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YARN TESTING  - 2

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  • 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.

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