Tear strength is used to evaluate the ability of fabric resistant to tear along the breach or damaged position in the process of use. Its unit is Newton, “N” for short. Generally, tear strength determines the durability of dyeing and finishing products. A part of fabric edge is applied a centered load, to make each inner yarn undertake maximum load to burst, and at last a crack comes. Tear strength is often used to measure the performance of tent and umbrella, but knitting fabric and elastic woven fabric will not test this indicator. There are many methods to measure fabric tearing strength, such as pendulum method, trapezoid method, tongue method, and wing method.
2.Tear strength test
- Apparatus and Material
Pendulum fabric tear strength tester (Figure 1), steel ruler, scissors, a few pieces of fabric, etc.
figure 1: Elmendorf Tearing Tester
- Test principle
The sample is fixed in fixture, with a cut on it. The pendulum in the position of max potential energy is released. When the movable clamps leave the fixture, the sample is torn along the notch. And the work that torn fabric makes in a certain length is converted to tear strength.
- Test method
a) Sample preparation
The sample is cut randomly in at least 3m from cloth edge, the length of which is no less than 1m. Got two sets of sample, and one is warp sample and the other is weft sample. Figure 2 shows the size of samples.
b) Test procedure
- Pendulum’s quality is chosen to make sample test results fall in the range of 15%~85% of corresponding ruler. The zero position of machine should be calibrated, and pendulum is rose to the starting point.
- The sample is clamped in the fixture. The long side of sample is parallel to top of fixture, while its bottom is placed in the base of the fixture. The opposite side of the groove is cut a notch of 20mm×0.5mm by a knife, leaving a tear length of 43mm×0.5mm.
- Push “Stop”, and release the pendulum. When pendulum makes a backswing, you should grasp the pendulum to prevent the pointer position from damaging.
- Read out data from measuring device, and ensure at least 5 times’ measurements in each direction.
- The average value of tear strength from each test direction is calculated, keeping 2 valid figures.
b.Other test methods
1) Trapezoidal method
- A trapezoid is drawn on the sample, and two hypotenuses of the trapezoid are clamped by clappers of strength tester. An increasing force is applied to the sample to tear along the width direction, and the average max tear strength is tested.
- 5 samples each from longitude and latitude is cut, with size of (75mm±1mm)×(150mm±2mm), as shown in Figure 3.
- The distance of two clamps is set to (25mm±1mm), and stretch speed is 100 mm/min. The appropriate loading range is chosen to fulfill that tear strength will fall on10%~90% of total span. The sample is clamped along the hypotenuse of trapezoid, to make the notch lie in the middle of two clamps, with the short side of trapezoid keeping tight and long side wrinkle. Power up the machine. As the down clamp is going down, each yarn of short side of the sample is stressed one after another, then the force spreads from notch line to long side of the trapezoid till sample is torn completely.
2) Tongue method
The tongue method is divided into single tongue method (commonly used) and double tongue method. The sample is mounted in tensile tester, to make the sample notch line straight between the up and down clamps. Power up the tester, and then the tension is applied towards notch; tear strength that tears to the regulated length is recorded.
- (Single) Tongue method
Two sets of samples are cut from longitude and latitude of test samples, at least 5 pieces in each set. Each two samples can not contain a same yarn from length and width direction. Samples are rectangles with length of 220mm±2mm and width of 50mm±1mm. And each sample should be cut a 100mm±1mm notch parallel to the length direction from the middle of width direction. The tear terminal should be marked at 25mm±1mm from center distance of sample to uncut end. As Figure 4 shown.
The length of span is set to 100mm, and stretched speed is 100mm/min. According to Figure 5, sample is installed in clamps, with each leg clamped in a clamp. The cutting line aligns at the center line of clamps. The uncut end is in free condition. Note that two legs by cutting end are ensured to fix in clamps, so that tear is parallel to notch when tear starts and in tearing direction.
- Double tongue method
The sample preparation of double tongue method is as shown in Figure 5. When clamping, sample tongue is clamped in the center of clamps, symmetrical, to make line bc visible. Two legs of sample, parallel to the direction of tearing, are clamped symmetrically in removable clamps, to make line ab and cd visible. Pay attention to ensuring each tongue is fixed in clamps, so that tear is parallel to tearing direction when tear starts.
Start the machine, tear is pulled to the end marks of sample (the position “*” in Figure 5). Tearing strength and tearing length of each sample is recorded. The observation should be done on whether tear is processed in the direction of applied force and whether yarns slip away from the fabric. If the sample doesn’t slip away from the clamps and tear is conducted along the direction of applied force, the test results can be acknowledged, otherwise, removed.
3) Wing method
In addition to the above methods, there is wing method. One side of the sample is cut to a specified shape of two wings, and then clamped it according to two wings inclining to tearing direction, with a mechanical tension focused on the notch to conduct a tear along the expected direction. Tear strength that tears to the regulated length is recorded.