It is the resistance of the fiber up to the breaking point when one end of the fiber is subjected to a fixed tensile force and the other end is subjected to a movable tensile force. The unit of strength is grams. One of the desired properties from the fibers that make up the yarn is strength.
For quality yarn, the elasticity and strength values of the fibers are important. For this, the elasticity and strength of the fiber to be used must be determined before the yarn is made.
When the wool fiber gets wet, its strength decreases and its elongation ability increases. Wetting fibers suffer a loss of strength between 10-25%. Strength determination of wool fiber can be determined when it is in the form of a single fiber or a bundle. Different devices are used for this purpose.
Determining Strength of a Single Lift
This method is a method of measuring the strength of individual fibers. Polkeit, Deforden and Schopper devices are used to measure the strength of single fibers. It is the most widely used Schopper strength measuring device for wool fiber.
Single fiber strength determination with Schopper device
In the Schopper device, the fibers are adjusted to break in 20 seconds. The operation of this device is provided by a hydraulic device. There are two display charts on the front of the device. The upper indicator chart is arranged to determine the strength of the fiber, and the lower one to determine the % elongation.
After the fibers whose strength is to be measured are attached to the jaws of the device, the action lever is activated starting from the point (0). In this way, an increasing force is applied to the fiber. The fiber that wants to resist this breaks from somewhere, at the moment of this break, the movement stops by itself and the indicator remains constant.
When the value read on the upper display chart of this device is applied to the formula below, the breaking strength of the fiber in grams is calculated.
Tensile strength = (S) value x weight / 100
If a weight must be added in order to get rid of the folds of the fiber caught between the jaws and to stretch it sufficiently in these trials, an additional amount of weight must be added to the result obtained with the above formula. Then the formula
Tensile strength=[ (S) value x weight / 100]+g.
Wool stretches by 10–30%.
When the spring ability of the wool is good, the product's resistance to wrinkling and crushing is high. Short wool fiber has a higher springiness.
Single fiber strength determination with WIRA device
In this method, the aim is to find the strength of a single fiber. It is calculated separately by taking 15 wool fibers. The selected single fiber is held by the ends and placed between the jaws of the WIRA strength measuring device and fixed in tension. The movable jaw moves forward, breaking the fibers. At this time, the fiber tends to break as it is subjected to a tension between the two jaws. The evaluation is made on the basis of the force sufficient to break the fiber and the amount of elongation when breaking the fiber.
The strength value at break is read from the upper left corner of the device. Each value read is written and all values found are summed and divided by 15. The result is reported as the fiber strength value.
Determination of Breaking Length in Clustered Fibers
Strength determinations of single fibers are very time consuming and tedious. Because it is not enough to test the strength of a single or a few fibers. In order to obtain an accurate result, the strengths of a larger number of fibers should be determined in this way and their average strength should be calculated at the end.
Determination of strength of bundled fibers with stelometer device
Before measuring the strength of the bundled fibers, a few tufts of fibers that can represent the lot are taken from the wool samples. The selected fiber bundles are first opened manually and combed to separate the ones shorter than a certain length, at the same time the fibers are brought parallel to each other.
Considering the average length of the fibers in the sample, the sample is cut into bundles of 25-50 or 75 mm in length.
25 mg of fiber for 30 mm length, 50 mg for 60 mm length and 75 mg for 90 mm length are weighed on precision scales.
However, 10% under or over tolerance may be acceptable for these bundles. Strips are adhered from the lower and upper ends in groups of 5, with 5 mm intervals on each fiber bundle.
Then the adhesive tapes connecting the bundles are cut and the bundles are separated. The strength is measured by breaking off the samples thus prepared.
As soon as the force applied to the sample clamped between the jaws of the device causes the beam to break, the number read on the indicator is recorded next to the weight of that sample.
When the breaking tests are finished, the value read at the break of each bundle is divided by its own weight and the breaking strength is calculated.
Strength determination of bundled fibers with Presley device
First of all, approximately 5-10 mg of fiber is taken and smoothed by hand or comb and made parallel.
2,3 mg of fiber remains, the parallel fiber is placed in the pressley clamp and compressed.
The hanging fibers from both sides are cut.
The clamp is placed on the presley strength device and the balance of the device is adjusted with the help of a spirit level.
Movement is given to the traveling weight on the sled. The device should be kept very balanced while the movement is given, care should be taken for the health of the experiment and not to damage the fibers.
When the weight reaches a certain point on the slide, the fibers break and the movement stops.
The value indicated by the weight on the slide shows the breaking load of that fiber group.
A complete break is achieved in a successful experiment.
Then, the fiber group is weighed on a precision scale.
As the number of tests increases, the results obtained better reveal the true strength value.
The breaking load value read on the scale of the Presley device is read in pounds.
Values other than 10-20 lb are not taken into account in the experiment.
At the moment of rupture, the value marked on the display of the device is read and recorded next to the weight of this bundle before.
When the test repetition is over, the breaking strength of each bundle is calculated by dividing the breaking value by its own weight.
In order to determine the strength with the Presley device, first the presley index (P:I) value is calculated using the formula below.
Breaking load (lb)
PI= Fiber group weight (mg)
The pressley index value is found by taking the average of the loads at break and the weights of the fiber groups. This is around +-0,5. Then the presley strength (PM) is calculated with the help of the following formula:
PM =(10,81 x PI)- 0,12
As a result of this formula, the presley strength is found as 1000 / inch². The following comments can be deduced from the obtained pressley strength values.
higher than 92; the sample is very very strong
>>> 87–92; the sample is very solid
>>> 81–86; sample intact
>>> between 75-80; sample of medium strength
>>> between 70-74; the sample is quite solid
>>> less than 70; the sample is weak.