YARN TESTING

INTRODUCTION: Yarn occupies the intermediate position in the manufacture of fabric from raw material. Yarn results are therefore essential, both for estimating the quality of rawmaterial and for controlling the quality of fabric produced. The important characteristics of yarn being tested are,

1. yarn twist
2. linear density
3. yarn strength
4. yarn elongation
5. yarn evenness
6. yarn hairiness etc.

SAMPLING: In order that the results obtained are reproducible and give reliable information about the material, the sampling must be true and representative of the bulk lot. The sampling procedure should be designed to take account of and to minimise the known sources of variability such as the variation between spindles, the variation along the length of the bobbin, etc. The procedure for sampling and the number of test carried out are given under each characteristic.

AMBIENT CONDITIONS FOR YARN TESTING: Some textile fibres are highly hygroscopic and their properties change notably as a function of the moisture content. Moisture content is particularly critical in the case of properties, i.e yarn tenacity, elongation, yarn evenness, imperfections, count etc. Therefore conditioning and testing must be carried out under constant standard atmospheric conditions. The standard atmosphere for textile testing involves a temperature of 20+-2 degree C, and 65+-2% Rh. In tropical regions, maintaining a temperature of 27+-2 degree C, 65+-2%RH is legitimate. Prior to testing, the samples must be conditioned under constant standard atmospheric to attain the moisture equillibrium. To achieve this it requires at least 24 hours.

TWIST: "Twist is defined asthe spiral disposition of the components of yarn, which is generally expressed as the number of turns per unit length of yarn, e.g turns per inch, turns per meter, etc.

• Twist is essential to keep the component fibres together in a yarn.
• The strength, dyeing, finishing properties, the feel of the finished product etc. are all dependent on the twist in the yarn.
• With increase in twist, the yarn strength increases first , reaches a maximum and then decreases.
• Depending on the end use, two or more single yarns are twisted together to form "plied yarns" or "folded yarns" and a number of plied yarns twisted together to form "cabled yarn".
• Among the plied yarns, the most commonly used are the doubled yarns, wherein two single yarns of identical twist are twisted together in a direction opposite to that of the single yarns.
• Thus for cabled and plied yarns, the direction of twist and the number of turns per unit length of the resultant yarn as well as of each component have to be determined for a detailed analysis.
• Direction of twist is expressed as "S"-Twist or "Z"-Twist. Direction depends upon the direction of rotation of the twisting element.
• Twist take up is defined as, "The decrease in length of yarn on twisting, expressed as a percentage of the length of yarn before twisting.

  LINEAR DENSITY OR COUNT OF YARN:

• The fineness of the yarn is usually expressed in terms of its linear density or count.
• There are a number of systems and units for expressing yarn fineness. But they are classified as follows

  DIRECT SYSTEM:

  1. English count(Ne)
  2. Metric count(Nm)
  3. French count(Nf)

  INDIRECT SYSTEM:

  1. Tex
  2. Denier
  1. Ne : No of 840 yards yarn weighing in One pound
  2. Nm : No of one kilometer yarn weighing in One Kilogram
  3. Nf : No of one kilometer yarn weighing in 0.5 kilogram
  4. Tex : Weight in grams of 1000 meter(1 kilometer) yarn
  5. Denier: Weight in grams of 9000 meter(9 kilometer) yarn
• For the determination of the count of yarn, it is necessary to determine the weight of a known length of the yarn. For taking out known lengths of yarns, a wrap-reel is used. The length of yarn reeled off depends upon the count system used.
• Another factor which determines the length of yarn taken for testing is the type of balance used. Some balances like quadrant balance, Beesley's blanace have been specially designed to indicate the yarn count directly from tests on specified short lengths of yarn and are very useful for determining the counts of yarn removed from the fabrics. The minimum accuracy of balance required is 0.001mg
• One of the most important requirements for a spinner is to maintain the average count and count variation within control. The term count variation is generally used to express variation in the weight of a lea and this is expressed as C.V.%. This is affected by the number of samples and the length being considered for count checking. While assessing count variation, it is very important to test adequate number of leas. After reeling the appropriate length of yarn, the yarn is conditioned in the standard atmosphere for testing before it's weight is determined.
• The minimum number of sample required per count is 20 and per machine is 2.

  YARN STRENGTH AND ELONGATION:

• Breaking strength, elongation, elastic modulus, resistance abrasion etc are some important factors which will represent the performance of the yarn during actual use or further processing. Strength testing is broadly classified into two methods
  1. single end strength testing
  2. skein strength or Lea strength
  Tensile strength of single strands of yarn:
• During routine testing, both the breaking load and extension of yarn at break are usually recorded for assessing the yarn quality. Most of the instruments record the load-elongation diagram also.
• Various parameters such as initial elastic modulus, the yield point, the tenacity or elongation at any stress or strain, breaking load, breaking extension etc can be obtained from the load-extension diagram.
• Two types of strengths can be determined for a yarn
  1. Tensile strength -load is applied gradually
  2. Ballistic strength - applying load under rapid impact conditions
• Tensile strength tests are the most common tests and these are carried out using either a single strand or a skein containing a definite number of strands as the test specimen.
• An important factor which affects the test results is the length of the specimen actually used for carrying out the test. The strength of a test specimen is limited by that of the weakest link in it.If the test specimen is longer, it is likely to contain more weak spots, than a shorter test specimen. Hence the test results will be different for different test lengths due to the weak spots.
• The amount of moisture in the yarn also influences the test results. Cotton yarn when fully wet show higher strength than when dry, while opposite is the case with viscose rayon yarns. Hence, to eliminate the effect of variation due to moisture content of the yarn, all yarn strengrth tests are carried out, after conditioning in a room where the standard atmospheric condition is maintained.
• The rate of loading as determined by the "time-to-break", which is the time interval between the commencement of the application of the load and the rupture of the yarn, is an important factor , which determines the strength value recorded by using any instrument. The same specimen will show a lower strength when the time-to-break is high, or higher when the time-to-break is low.
• The instruments used for determining the tensile strengh are classified into three groups, based on the principle of working.
  1. CRT - Constant rate of traverse
  2. CRE - Constant rate of extension
  3. CRL - Constant rate of loading
• In the instruments of CRE type, the application of load is made in such a way that the rate of elongation of the specimen is kep constant. In the instruments of the CRL type,the application of load is made in such a way that the rate of loading is constant througout the duration of the test. This type of instruments are usually preferred for accurate scientific work. In the CRE and CRL types of instruments, it is easy to adjust the "time-to-break" while this adjustment is not easy in the CRT types of instruments.
• The uster Tensorapid applies the CRE principle of tensile testing. Constant Rate of Extension describes the simple fact that the moving clamp is displaced at a constant velocity. As a result, the specimen between the staionary and the moving clamp is extended by a constant distance per unit of time and the force required to do so is measured. Apart fron single values, this instrument also calculates mean value coefficient of variation and the 95% confidence range of maximum force, tenacity,elongation and work done
• The total coefficient of variation describes the overall variability of a tested lot, i.e the within-sample variation plus the between-sample variation. If 20 individual single-end tensile test are performed on each of ten bobbins or packages in a sample lot, the total coefficient of variation is calculated from the pooled data of the total number of tests that were carried out.
• In tensorapid, the breaking tenacity is calculated from the peak force which occurs anywhere between the beginning of the test and the final rupture of the specimen. The peak force or maximum force is not identical with the force measured at the very moment of rupture. The breaking elongation is calculated from the clamp displacement at the point of peak force. The elongation at peak force is no identical with the elongation at the very moment of rupture(elongation at rupture).
• The work to break is defined as the area below the stress/strain curve drawn to the point of peak force and the corresponding elongation at peak force. The work at the point of peak force is not identical with the work at the very moment of rupture.
• To compare tensorapid test results with other results,
  1. a measurement must be performed according the CRE princple
  2. testing speed must be exactly 5 m/min
  3. the gauge length or the length of the specimen should be 500 mm
  4. the pretension should be 0.5 cN/tex
• There are two fundamental criteria which affect the compatibility between different measurements of tensile yarn properties.
  1. testing conditions, i.e the testing principle(CRE,CRL), testing speed, gauge length, and pre-tensioning.
  2. the second criteria,which also affects the magnitude of the differences, relates to the specific stress/strain characteristic of the yarn itself, which is determined by the fibrous materials, the blend ratio, and the yarn construction.

  Skein strength or Lea strength:

  The skein breaking strength was the most widely used measure of yarn quality in the cotton textile industry. The measurement of yarn quality by this method has certain drawbacks. Firstly, in most of the subsequent processing, such as winding, warping or weaving, yarn is used as single strand and not in the form of a skein except occasionally when sizing ,bleaching, mercerising or dyheing treatments are carrried out on hanks. Secondly, in the method used for testing skein strength, the rupture of a single strand at a weak place affects the result for the whole skein. Further, this method of test does not give an indication of the extensibility and elastic properties of a yarn, the characters which play and important role during the weaving operations. However, since a large size sample is used in a skein test as against that in a single strand test, the sampling error is less. The skein used for strength test can be used for determination of the linar density of the yarn as well.

• In addition to the factors influencing the yarn strength, the size of the skein(lea) will affect to a large extent the strength recorded. The usual practice is to use a lea(120 yards) of yarn prepared by winding 80 turns on a wrap-reel having a perimeter of 1.5 yards(54 inches), so that during a test, there are 160 strands of 27 in.(") length. There are different systems in use. But the actual breaking strength recorded on the machine would depend on the type of skein used as both the number of strands and test length may differ. The instruments most commonly used for this test is CRT type, where the bottom hook moves at 12 inches per min.
• After findingout skein strength, broken skeins are also weighed to determine the linear density. The most common skein used is the lea and the results of lea strength tests are expressed as C.S.P., which is the product of the linear density(count)of the yarn in the English system (Ne) and the lea breking strength expressed in lbs. In view of the fact that C.S.P. is much less dependent on yarn count than on strength, especially when count diffferences are small, C.S.P. is the mostg widely used measure of yarn qauality.