Three standards for tensile testing machine
The tensile properties of polymer, stretching, tensile machine, stroke, fixture, plastic and rubber are one of the most important and the most basic properties in their mechanical properties. It determines the application occasions of this kind of plastic and rubber to a great extent. The tensile properties can be tested by tensile test.
1. Tensile properties of polymers
When used as materials, polymers are required to have necessary mechanical properties. It can be said that for most applications of polymers, mechanical properties are more significant than other physical properties.
Polymers have mechanical properties with a wide range of variability in all known materials, which is due to the fact that polymers are composed of long-chain molecules with obvious relaxation characteristics of molecular motion. For example, polymer materials have quite high elongation, and the elongation at break of PE is generally 90% ~ 950% (among which the elongation of linear low density polyethylene LLDPE is higher). Through special manufacturing process, the elongation of some materials can be above 1000%, while the elongation at break of ordinary polymer materials is mostly between 50% ~ 100%. Generally, heat shrinkable film and tensile film have high requirements for the tensile properties of materials.
2. Tensile test
Tensile test (stress-strain test) is generally to clamp both ends of the material sample on two clamps with a certain distance. The two clamps separate and stretch the sample at a certain speed, and measure the stress change on the sample until the sample is damaged.
Tensile test is one of the most widely used methods to study the mechanical strength of materials, which requires a constant speed tensile testing machine. According to different load measurement methods, tensile testing machines can be divided into pendulum tensile testing machines and electronic tensile testing machines. At present, electronic tensile testing machines are widely used.
3. Selection index of electronic tensile testing machine
As the flexible packaging materials are mainly high polymer or its related materials, as mentioned above, the elongation of high polymer materials is far better than that of metal, fiber, wood, plate and other materials. Therefore, the tensile machine for detecting the tensile properties of high polymer is different from the usual tensile machine for detecting the tensile properties of materials, Special attention should be paid to the effective stroke of the electronic tensile machine and the sample fixture.
3.1 effective stroke
During the tensile test, although the size of the sample used is small, the elongation of the material is generally high. Therefore, a tensile machine with a large stroke is required to detect the tensile properties of flexible packaging materials, otherwise the operation of the fixture may exceed the service limit of the stroke and cause damage to the equipment.
It should be noted that in the calculation of elongation, we only collect the elongation between two marking lines on the sample. The markings are drawn on the prepared sample by printing or hand (the addition of markings will not have any impact on the sample), and what is the distance between the markings? Most of the distances given by different standards have certain differences, and different sample sizes are often given for different materials in the same standard, so the distances between markings are also different, but this is conducive to detecting materials with very large or very small elongation and obtaining test results. For plastic film, the distance between markings is usually 25 ~ 50 mm.
Because the deformation and elongation of the sample in the tensile test is not only within the marking line, all the samples between the two fixtures will get different degrees of tensile deformation. In the standard, the initial distance between the clamps corresponding to the marking distance is within 80 ~ 115mm. If the samples between the two clamps can maintain the same elongation and are assumed to be 500%, the effective stroke of the tensile machine shall be 480 ~ 690mm. If the elongation is 1000%, the effective stroke of the tensile machine shall be at least 880mm to ensure the normal test.