Common Knowledge of Universal Testing Machine Fixtures

Common Knowledge of Universal Testing Machine Fixtures

1.Universal Testing MachineThree essential elements it should have:

Loading device, fixture, force display device, and recording.

The importance of fixtures in a universal testing machine lies in the fact that by clamping the sample with the fixture and applying force, the force display device and recording can be used to determine whether the material is qualified and meets the predetermined performance indicators. Without reliable fixtures, these judgments cannot be made. Fixtures area frequently changing part in the universal testing machine based on the variation of material samples. Different materials require different fixtures, which is an important factor in whether the test can proceed smoothly and the accuracy of the test results. Proper and correct use of fixtures is beneficial forthe accuracy of test results. With the development of science and technology, the requirements for materials in various industries are becoming increasingly high, leading to the continuous emergence of new materials, thus raising higher demands for the design of fixtures. According to the testing methods of fixtures, they can be divided into: tensile fixtures, compression fixtures, bending fixtures, peeling fixtures, shear fixtures, etc., among which tensile fixtures account for about 80% of the total fixtures.2. Basic Performance of Fixtures1. Strength requirements:When clamping the sample with the fixture and then applying force, the size of the universal testing machine that the fixture can withstand is a very important indicator. It determines the size of the fixture structure and the labor intensity of fixture operation. The sample material can be divided into metal and non-metal, and the shape varies in size. The test force that the sample can withstand ranges from a few centinewtons to several tons, such as ordinary steel. The largest electronic universal testing machine in the country has a test force of 600KN, 0.5 grade machine, with sample sizes ranging from a diameter of φ0.006mm for fine wire to a diameter of 1m for PVC pipes, etc. This requires selecting and designing different fixtures based on different test forces and sample shapes.2. Material requirements.①. For general metal and non-metal samples, the jaws of the fixture directly contact the sample, and high-quality alloy structural steel, high-carbon alloy steel or low-carbon alloy steel, cold work mold steel, etc., are generally selected. Appropriate heat treatment processes such as quenching and tempering, carburizing quenching, etc., are used to increase their strength and wear resistance. Sometimes special steel materials are also embedded at the jaw or the jaw surface is coated with tungsten carbide.

②. For some fixtures with small test forces, the surface in contact with the sample uses soft rubber or similar materials.

③. The fixture body generally uses high-quality medium carbon steel and alloy structural steel, and appropriate heat treatment processes are used to enhance its mechanical properties. Sometimes, to reduce weight, aluminum alloys and other non-ferrous metals and special metals are also used. Sometimes cast structural cast steel, cast aluminum, etc., are used.

3. Structural requirements:The design of fixtures mainly depends on the testing standards of materials and the shape and material of the samples. The aforementioned testing standards refer to ISO, ASTM, DIN, GB, BS, JIS, etc., as well as enterprise standards, industry standards, etc. These standards generally have strict regulations on sample preparation and testing methods, and we can design different fixtures based on the differences in samples and testing methods. For fixtures used for special samples, the design is mainly based on the shape and material of the samples.3. Structure of FixturesFixtures do not have a fixed structure, which is a significant difference from the main machine. The main machines are similar both domestically and internationally, while the differences in fixtures between domestic and foreign are substantial, and there are also significant differences between different companies. This mainly depends on the overall level of the company and the accumulation of experience by the designers. Foreign fixtures, such as those from INSTRON, MTS, ZWICK, etc., are generally finely crafted and have high reliability, but they are also more expensive and positioned in the high-end market. Domestic fixtures, such as those from SANS, have a large market share in the domestic market due to their wide industry involvement and can to some extent replace foreign fixtures, positioning them in the mid-to-high-end market. However, there is still a certain gap between domestic and foreign levels in the use of fixtures for new materials and special materials. The force increases with the test force.4. Standards for Judging the Applicability of FixturesDetermining the applicability of fixtures is quite difficult due to the special structure of fixtures. For a certain fixture, it is hard to determine which type of sample it is more suitable for, but it cannot be said that there is no way. However, there are several reference points:

1. The fixture is easy to use and safe.

2. Reliable clamping with no slipping.3. During the testing process, the sample breaks well.4. Low data dispersion.

5. Difficulties in Fixture Design

1. Samples such as steel wires and steel strands have high hardness and relatively loose internal structures, so the force is uneven during tensile testing, and the jaws that clamp the samples are also prone to wear.2. Large test forces and large diameter nylon ropes, etc., have significant deformation during testing, making clamping difficult, so the design of fixtures is also a challenge.

Tai'an Tiancheng

Testing EquipmentISO、ASTM、DIN、GB、BS、JIS…等，还有企业标准、行业标准等，这些标准中一般都对试样制样及试验方法都有严格的规定，我们可以根据试样及试验方法的不同设计不同的夹具。对于特殊试样使用的夹具，主要根据试样的型状及材质设计夹具。

三、夹具的结构

      夹具没有固定的结构，这和主机有明显的区别，主机国内、国外的大同小异，而夹具国外的、国内的区别很大，不同公司间也有大的区别。这主要取决于公司的整体水平，设计人员的经验的积累。国外的夹具，如INSTRON、MTS、ZWICK等公司的夹具一般做工细致,可靠性较高，但价格较高，处在高端市场，而国内的，如SANS的夹具，由于涉足行业广，在国内的市场分额大，在一定程度上可以取代国外的夹具,处在中高端市场,但在一些新材料,特种材料用夹具上国内与国外水平还有一定差距。力大，随试验力增加。夹紧力随之减小。

四、夹具适用性的判断标准

      夹具适用性判定很难界定，由于夹具结构的特殊性，对一种夹具，难以确定它更适合哪种试样，但却不能说没有办法。但有几个参考点：

     1、夹具使用方便、安全。
     2、夹持可靠，无打滑现象。
     3、在试验过程中，试样断点好。
     4、数据离散性小。

     五、夹具在设计中所存在的难点

     1、钢丝、钢绞线等试样的硬度高，内部结构相对松散，所以在拉伸试验过程中受力不均匀，夹持试样的钳口也易磨损等。

     2、大的试验力和大直径的尼龙绳等，由于试验中变形过大，夹持困难，所以夹具的设计也是一个难点。

泰安市天成检测设备The integrity, strength, and product quality of the limited company have been recognized by the industry. Friends from all walks of life are welcome to visit, guide, and discuss business.