Tensile Properties of Textile Fibres

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   ; Tan α ↑↓      →    extension ↓↑
                                                      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
                Total work done in total extension = (Work done in elastic extension + Work done in
                                                                          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.


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 extensometer
·   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