FIBRE STRENGTH:

The different measures available for reporting fiber strength are
1. breaking strength
2. tensile strength and
3. tenacity or intrinsic strength
Coarse cottons generally give higher values for fiber strength than finer ones. In order, to compare strength of two cottons differing in fineness, it is necessary to eliminate the effect of the difference in cross-sectional area by dividing the observed fiber strength by the fiber weight per unit length.

The value so obtained is known as "INTRINSIC STRENGTH or TENACITY". Tenacity is found to be better related to spinning than the breaking strength.
The strength characteristics can be determined either on individual fibers or on bundle of fibers.

SINGLE FIBRE STRENGTH: The tenacity of fiber is dependent upon the following factors
chain length of molecules in the fiber orientation of molecules size of the crystallites distribution of the crystallites gauge length used the rate of loading type of instrument used and atmospheric conditions
The mean single fiber strength determined is expressed in units of "grams/tex". As it is seen the unit for tenacity has the dimension of length only, and hence this property is also expressed as the "BREAKING LENGTH", which can be considered as the length of the specimen equivalent in weight to the breaking load. Since tex is the mass in grams of one kilometer of the specimen, the tenacity values expressed in grams/tex will correspond to the breaking length in kilometers. BUNDLE FIBRE STRENGTH: In practice, fibers are not used individually but in groups, such as in yarns or fabrics. Thus, bundles or groups of fibers come into play during the tensile break of yarns or fabrics. Further, the correlation between spinning performance and bundle strength is at least as high as that between spinning performance and intrinsic strength determined by testing individual fibers. The testing of bundles of fibers takes less time and involves less strain than testing individual fibers. In view of these considerations, determination of breaking strength of fiber bundles has assumed greater importance than single fiber strength tests

More on: Test Method for Breaking Strength of Yarn in Skein Form
ASTM D1578 - 93