Tensile Properties of Textile Fibre

Tensile Properties of Textile Fibre

Load : The application of a load to a specimen in its axial direction causes a tension to be developed in the specimen.

The load is usually expressed in gm wt or pounds  wt.

Breaking Load: The load at which material breaks is called breaking load. It is usually expressed in gm –wt or lb-wt.

Stress: Stress is the ratio between the force applied and X-sectional area of the specimen.

So, Stress= Force applied/(x-sectional area)                                 

Units: Dynes/ cm²

Mass stress: Mass stress is the ratio of the force applied to the linear density (mass per unit length).

So, Mass stress=           Force applied/(Linear density)

Units: gm-wt/denier or gm –wt/tex.

Breaking Length: The breaking length is the length of the specimen which will just break under its own wt when hung vertically.

Unit of breaking length is kilogram.

Problem: 100 denier Rayon yarn break at a load of 185 gm, what is breaking length?

          Breaking length=18/1000   X  9000/100

                                    =16.65 km.

Strain: The strain is the term used to relate the stretch or elongation with the initial length.

          Strain= Elongation on or change in length / initial length

                     = BC/AB

Extension: By expressing the strain as a percentage we obtain extension.

Extension= Extension / initiallen   X 100%

Extension is sometimes referred to as the strain percent.

Elastic Recovery: It is a property of a material by which it tends to recover its original size and shape.

Elastic recovery = Elastic Extension / Total Extension

Work of Rapture: This is a measure of the “tough” the mtl. It is the energy or work required to break the specimen. The work of rupture will be proportional to the cross section of the specimen and to its length.

Work of Rapture may be expressed in gm – cm per denier per cm length. A work of Rapture value will indicate the resistance of the mtl to sudden shock.

Work Factor: If the fibre obeyed Hook’s law throughout the test, from zero load to breaking load, the stress strain curve would be a straight line. The ratio –Area under the curve/ (Breaking the stress X Breaking strain) would be equal to one half. For a particular curve this ratio is known as work factor.

Load-elongation curve:

In load-elongation curves of a 250 denier viscose rayon yarn and a 30 denier nylon yarn. The test length in each case was 20 in and the yarns were tested on a Scott Serigraph which operates on the inclined-plane principle to give a constant rate of loading.

Stress-strain curve:

The stress-strain curves derived from the load-elongation curves. The general shape of the curves remains the same but their relative positions have changed. The superior strength of the nylon is more clearly seen and the compression between the two types of fiber made easier.

Stress= Load / Denier

Strain= Elongation/ (Total length X Test Length)

Factors affecting yarn strength:

1. Staple length: Longer staple cotton gives higher strength with synthetics where much longer staple lengths than cotton are available, the increase levels off after the optimum length.
2.     Fiber Fineness: Fine fiber gives greater yarn strength than coarse fibers when spun into a given size. 
3.     Fiber strength: Logically, a strong fiber produces a stronger yarn than a weak fiber.
4.     Twist: For any single spun yarn, there is always a twist that gives maximum strength. A twist less than or greater than this optimum amount results in a yarn of lower strength. 
5.     Evenness: the greater the uniformity of a spun yarn, the higher is its strength and the more uneven a yarn, the lower is its strength.
6.     Fiber length distribution: Variations in the distribution of fiber lengths will cause a variation in yarn strength. The greater percentage of short fibers, the lower the strength of the yarn.
7.     Fiber finish: The type and amount of chemical finish applied to fibers, particularly the man made fibers, has a very definite effect on the strength of the yarn, as well as on the processing characteristics  of the staple.
8.     Maturity: If maturity of fibre increases yarn strength also increases.

Factors affecting the tensile properties of textiles:

1.       Test specimen length.

2.       The capacity of the machine.

3.       The effect of humidity and temperature.

4.       The previous history of the specimen.

5.       The form of the test specimen.

6.       The time of loading and the time to break the specimen.

1.       Test Specimen length:

 If we tested the specimen at a gauge length AB the strength recorded would be that of the weakest point and the value would be S1. If we had tested the specimen in two breaking loads, S1 and S2, the mean of which would be higher than S1. Hence, by testing the yarn at a shorter gauge length the apparent yarn strength has increased. This effect is known as the “weak link” effect.

2.       The capacity of the machine:
If a weak specimen is tested on a high-capacity machine the time to break it will be short, and therefore an optimistic strength result will be produced .The capacity of the machine should be chosen so that the time required to break the specimen is close to the recommended time.

3.       The effects of humidity and temperature:
The mechanical behavior of textile fibers and fibers structures is influenced by the amount of moisture in the specimen. The moisture relationships of the various fibers types differ and naturally. The degree to which the fibers properties are modified will vary. The stress-strain curve for a hydrophobic material such as Terylene and when tested in the dry state will be similar to curve obtained from a wet test. On the other hand the curve obtained when testing say acetate rayon dry and wet will exhibit significant differences.

4.       The previous history of the specimen: The mechanical properties of a specimen before and after straining changes mention ably. Chemical treatment may also affect the tensile properties of the specimen.

5.       The form of the test specimen: The test specimen is a composite structure built up from individual fibers or filaments. Changes in the twist factors used cause changes in the yarn strength, elasticity, liveliness, luster, and other yarn characteristics. In case of fabric, the warp way properties differ from weft properties.

6.       Time of loading and breaking: A rapid test produces a higher breaking load than a slow test.
Let, F_T=the breaking load for a time to break of T sec, and 
     F₁₀=the breaking load for a time to break of 10 sec.
     Then,
            F_T=F₁₀(1.O9-0.O9 log T)
 By rounding off the figures we obtain,
            F_T=F₁₀(1.1-0.1 log T)

Types of Tensile Strength Testing Instruments:

Principle	Application	Type of instrument
1.     pendulum lever	Single fibre Fibre bundle Single thread Lea or skein Fabric(vertical &hori.)	Balls Magazine Hair tester Stelometer Good brand, Bear, Scott
2.     Balance	Single fiber Fibre bundle Fabric	Barratt, loading by solenoid. Pressley Good brand
3.     Spring	Single fibre Single yarn Fabric	Cambridge Extensometer, -Shorter Hall, flat spring. -Harrison , torsion bar.
4.     Inclined plane	Single fiber Fabric	Uster Automatic Uster I.P.4
5.     Ballistic or impact	Fiber bundle Yarns and fabric	Lang Good brand
6.     Strain gauge or 7.     Transducer	Yarn	Straino meter,
8.     Constant –tension winding test	Running yarn	B.C.I.R.A.
9.     Dynamic tester	Special research instruments for measuring the modulus of dynamic elasticity and stress- strain properties at high speed	Ballou and Siverman
10.          Hydraulic tester	Bursting tester for fabrics	Good brand Catalogue

Muhammad Sazzad Hussain
B.Sc. in Textile Engineering
Daffodil International University
Email : textileknowledge99@gmail.com
Phone Number : +8801684380984