Fiber Strength And Fiber Elongation

1. Fiber Strength 

Fiber Strength is very often the dominating characteristics. This can be seen from the fact that the nature produces countless types of fibers, most of which are not usable for textlies because of inadequate strength. The minimum strength for a textile fiber is approximately 6 cN/tex.

Some significant breaking strength of fibers:

• Polyester   = 35-60 cN/tex
• Cotton       = 15-40 cN/tex
• Wool         = 12-18 cN/tex

In relation to cotton, the strength of fiber bundles was measured and stated as the Pressley value.

The following scale of values is used:

·         93 and above = Excellent.

·         87-92             = Very strong.

·         81-86             = Strong.

·         75-80             = Medium.

·         70-74             = Fair.

·         Under 70        = Weak.

Conversion to physical units should be avoided because the measuring procedure is not very exact.

Today the fiber bundles are commonly tested with HVI instrumentation. Depending on the used calibration stand- ard (USDA- or HVI-calibration cottons) the strength is expressed in g/tex (cN/tex).

For the commonly used HVI-CC calibration the following scale of values is used (1/8 in. gauge strength g/tex)*:

• 32 and above = very strong
• 30-32             = strong
• 26-29             = base
• 21-25             = weak
• 20 and below = very weak

Except for polyester and polypropylene fiber, fiber strength is moisture-dependent. It is important to know this in processing and also in testing. Since fiber moisture is dependent upon the ambient-air conditions, it depends heavily on the climatic conditions and the time of expo- sure before operation. Whereas the strength of cotton, linen, etc., increases with increasing moisture content, the reverse is true for polyamide fiber, viscose and wool.

     2. Fiber elongation

      Three concepts must be clearly distinguished:

• permanent elongation: that part of the extension through which the fiber does not return  on relaxation;
• elastic elongation: that part of the extension through which the fiber does return on relaxation;
• breaking elongation: the maximum possible extension of the fiber until it breaks, i.e. the permanent elongation and the elastic elongation together.

Elongation is specified as a percentage of the starting length. The elastic elongation is of decisive importance since textile products without elasticity would hardly be usable. They must be able to deform (e.g. at knee or elbow) in order to withstand high loading (and also during processing), but they must also return to shape. The fiber elongation should therefore be at least 1-2% (glass fibers), and preferably slightly more. The greater crease-resistance of wool compared with cotton arises, for example, from the difference in their elongation:

 • cotton 6-10%;

 • wool 25-45%.

   The following scale represents the cotton fiber elongation *:

 • below 5.0%  = very low;

 • 5.0-5.8%      = low;

 • 5.9-6.7%      = average;

 • 6.8-7.6%      = high;

 • above 7.6%  = very high.

   Man-made fibers show higher elongation values from about 15 to 30%. For functional textile goods, still higher elonga- tions are necessary sometimes, but they make processing in the spinning mill more difficult, especially in drafting opera- tions. Higher elongations are needed for sportswear, hois- ery, corsetry, and stretch products. If a fiber is subjected to tensile loading, demands are made on both its strength and elongation. Strength and elongation are therefore insepara- bly connected. This relationship is expressed in the so-called stress/strain diagram. For each type of fiber, there is a typi- cal curve. In blending, it should be ensured that the stress- strain curves of the fibers to be blended are similar in shape. Measurment of elongation is difficult and time consuming.

  *Reference:

  Cotton fiber chart 2006.

  ITMF International Textile Manufacturers Federation