Tensile
Properties of Textile Fibres
The behaviors shown by
textile materials (fibre, yarn, fabric etc.) when it is subjected to load or tension, are known as tensile
properties.
a)
Breaking load:
The load required to break
a specimen is termed as breaking load. Breaking load depends on fibre type,
nature of fibre bonds, crystallinity, orientation etc. Breaking load is usually
expressed by kilogram, gram, pound, Newton etc.
b)
Tensile strength:
The term “Tensile” has been derived from the word
“Tension”. Tensile strength is very important property of textile materials
which represents the ratio between force required to break a specimen and
cross-sectional area of that specimen.
Tensile strength = Force required to break a specimen
Cross-sectional area
c) Tenacity:
Tenacity can be
defined as the ratio between breaking load and linear density of specimen.
Tenacity of a specimen may be expressed as the units of gram/tex, gram/denier,
Newton/tex etc.
Tenacity = Breaking
load
Linear density
d) Breaking extension:
The load
required to break a specimen is a useful quantity. Breaking extension of a
specimen can be defined as the actual, percentage increase in length upto breaking.
So, it can be said that, the length of a specimen which extends for applying
load before breaking is known as breaking extension and it is usually expressed
as the percentage.
Breaking
extension (%) = Elongation at break x 100
Original length of
specimen
e) Initial modulus:
The tangent of angle between
initial curve and horizontal axis is equal to the ratio of stress and strain.
In engineering science, this ratio is termed as initial modulus but in textile
science it is known as initial young’s modulus. Initial modulus of textile
materials depends on chemical structure, crystallinity, orientation of fibre
etc.
Initial modulus, tanα = Stress
Strain
f) Work of rupture:
The energy required to break a specimen or
total work done for breaking a specimen is termed as work of rupture and is
expressed by the units of joule, calorie etc. If applied force ‘F’ increases
the length of a specimen in small amount by ‘dl’, then we have-
Work done = Force X
Displacement
=
F X dl
g) Work factor:
Work factor can
be defined as the ratio between work of rupture and the product of breaking
load and breaking elongation.
So, Work factor
= Work of rupture
Breaking load x Breaking
elongation
If the fibre obeys hook’s law, then
the load-elongation curve would be a straight line and the work of rupture = ½
x Breaking load x Breaking elongation
So, in an ideal case, the work
factor, Wf = 1/2, whereas, Wf >1 for top curve and Wf <1
for bottom curve.
h) Work recovery:
The ratio between work returned
during recovery and total work done in total extension is known as work
recovery.
Work recovery = Work returned
during recovery
Total work done in total extension
Total extension = Elastic
extension + Plastic extension
i) Elastic recovery:
The power of recovery from an
immediate extension is called as elastic recovery. Elastic recovery of fibres
depends on type of fibres, fibre structure, type of molecular bonds and
crystallinity of fibres. Elastic
recovery can also be defined as the elastic extension against total extension
and expressed as the percentage.
So, Elastic recovery (%) = Elastic
extension x 100
Total extension
j) Creep:
When load is
applied on a textile fibre, an instaneous strain is occurred in the fibre and
after that strain the fibre strain will be lower with passing time that means
slow deformation will be occurred. This type of behavior of textile fibre is
known as creep. The formation of crease
marks on cloth depends on creep behaviors of fibres. Creep is usually
classified into two classes-
i) Temporary
creep:
This type of creep is temporarily occurred
in fibre. So, after removing load it is possible for textile fibre to recover
it’s original shape. Here, elastic
deformation is occurred and fibre does not break, only molecular chains of
fibre get stretched.
ii) Permanent creep:
This type of creep is permanently occurred
in fibre. So, after removing load it is not possible for textile fibre to
recover it’s original shape. Here,
plastic deformation is occurred and molecular chains of fibre break, hence the
whole fibre breaks.
Stress-strain
curve:
A yield
strength or yield point is the material property defined as the stress at which a material begins to
deform plastically. Prior to the yield
point the material will deform elastically and will return to its
original shape when the applied stress
is removed. Stress at yield point is known as yield stress and strain at yield
point is known as yield strain.
Factors
affecting the results of Tensile Testing:
· Materials and
it’s conditions:
- The chemical
treatment to which it has been subjected
- The mechanical
treatment that it has received
- Amount of
moisture that it contains
- Temperature in
the testing atmosphere
· Arrangement
& Dimension of the specimen
· Nature &
Timing of the test
Methods or Principles of Tensile
Experiment:
01. Constant rate
of loading (CRL):
A specimen is
gripped between two jaws-top jaw which is fixed and bottom jaw which is
moveable. The load on specimen is initially zero, but increase at constant
rate. By adding constant rate of water in a container which is attached to the
bottom jaw, may increase the load gradually. Thus, constant rate of flow gives
the constant rate of loading. The function of this applied force is to extend
the specimen until it eventually breaks down. Thus, loading causes the
extension.
02. Constant rate
of elongation (CRE):
A specimen is
gripped between two jaws-top jaw which is fixed and bottom jaw which is
moveable to downward direction at a constant velocity by means of a screw
mechanism. Initially the tension on specimen is zero. But, when the bottom jaw
moves downwards at a constant rate, the specimen is extended and an increasing
tension is developed until the specimen finally breaks down. In this case, the
extension causes loading.
Types of Tensile Testing Instruments:
·
Cambridge
·
Scott inclined plane tester
·
Cliff tester
·
Pressly fibre strength tester
·
Instron tensile strength tester
·
Lea strength tester
Tensile properties of
fibres:
|
Fibre
|
Tenacity
(N/tex)
|
Breaking extension (%)
|
Work of rupture
(mN/tex)
|
Initial modulus
(N/tex)
|
|
Cotton
|
0.19-0.45
|
5.6-7.1
|
5.1-14.9
|
3.9-7.3
|
|
Jute
|
0.31
|
1.8
|
2.7
|
17.2
|
|
Silk
|
0.38
|
23.4
|
59.7
|
7.3
|
|
Nylon
|
0.47
|
26
|
76
|
2.6
|
|
Polyester
|
0.47
|
15
|
53
|
10.6
|
|
Wool
|
0.11-0.14
|
29.8-42.9
|
26.6-37.5
|
2.1-3.0
|
|
Viscose
|
0.18-0.27
|
15.2-27.2
|
18.8-30.6
|
4.8-6.5
|
