Break.
The units being, gms per denier or gms per tex.
The value used for the denier of a specimen is usually
the nominal denier before testing, no account being
taken of the decrease in denier as the specimen is
stretched and becomes finer. For some purposes this
change is noted and suitable correction made.
It is useful to note that by expressing the breaking
strength of different materials in terms of tenacity,
comparisons can be made directly between specimens
of varying fineness.

Breaking Length:
The ‘ Breaking length’ is the length of the
specimen which will just break under its’ own
weight when hung vertically. The unit for breaking
length is the kilometre.
Eg: If, for example, a 100 Denier viscose yarn
broke at a load of 185 gms, the breaking length
would be;
185 x 9000 =16.65 Km
100 x 1000

Strain:
When a load is applied to a specimen a certain amount of
stretching takes place. The amount will vary with the
initial length of the specimen. The ‘Strain’ is the term used
to relate the stretch or elongation with the initial length.
Strain = Elongation
Initial Length.
Extension
By expressing the strain as a percentage, we obtain the
extension,
Extension = = Elongation x 100
Initial Length.
The extension is sometimes referred to as the ‘ Strain
Percent’.

Breaking Extension:
The Breaking extension is the extension of the
specimen at the breaking point.
Load Elongation Curve:
An extremely important curve is produced when the
load on a specimen is plotted against the elongation.
This curve describes the behaviour of the specimen
from Zero load and elongation up to the breaking point.
.
From this load elongation curve much important
information can be gained, initial young modulus , work
of Rupture, yield point etc.

The Stress-Strain curve:
For some purposes, it is more convenient to convert
the load-elongation curve to a stress-strain curve, there
by enabling more direct comparison to be made
between different types of materials and structures.

The Load elongation curve of 250 Denier viscose rayon
and a 30 Denier Nylon yarn is shown. The test length in
each case was 20 Inches and yarn was tested on a Scott
Serigraph which operates on the inclined plane
principle to give a constant rate of loading. To convert
the curves to stress strain curves we must draw up a
table of values of stress strain derived from
measurements on the load-elongation curves. Stress
values are obtained by dividing load values by the
denier; strain values are calculated by dividing the
actual elongation by 20 in the test length. For instance,
at 1” elongation the load on the viscose was 272 gms.
The stress at this point was therefore 272 / 250, 1.08
g / denier. The corresponding strain was 1/20 or 0.05.

The figure following shows the stress strain curves derived from
the load elongation curves. The general shape of the curves
remain the same but the relative positions have changed. The
superior strength of the nylon is more clearly seen and the
comparison between the two types of fiber made easier.

Why in the Stress-Strain curve, different types of materials have
different types of curves?
The stress strain curves of fiber structures such as fabrics and
yarn will reflect the characteristics of the fibers are fibers from
which they have been built , modified according to the methods
used in their construction, eg Twist, weave, Physical and
chemical finish, etc. A study of the properties of the fiber is there
fore of paramount importance. The stress-strain curve of a fiber
is governed mainly by its molecular structure;.
When an external force is applied to a material it is balanced by
internal forces developed in the molecular structure of the
material.

The arrangement of molecules with in the fiber in a
amorphous and crystalline form with varying degrees. In the
early stages of stretching the material, the elongation is mainly
concerned with the deformation of the amorphous regions, in
which primary and secondary bonds are stretched and
sheared. If the stress were removed at this stage most of the
extension would be recovered and the material would exhibit
elastic properties.
By increasing the stress further, the stress-=strain curve bends
sharply and large strains or extensions are produced by small
increase in stress.
The stress-strain curve begins to bend towards the stress axis
un till finally the breaking point is reached.
Since different materials have different molecular structures it
follows that their stress-strain curves will be different.

The initial Young’s Modulus:
A particularly important part of stress-strain curve is the initial
portion starting at Zero stress and strain. The initial portion of
stress-strain curve is fairly straight , indicating a linear relationship
between stress and strain.
In other words, the material behaves like a spring . The
significance of this is that when the load is removed, the material
recovers its’ original length or nearly so.
The tangent of the angle between the initial part of the curve and
the horizontal axis is the ratio stress;strain. In engineering science
this ratio is termed “ Young's’ modulus”. It gives a measure of the
force required to produce a small extension.
A High modulus indicates inextensibility and a low modulus great
extensibility. In textiles we use the term initial young modulus and
it describes the initial resistance to extension of a textile material.

Count Strength Product ( CSP)
As we know, the strength has a bearing on the linear density of
the material. In considering the yarn strength to judge its tensile
properties, it is CSP that is normally referred. The count strength
product will reflect how the yarn in a bunch behaves instead of a
single yarn, in consideration its Linear density, that is either
Count, Denier or Tex.
The strength of a bunch of yarn is measured by a instrument
called Lea-Tester.

Chapter 5 Tensile Strength Testing of textiles

LEA TESTING (CSP)
YARN COUNT
It indicates the weight per unit length or length per unit weight.
In English system count denotes the number of hanks of 840
yards that will weight one pound.
LEA
A continuous length of yarn in the form of a coil of 80 loops
made on the reel of girth of 1.5 yards. Ie 120 Yards

YARN STRENGTH
It is measured in terms of lea strength or skein strength and
expressed as count strength product. The CSP indicates the
number of leas required to break under their own weight when
the leas are tied one over and hung vertically.
LEA TESTER
The lea tester is a simple and versatile machine. They are made
in different capacities upto 100,200,300 and 500 lbs. The unit
records tensile strength and extension at breaking. While
carrying out the test, an instrument of suitable capacity is
chosen so that most of the test readings fall between 25 and 75
persent of the maximum value that can be registered on the
instrument. The rate of traverse is 12” per minnute.
Sampling
Take 10 cops or 5 cones per sample.

Automatic Wrap Reel
It is designed for preparing leas of 120 yards of definite length
with uniform tension, it helps in preparing more number of leas
in short time.
Preparation of test specimens
1. Number the selected cones/cops and fix them on the bobbin
holder of the wrap reel.
2. Reel out the required length of 120 yards for wrap reel.
3. Cut and tie the trailing end of the lea to its leading end.
4. Similarly take 30 leas making a total of 40 leas from the
same 10 bobbins.
5. Condition the sample in a conditioning box for about 12
hours.
6. Determine the mass in grams of the leas and calculate the
count
Cotton count = 64.80 / Weight of lea in gms

Lea strength test procedure
1. Bring the hooks of the testing machine to zero position.
2. Take a particular lea with count known and fix it on to the
hooks and carefully separate the yarn to avoid overlapping on
individual strands.
3. Start the m/c and carry out the test up to rupture.
4. The lea strength is automatically recorded in the system. The
count CV%, the strength CV%, Lea CSP, maximum and minimum
values of count and strength are obtained in the form of
printouts.
5. Similarly find the breaking load of the remaining leas and
record them against their respective counts.
6. Calculate the avg. breaking load of the, avg. linear
density(count) of all the observations taken, coefficient of
variation (CV) of breaking load and CSP
7. The product of average strength to the Avg count of the lea
will give you the CSP ( Count strength Product)

Chapter 5 Tensile Strength Testing of textiles

More Related Content

Similar to Chapter 5 Tensile Strength Testing of textiles (20)

Recently uploaded (20)

Chapter 5 Tensile Strength Testing of textiles