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Testing Error —— the Most Troubling Problem in the Tensile Strength Test of the Textiles
After the fabric is made into a textile, it is subjected to various forms of external force during the actual wearing process, such as stretching, compression, bending and the like, wherein the stretching is the main form of action leading to the damage of the textile. The ability of the fabric to resist the tensile break is called tensile breaking strength. The tensile strength test is to draw a fabric sample of specified dimensions at a constant speed until the specimen is broken, and which is usually used to assess the effects of sunlight, washing, wear and various finishing on the inherent quality of the fabric.
However, the biggest problem of testing fabric tensile strength is that it is easy to produce errors in measurement. If the error is large, it will not only take more time to re-test but also cause customers to distrust us. It will even affect our wrong use of textile fabric and cause great loss of production. Therefore, it is very important to obtain accurate test results.
So What are the Factors that Affect the Test Results of Fabric Tensile Strength?
Specifically, the measurement error mainly comes from the following four aspects:
1 External conditions mainly refer to the constant changes of air temperature, air pressure, air humidity and clarity, wind force and atmospheric refraction in the observed environment, which lead to errors in the measurement results.
2 There is no guarantee that the structure of the instrument can satisfy all kinds of geometric relations in the process of processing and assembling of the instrument, and this kind of instrument will inevitably bring error to the measurement.
3 The operation method, the measurement method backwardness, the imperfection causes the measurement error.
4 The observer’s own condition, because the observer’s technical proficiency is different, which will also produce errors in the operation, such as alignment, leveling and aiming.
How to Reduce Test Error Effectively?
1 Treat the sample of printing and dyeing products as required before the tensile strength test.
Test sampling shall be conducted in accordance with the sampling methods specified in the product standards or agreed upon by the parties concerned, and there should be no defects affecting the test results on the samples. Before the test, the sample should be placed in the laboratory conditioning chamber under the standard condition that the temperature is (22±3)℃ and the relative humidity is (65±3)% for 24H until equilibrium. For the samples with high moisture content and high moisture regain, such as cotton fiber, should be pre-humidifying in order to ensure that the sample is balanced by moisture absorption during the humidification period. The data obtained under other conditions should be specified in order to ensure the objectivity of the test data.
2 The pre-tension of the gripper reduces the measurement error caused by the installation.
The pre-tension has a great influence on the elongation, usually, the tension is too large, the elongation is small, on the other hand, the elongation is larger. The purpose of pre-tension is to reduce the error caused by the sample loading. Apply a smaller tension to the specimen in advance so that the specimen is just straightened and clamped, and the elongation of the specimen is not caused or the amount of elongation produced is very small, which can be ignored.
For the pre-tension, there are still some difficulties in the practical operation. The usual method is to fix the upper end of the sample first, then clip a weight value at the lower end of the sample equal to the pre-added tension worth the clip or weight, and then fix the lower end of the sample. In fact, because of the structure of the tension machine, it is difficult to operate easily and easily cause errors, so this method is not suitable.
In order to solve this problem, we add the pre-tension control program in the software of the tension machine. When setting parameters, enter the specified pre-tension value. At the beginning of the test, the clamp moves at a very low speed, and the tensile force of the specimen increases with the increase of the displacement. When the tensile force is increased to the pre-tension set, the clamp begins to draw the specimen at the set speed, and the measurement of the sample elongation is also started here, and take this as the starting point to calculate the displacement of the sample. In this way, from the technical point of view, the error caused by the sample loading will be kept in the minimum range.
3 Moderate range
Choose a sensor with the right measurement range. If the sample strength is small, we should not select the force sensor with too large measuring range, because the resolution of the sensor is difficult to distinguish the exact value of the tensile strength of the fabric, resulting in the measurement error. In general, sample strength is more accurate within 20% – 90% of the sensor range. The range of TF001 fabric tensile tester is 2500N that suitable for measuring the tensile strength of most common fabrics, and in addition, the TF002 fabric tensile tester offers range customization, which meets different testing requirements, up to 2T (20KN) strength.
4 Pneumatic clamp reduces measurement error due to excessive or small clamping force.
In the process of drawing, the clamping force on the sample should not be too small or too large. When the clamping force is insufficient, the sample will slip and lead to the measurement error, and if the clamping force is too large, the sample will be broken and the measurement error will also be caused, which has a great influence on the test results. A large number of test results show that the error caused by this error can reach 10%, and some even exceed 20%, which is far beyond the allowable range of textile test error and must be paid attention to.
Except for ASTM D5034 / 5035, most of the test methods have no specific requirements for clamping force of clamp. Because of the different thickness and structure of the tested samples, the clamping force of the clamps must be different. In fact, it is the best clamping force to make the sample doesn’t slip at the time of the drawing. In the past, the operator’s experience and strain ability will affect the accuracy of the test, but now the appearance of pneumatic clamp solves this problem. According to the thickness of the specimen, the pneumatic clamp exerts a certain clamping force on the specimen, and the force on the specimen is uniform. Clamp with a gasket to hold the sample firmly, thereby minimizing the error.
5 The closed-loop controller reduces measurement error caused by fitting offset delay
The force measuring the structure of strength tension machine is usually composed of sensor, direction joint, upper and lower clamps, and there must be a concentric problem between them. In the tensile test, the center line between them and the center line of the sample will deviate. When the deviation is small, the test error is within the allowable range of the precision of the test instrument, and it has little effect on the test result. However, the deviation of the whole machine is accumulated by the offset error of each component. Therefore, it is necessary to consider the design, machining and installation precision of the tension machine, and to control the test error of the tension machine within the minimum range.
The TF001 textile tension strength machine adopts the principle of the closed-loop controller, sensors, directional joints, upper and lower clamps and other fittings connected by the gap-free synchronous belt, the centerline has almost no deviation, greatly improve the stability of the test results, and reduce the test error to the lowest.
Textiles can be seen everywhere in life, no matter what kind of textiles, they will face the problem of breakage. Therefore, it is necessary to test the tensile fracture strength of textiles before making the finished products in order to evaluate the service life of textiles. In addition, fabric strength not only affects the service life of textiles, but also has an important effect on process processing, and even affects the shipment of products. Therefore, when textile enterprises purchase the fabric, it is necessary to test the fabric strength to judge whether incoming materials is in accordance with the production requirements. As a professional supplier of textile instruments, TESTEX Textile Testing Equipment is a reliable choice for testing textile quality, for more information, please click on https://www.testextextile.com/