What Are the Methods for Testing the Burst Strength of Knitted Fabrics?

Burst strength, or puncture strength, top breaking strength, directly reflects the durability index of knitted fabrics when deformed and ruptured by external pressure. Puncture strength is one of the important assessment indexes for the quality of knitted fabrics. If the puncture strength is unqualified, the knitted fabrics can only be downgraded or scrapped even if the other properties are good. How can we accurately test the puncture strength of fabrics to provide a fair appraisal for customers and manufacturers? In this article, we will share the burst strength test methods, and the main factors affecting the results of puncture strength testing as well as the improvements.

Puncture strength is the maximum vertical force required to make the knitted fabric to bulge and expand till breaks. It is also called burst strength. Burst strength is similar to the force applied to the knees, elbows, gloves, and socks of apparel fabrics. The puncture test is particularly suitable for knitted fabrics, three-way fabrics, non-woven, and parachute fabrics. Certain knitted fabrics, such as weft knitted fabrics, have the characteristics of longitudinal extension and transverse contraction, and the two directions have a large influence on each other. If the tensile test is used, it is necessary to test the longitudinal, transverse, and diagonal directions separately, but if you choose the burst strength test, you can have a one-time comprehensive evaluation of the strength in the longitudinal, transverse, and diagonal directions.

What is the broken mechanism of fabric under puncture strength or burst strength? In the broken process, the force is multi-directional, for general woven fabrics and knitted fabrics, strength and deformation are anisotropic, extending in all directions under the action of the puncture force. Then the shear stress compounded by the tension along the warp and weft or in the straight and horizontal directions will cause the yarn to break (at the point where the deformation is greatest and the strength is weakest). The fissure is generally right-angled or linear.

 

1. Commonly Burst Strength Test Equipment, Test Standards, and Test Methods 

1.1 Hydraulic Method for Burst Strength Test

The hydraulic method is based on the principle that a specimen of a certain area is clamped to an extendable diaphragm and a liquid pressure is applied underneath the diaphragm. Then, the volume of the liquid is increased at a constant rate, causing the diaphragm and the specimen to expand until the specimen ruptures and the bursting strength and expansion are measured.

The most commonly used hydraulic burst strength test machines are the Mullen C and Mullen A. Both machines utilize the liquid pressure of glycerin to create an uplift effect through the elastic membrane to complete the burst test. The main difference between the Mullen C and Mullen A is that they have different ranges, the range of the Mullen C is from 2 to 200 PSI, while the range of the Mullen A is from 10 to 500 PSI. However, when using the burst strength tester, it is only permitted to test up to 15%-75% of the full range, otherwise, it is easy to burst the elastic membrane. For some high elastic fabrics, the customer is required to break the fabric, if the fabric can not be broken on the Mullen C, it is necessary to take the fabric to the Mullen A. The common standard methods for the Mullen-type hydraulic burst strength machine are the American Standard (ASTM) and European Standard (ES). The American Standard is ASTMD 3786 in PSI and the European Standard is ISO 13938-1 in KPA.

For the sample requirements of the Mullen C hydraulic burst strength test, knitted fabrics, and non-woven fabrics, the sample should be round and the size should be 125 square millimeters. The sampling should be representative, cut along the diagonal of the fabric, and take 5 samples for each type of fabric. For the burst strength test of garment seams, the sampling requirement is to cut each seam into a square of 125mm x 125mm and make its four sides parallel or perpendicular to the seams, so that the seams are exactly in the center of the specimen, and the same seam line is generally tested in one or two.

1.2 Pneumatic Method for Burst Strength Test

The testing principle of the pneumatic method is that the specimen is clamped on an extendable diaphragm and gas pressure is applied underneath the diaphragm. Then, the volume of gas is increased at a constant rate, so that the diaphragm and the specimen are expanded until the specimen ruptures and the bursting strength and expansion degree are measured. The commonly used pneumatic fabric bursting strength tester is the AirBurst pneumatic bursting strength tester, which utilizes an air pump to provide gas pressure to burst the fabric through the bulging of the elastic membrane. It consists of an air pump and a tester, and the test results can be presented on the screen of this machine. The tester contains a test cup, an elastic membrane, and a safety cover. There are two kinds of test cups in common use. The large cup, with an area of 50 square centimeters, has a permissible displacement of 70.50 mm and a test time of 20±5 seconds; the small cup, with an area of 7.3 square centimeters, has a permissible displacement of 22.50 mm and a test time of 20±5 seconds. There are many standard methods of testing in the AirBurst Pneumatic Bursting Strength Tester, but the commonly used methods are ASTMD3786, in PSI, for American Standard and ISO 13938-2, in KPA, for European Standard.

 

For the sampling requirements of the Pneumatic Fabric Bursting Strength Instrument, for 7.3 square centimeters of the test cup, the sample of knitted fabric, the non-woven fabric should be round and the diameter is more than 10 cm, sampling to be representative that cut along the diagonal, each type of textile are generally taken 5 samples. For the burst strength test of garment seams, the sampling requirement is to cut each piece of fabric sample into a square of 10cm x 10cm and make its four sides parallel or perpendicular to the seams, so that the seam line is exactly in the center of the sample, and the same seam is generally tested in one or two.

1.3 Steel Ball Method for Burst Strength Test

The testing principle of the steel ball method is: a certain area of the specimen is clamped in the fixed base of the circular specimen, and the spherical top bar is vertically pressed against the specimen with a constant moving speed, so that the specimen is deformed until it breaks, and the top breaking strength or puncture strength is measured. The commonly used fabric strength meters for the steel ball method are the SmartPull fabric tensile strength machines. It utilizes the surface of a steel ball to break the fabric. For this test, American standards are commonly used. For example, in ASTMD 3787 and ASTMD 6797, both of which are in pounds (LBF), the breakthrough speed is 12 inches/min, and the diameter of the steel ball ejector is 25 mm. In the national standard, we use the top break speed is 100mm/min, the unit is Newton (N), and the diameter of the steel ball top bar is 20mm.

 

For the sampling requirement of the SmartPull Multi-functional bullet-type electronic fabric strength machine, take 5 circular samples of an area greater than or equal to 100 square centimeters, the specimen should be representative, with the test area that should not be folded, pleated, and avoid the edge of the fabric, preferably take the samples along the diagonal of fabric.

2. Factors Affecting the Results of the Fabric Burst Strength Test

2.1 Internal Factors of the Fabric Itself

This factor is formed during the manufacturing, dyeing, finishing, and post-treatment of the fabric and has nothing to do with the tester. There are several aspects as follows:

The effect of breaking strength and breaking elongation of the yarn, when the breaking strength and elongation of the yarn in the fabric are larger, the top breaking strength of the fabric is larger.

The effect of fabric thickness. When all other things are the same, the fabric is thicker, the burst strength is larger.

The warp or weft density of the fabric. When other conditions are the same, the density of the warp and weft of the fabric is different at the same time, the fabric will tear along the direction that the density is small the crack is linear, and the fabric top breaking strength is small.

Yarn hook strength. In knitted fabrics, when the crochet strength of the yarn is large, the top-breaking strength of the fabric is large.

Yarn fineness and loop density in knitted fabrics also affect the breaking strength of knitted fabrics. Increasing the yarn density and loop density increases the breaking strength.

The effect of the treatment of the fabrics in boiling, brushing, dyeing, enzyme washing, and qualitative resin finishing. Under the influence of these different processes, the fabric’s breaking strength decreases, and the extent of this decrease varies with different treatments and materials used. Generally speaking, fabrics with darker colors have a lower breaking strength than those with lighter colors.

2.2.1 Impact of Test Methods Used

The test results are affected by the use of different test methods. Generally speaking, the burst strength measured by the hydraulic and pneumatic methods is much more stable than the burst strength measured by the steel ball method. If there are no special requirements, the tester should use either the hydraulic or pneumatic method. For breaking forces, less than 80 KPA, the difference between the pneumatic and hydraulic methods is not significant, but for breaking forces, greater than 80 KPA, the hydraulic method is generally more stable.

The same test method, but with different test ranges, will also have deviations in the test results. The practice has shown that for the same hydraulic method, the test results will have some deviation if different ranges are used, and generally speaking, the test results will be more accurate if a smaller range is used. Therefore, the tester should use the smaller range if it is possible to do so. Since the range of the Mullen C machine is smaller than that of the Mullen A machine, the results of the Mullen C machine will naturally be more accurate than those of the Mullen A machine.

2.2.2  Operator Effects

Influence of the tester’s reading. The readings of the Hydraulic Burst Strength Tester may be skewed due to the fact that different people, do not read directly in front of the counter.

Influence of the tester by choosing the wrong standards or method. Especially in the case of pneumatic fabric bursting testers, the results can be greatly affected by the wrong standards and methods, the wrong test cup, or an incorrectly calibrated tester.

The influence of irregular operation by the tester. Especially in the case of hydraulic bursting strength testers, if the tester fails to return the valve handle to its original position in time at the moment of bursting, the bursting strength will be too large.

Influence of tester’s sampling. Each sample should not only be representative but also be taken within the specified range. If the tester takes the sample at the edge of the fabric or at a place where the fabric has been stretched, the burst strength will usually be smaller.

 2.2.3  Influence of Testing Environment

In the burst strength test, the test environment also has a great influence on the BS test results, so the tester needs to choose the corresponding test environment in strict accordance with the standard.

3. Measures to be Taken to Ensure the Accuracy of the Test Results

There are many factors affecting the results of the fabric burst strength test. The internal factors affecting the results of the fabric test have already been formed before the fabric test and cannot be changed by the tester, but the external factors affecting the results of the fabric test can be avoided by the tester.

3.1 Selection of Appropriate Test Methods, Standards and Instruments

The test results will be more accurate if a hydraulic burst strength tester or a pneumatic fabric strength tester is selected and the appropriate standardized method is used.

3.2 Selecting the Appropriate Measuring Range

The strength of fabrics varies from one material to another and from one structure to another. The tester must estimate the strength of the fabric before testing and then select the correct range. If the fabric is not out of the small range, the tester should choose a small range to minimize the test deviation.

3.3 Instrument Servicing and Calibration

The instrument needs to be serviced, maintained, and calibrated on a regular basis. Before each test, the burst strength tester should be calibrated to see if the breaking strength is within the range of use, if not, the instrument must be serviced and calibrated so that the test results will be accurate.

3.4 Tester’s Work State

In the hydraulic type burst strength test, the tester should carefully observe the changes in the fabric, especially in the fabric top broken moment, and timely move the valve handle back to the original position, to avoid strength bias (but if you choose HydroBurst, you could ignore that since this machine can test the burst strength automatically without the valve handle). In the steel ball method of bullet-type electronic fabric strength machine test, the tester needs to tighten the sample gripper, and each time the degree of force is basically the same, in order to prevent the fabric from slipping and affecting the test results.

3.6 Correct Sampling

The sample should be representative, the test area should avoid folding mouth wrinkles, and avoid the edge of the fabric, it is best to extend the diagonal sampling.

3.7 Test Environment

The test should be strictly by the standards to select the appropriate test environment. For example, in the U.S. standards, it requires the standard test environment: temperature is (21 ± 1) ℃, and relative humidity is (65 ± 2) %.

Conclusion

In this article, we have discussed knitted fabrics burst strength testing and the main factors affecting the test results as well as improvement measures. As long as we have a good understanding of the correct test method and can overcome the negative factors affecting the results of the fabric burst strength test, we can ensure the accuracy of the test results.

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