Fibre strength testing
Fibre strength is an important category of fibre properties. It is important to estimate fibre characteristics, yarn structure, and fabric properties.
To measure fibre strength, factors like ‘breaking strength’, ‘tensile strength’, and ‘tenacity or intrinsic strength’ are useful.
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| Filament fibre strength |
Breaking strength
Breaking strength indicates the maximum tension that the fibre can sustain before it breaks from a particular point.
For cotton fibre, breaking strength depends upon various factors such as the inherent strength of the material, the number and intensity of weak places on the fibre surface, and the area of cross-section of the fibre.
However, the cross-section area of the fibre is the main reason behind the fibre strength as a result, coarse cotton generally records higher values for fibre breaking strength than finer ones.
Therefore, to compare the strength of two types of cotton (One is coarser and another is finer), it is vital to eliminate the effect of the difference in cross-sectional area by calculating the values of the fibre strength per unit cross-sectional area.
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| Force elongation curve |
Tensile strength
Tensile strength is the maximum force recorded in extending a test piece up to the breaking point. (This is generally known as the strength of the fibre.)
The force required to break a specimen is directly proportional to its cross-sectional area.
The tensile force recorded at the moment of rupture is often considered as the tensile strength at break.
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| Force extension curve |
Tenacity or Intrinsic strength
Tenacity is the specific stress of fibre which is corresponding with the maximum force on a force/extension curve.
Factors affecting Fiber strength
Fibre strength depends on factors like time taken to apply the rupture on the specimen, RH value, fibre maturity atmospheric contain, and Specimen length.
In some points where fibre is weaker or band, this kind of point generally lowers the fibre strength, However, if other places are stronger but fibre can easily break from the weak points.
Some structural imperfections also deteriorate strength value.
Atmospheric conditions or humidity affect the strength of fibres. Generally with the increase in RH value, the Strength of fibres increases.
Single fibre strength and bundle fibre strength also affect the test results. Generally, strength will increase concerning several fibres.
Measurement of strength
Fibre strength is generally measured by three principles.
- The constant rate of the traverse - CRT
- The constant rate of specimen elongation - CRE
- The constant rate of loading - CRL
The constant rate of the traverse - CRT
In the instruments with the CRT type principle, strength is measured with a constant rate of the traverse.
In this method, the specimen is held between two clamps, one of which is the lower clamp (in a vertical arrangement) is traversed at a constant speed to exert a load on the specimen.
While the other clamp works against a spring or a pendulum which is suitable in weighted.
The load on the specimen is measured by the extension of the spring or the swing of the pendulum, during testing.
In these instruments, the rate of loading is directly affected by the extensibility of the specimen and hence it will vary with the type of specimen under test.
In the case of instruments using a pendulum principle, the errors due to inertia are appreciable.
As there is a time lag before the pendulum is accelerated to the required speed, there will be a lag in the rate of loading in the initial stages, while at the later stages there is a likelihood of the pendulum overshooting beyond the breaking load of the specimen.
In these instruments, it is quite difficult to adjust the ‘time-to-break’.
The constant rate of specimen elongation - CRE
In the instruments with the CRE type principle, strength is measured with a constant rate of elongation.
In this method, CRE can be estimated by modifying the CRT instruments suitably. (In this modification, do render negligible movement of the upper clamp.)
For this purpose, the upper clamp is fixed to stiff support and the load on this support is measured by suitable means such as a strain gauge as in the case of the Instron Tester.
In these instruments, it is easy to adjust the ‘time-to-break’, and the final result of this test is more accurate than the CRT method.
(However, CRT is not able to give accurate results through this principle because it is widely used given the ease of operation and rapidity of testing.)
The constant rate of loading - CRL
In the instruments with the CRL type principle, strength is measured with a constant rate of loading.
In this method, CRL can be accomplished by the sliding of weight down an inclined plane or by the extension of a spring or flow of water at a constant rate, etc.
The CRL type of instrument is generally suitable for accurate scientific work given the case of interpretation of the results obtained with these instruments.
In these instruments, it is easy to adjust the ‘time-to-break’, and also gave more accurate results than the CRT method. It is because the rate of loading is dependent on the extensibility of the specimen.
Some instruments can be made to function with more than two principles. I.e. - Instron is working on the CRE and the CRL principles.
Questions -
- What is breaking strength?
- Draw force-elongation curve.
- What is tensile strength?
- Draw a force-extension curve.
- What is tenacity/intrinsic strength?
- Which factors affect fibre strength?
- On which principles, strength is measured?
- What is a constant rate traverse?
- What is the constant rate of specimen elongation?
- What is the constant rate of loading?
References
Cenote, M. (2015). Google Books. In The SAGE Guide to Key Issues in Mass Media Ethics and Law (pp. 847–858). SAGE Publications, Inc.
Fundamental textile testing: Mechanical and Physical Tests. (n.d.). Fundamental Textile Testing: Mechanical and Physical Tests. from https://www.eurofins.com/textile-leather/articles/fundamental-textile-testing-mechanical-and-physical-tests/
Handbook of textile testing and quality control - Google search. (n.d.). G.Co. from https://g.co/kgs/RvVMgm
NPTEL IIT. (n.d.). Note.Ac.In. from https://nptel.ac.in/courses/116102029
Elsevier. (2016). Performance testing of textiles (1st ed.). Woodhead Publishing.
Houck, M. M. (2009). Identification of Textile Fibers (M. M. Houck, Ed.; 1st ed.). Woodhead Publishing.
Textile School. (2010, October 27). Fibre Identification - tests to identify a fibre. Textile School. https://www.textileschool.com/321/fiber-identification-tests-to-identify-a-fibre/
Trivedi, Y. (2020, April 10). Identification of Textile Fibers. Textilesphere.com. https://www.textilesphere.com/2020/04/identification-of-textile-fibers.html
(N.d.). Textilelearner.net. from https://textilelearner.net/classification
