Determination of Yarn Tensile Strength

Tensile Properties of Yarn:
Tensile property of yarn is an important parameter in determining the suitability for any particular application. So, it is utmost importance to determine this characteristic accurately. There are three basic principles for measuring yarn tensile strength. A yarn’s tensile property can be defined as the maximum force/load that is required to break the material. It is a parameter of vital importance regarding the fabrication of yarn because it directly influences the strength of the developed fabrics. Two different approaches are used to measure yarn strength. In the first approach a single yarn strength is determined. Normally Newton (N) and cN units are used. The amount of force required by an object having mass of one kilogram to accelerate it to one meter per second square is called as one Newton force. A single yarn strength provides information about the warping machine and loom efficiency. In order to calculate the combined strength effect of the yarn, the count lea strength product (CLSP) of the yarn is calculated. A lea of 120 yards is made using the wrapping reel, and the weight of the lea is determined in order to calculate the yarn count by using the formula: number of hanks/pound. The lea strength is determined by using a lea strength machine that has two jaws, one fixed and the other attached to the load. Using the constant rate of loading principle, the tensile strength of the lea can be determined.

Types of Tensile Strength Testing Machines:
On the basis of the working principle, tensile strength testing machines can be categorized into three major categories.

Constant Rate of Extension (CRE):
Here, the rate of elongation of the test specimen is kept constant and the load is applied accordingly. The working principle of the Tensorapid-4, which is used to evaluate the tensile strength of a single yarn, is a constant rate of elongation.

Constant Rate of Loading (CRL):
The machines apply the load on the test sample, which is increased constantly with time. The specimen is free to elongate and its extension depends on its properties for any applied load. The working principle of the lea strength machine is of this category.

Constant Rate of Traverse (CRT):
In this type of machine two pulling clamps are used to evaluate the tensile strength of the sample. One clamp moves with constant speed and application of the load is done by the second clamp, which is responsible for the activation of a load measuring mechanism. Normally old machines use this mechanism, such as the old fabric tensile strength testing machine.

Determination of Tensile Strength of Yarn:
Tensile strength is a crucial mechanical property of yarn that measures the maximum tensile or pulling force that a yarn can withstand without breaking. It is a key factor in determining the suitability of a yarn for various applications, such as textiles, sewing, knitting, or industrial uses. Tensile testing of yarns is used to determine the breaking force, elongation, and toughness properties of the yarn.

As yarn forms the basis of production of all types of fabrics it is essential that the strength and elongation of the yarn is monitored by a yarn strength testing system to ensure that it is suitable for today’s high-speed production techniques and its performance matches the requirements of the finished product.

• Tensile: The tension capacity in a material or tension exerted on a material.
• Strength: The property of a material that resists deformation induced by external force.
• Tensile strength: The strength of a material under tension which is distinct from compression, fusion or shear.

The theoretical treatments of yarn tensile strength primarily aim at exploring the various effects of the critical parameters influencing yarn strength. In practice, yarn mechanics is a desirable subject in the context of two key aspects: (i) estimating yarn strength, and (ii) predicting yarn strength. These two aspects are typically handled using empirical models in which yarn strength is related to some processing parameters or fiber properties.

Tensile strength of yarn by skein method:
This method involves the reeling of the yarn onto a skein more commonly known as a lea through a wrapping reel, which is used for measuring linear density, the two loose ends being tied together. This lea is mounted on two jaws of a tensile strength tester. After that the lea is subjected to increasing extension and the force applied is recorded. As one portion of the lea is broken from a point in the weakest region, the maximum force applied is noted in kilograms or pounds. The strength of at least 10 leas of the same count is measured by using the above method from which the mean is calculated. The British Standard determines a hank or lea of 100 wraps of 1 m distance across. This is tested at a certain rate, namely, up to the point at which it breaks within 20 ± 3 s. Alternatively, a consistent velocity of 300 mm/min may be used. On the off chance that the yarn is spun on the cotton, worsted frameworks of 10 skeins ought to be tested with 20 leas for woolens. The strategy is not utilized for persistent fiber yarns.

Count strength product or CLSP:
The check quality item (CLSP) is a term utilized for staple spun yarns of cotton and the lea (hank) quality. It depends on determining the quality of the lea made on a wrapping reel having a circumference of 1.5 yards; a 80 turns lea has a total length of 120 yards. The quality is commonly noted in pounds force (lbf). This strength, measured in pounds, is then multiplied with the English cotton count of that yarn to get the CLSP.

CLSP(Count Lea Strength Product) = Count(Nec) x Lea strength(in pounds)

Tensile strength by single yarn method:
The tensile testing of yarns is commonly done in accordance with ISO 2062 and ASTM D2256. These tests are used to determine the breaking force, elongation, and toughness properties of the yarn. The breaking tenacity, a ratio of the breaking force to yarn linear density, is also a common property for evaluating the strength of a yarn’s material and for comparison and validation purposes. It is necessary to clamp the yarn test specimen so that the machine loading axis is aligned with the specimen axis. This alignment is most easily achieved and repeatable using capstan style grips. Sharp edges or changes in the path can cause specimen failure to occur outside the gauge section and far below the actual strength of the yarn. Capstan grips also help to avoid this, as the yarn never encounters sharp changes in geometry. Since elongation properties are important for the product application of yarn materials, it is necessary to prevent slippage of the yarn during testing, which is accomplished through the even distribution of the load over the capstan instead of using only a set of clamp jaws.

Mostly the test standards are similar. In order to obtain more precision in results, the tests are performed many times so that accurate results are obtained. According to British standards the following number of tests should be performed:

a. For a single yarn

• For continuous filament yarns perform 20 tests
• For spun yarns perform 50 tests

b. For cabled and plied yarn perform 20 tests

Test Procedure:
Before the start of the test, the atmospheric conditions of the laboratory should be maintained according to standard. The settings of the machine should also be accurate and meet the demands of the standard. Mostly the USTER TENSORAPID/USTER TENSOJET testing machine is used for this purpose. The gauge length for the test is 500 mm and pretension is set to 0.5 cN/Tex.

First of all the conditioned yarn is fixed into the USTER TENSORAPID/ USTER TENSOJET and is adjusted between the two jaws of the machine, one of which is movable and the other stationary. It works at a speed of 5000 mm/s and the gauge between the two jaws is 500 mm. The machine is turned on and the test is started. The tests are performed automatically and stop after 20 have been completed. After completion the result is printed, which gives the value of the tensile strength and its coefficient of variation.

We require a large number of tests to be performed in less time and with higher efficiencies and accuracy levels. To meet this requirement USTER Technologies produces the USTER TENSORAPID/USTER TENSOJET testing machine, which is frequently used for measuring the single yarn strength. USP or USTER statistics enable us to compare the results of single yarn strength, whether they fall within an acceptable range or not. Mostly, the mean strength is not so important, though the frequency of any weak place is. Due to these weak places, yarn breakage occurs during subsequent processes of weaving and causes low production efficiencies or fabric faults that must be avoided to obtain high quality and more production. In high speed production the weak places still cause problems even if they occur after hundreds of meters. Therefore, in such cases, the coefficient of the strength of a single yarn is of greater importance than the mean value.

In order to check greater lengths of yarn the speeds of the machine are kept higher, otherwise the tests would take longer if the standard test time of 20 s was used. The greater the number of tests, the better will be the statistical prediction of weak spots and more precise results of tensile strength will be obtained.

Conclusion:
Determination of the yarn tensile strength is a critical aspect in assessing its mechanical performance and suitability for various applications. Tensile properties of yarn influenced by factors such as fiber type, fineness, yarn structure, processing methods, and environmental conditions, provides valuable insights into the yarn’s ability to withstand pulling forces without breaking. Understanding of the tensile strength of yarn helps optimize yarn selection for specific purposes, ensuring the durability and performance of the final products.

References:

1. Structure and Mechanics of Textile Fibre Assemblies Edited by P. Schwartz
2. A Practical Guide to Textile Testing By K. Amutha
3. Handbook of Tensile Properties of Textile and Technical Fibres Edited by A. R. Bunsell
4. Fundamentals of Spun Yarn Technology by Carl A. Lawrence

Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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