How to measure yarn strength

Definition of yarn strength

Yarn strength is an index indicating the tensile strength of the yarn, and it has absolute strength and relative strength. When comparing the strength of yarns with different linear densities, the relative strength is used.

Absolute strength (breaking strength): The external force that the yarn can withstand when it is stretched to break. There are: ① single yarn strength; ② strand strength; ③ strand strength, etc. The unit is Newton, gram force, kilogram force or pound force.

The relative strengths are: ① breaking strength; ② specific strength; ③ breaking length; ④ quality indicators.

Breaking strength: The external force that the yarn can withstand per unit cross-sectional area when the yarn is stretched to break, the unit is N/mm2.

Specific strength: The external force per linear density yarn can withstand when the yarn is stretched to break, the unit is N/tex.

Breaking length: The length of the yarn when its weight is equal to its breaking strength, in km. In the Tex series, it is equal to the ratio of the strength (gram force) of a single yarn or strand to its Tex, which is the specific strength;

Quality index: An index indicating the relative strength of the yarn. When the yarn linear density is in the inch system, it is equal to the product of the yarn strength (pound force) and the single yarn inch number; when the yarn linear density is in the tex system, it is equal to the yarn strength (kg force) and its single yarn characteristic. 1,000 times the quotient of X.

Yarn strength is a reflection of the inherent quality of the yarn, and it is a necessary condition for the yarn to have processing performance and end use. Quality indicators are currently the main basis for determining pure spinning or blended yarn products of cotton yarn or cotton-type chemical fiber, so yarn strength is one of the most important routine inspection items in textile production.

Yarn strength utilization factor

The strength of a single yarn is always less than the sum of the breaking strength of each fiber in its section. The ratio of the two is called the fiber strength utilization factor in the yarn. Cotton yarn is usually 0.40～0.50, and wool yarn is usually 0.20～0.30.

The yarn strength is always less than the sum of the individual yarn strengths in the yarn. The ratio of the two is called the yarn ratio. Generally, cotton yarn is 0.70～0.78, and wool yarn is 0.40～0.82.

The strength of the strands during the reverse twisting is generally higher than the sum of the strengths of the individual strands, and the ratio of the double-strand cotton is 0.95 to 1.35. The ratio of the yarn strength converted from the fabric strength to the yarn strength before weaving is called the yarn strength utilization factor in the fabric, and its value is greater than 1. The warp direction of cotton poplin fabric is about 1.155, and the weft direction is about 1.115.

Main factors affecting yarn strength

1) Fiber properties

The higher the fiber strength, the lower the linear density and the longer the length, the higher the yarn strength. The natural twisting of cotton fiber and the crimping of wool and chemical fibers will increase the cohesion between the fibers when the yarn twist is not large, thereby increasing the yarn strength. The lint content has a great influence on the strength of the yarn. In cotton yarn, the rate of short-staple under 16 mm increases by 1%, and the strength of cotton yarn decreases by 1% to 2%.

2) Yarn structure

When the fiber is transferred in the radial direction of the yarn, the yarn strength can be increased when the structure is uniform; when the fibers are arranged in layers and are folded, buckled, and hooked, the yarn strength decreases; below the critical twist, the yarn strength increases with the increase in the twist , After exceeding the critical twist, it will decrease as the twist increases. Pure cotton yarn is easy to break at the details and large neps; polyester-cotton yarn is easy to break at the turning points of thick and coarse details.

3) Spinning method

Yarns spun by different spinning methods have obvious differences in strength due to their different structures. The strength of thick airflow yarn is about 5%-10% lower than that of ring yarn of the same size, but the density of airflow yarn is uniform, and the weakest node is not lower than that of ring yarn. The strength of electrospinning is similar to that of airflow yarn. The in-phase self-twisted yarn has a low strength due to the untwisted area on the yarn; the out-of-phase self-twisted yarn has increased strength due to the staggered untwisted area on the yarn. The strength of twisted self-twisted yarn is further improved due to the reasonable distribution of twist.

4) Blending ratio

In the two-component blended yarn, when the blending ratio of one component gradually increases from zero, the strength of the blended yarn generally decreases. After the minimum value is reached, if the proportion of this component is increased, the yarn strength gradually increases. The greater the difference in elongation at break between the two mixed fibers, the more obvious this trough phenomenon of strength.

5) Large temperature and humidity

The higher the temperature, the lower the yarn strength; the higher the relative humidity, the higher the moisture regain of the yarn, the higher the strength of cotton and hemp yarns, the lower the strength of wool and man-made fiber yarns. Therefore, the strength test should be carried out after equilibrium under the specified standard state (temperature of 20 ℃, relative temperature of 65%), otherwise the measured strength must be corrected for temperature and moisture regain.

Yarn strength test

The strength of a single yarn or strand is measured on a pendulum-type yarn strength tester, a bevel-type yarn strength tester or an electronic strength tester. The length of the sample is specified as 500 mm. At present, the new yarn strength testing machine can automatically perform strength testing and print out the average value of strength and elongation and its coefficient of variation.

The yarn strength is measured on a pendulum-type yarn strength tester. The loop length of cotton yarn or cotton-type chemical fiber pure or blended yarn is 1 m, and every 100 loops is 100 m; the loop length is 1m, carded wool yarn 20 turns means 20 m is a strand, and combed wool yarn is 50 turns means 50 m is a strand.