Selection of test machine fixture

1. The testing machine must have three elements:
With force device: there are clamps; force value display device and record. It can be seen the importance of the fixture in the testing machine. We clamp the sample (or product) through the fixture, and judge whether the material (or finished product) is qualified and reaches the predetermined performance index through the force adding device, the force value display device and the record. Reliable fixtures, these cannot be judged. The fixture is a part of the testing machine that often changes according to the change of the material sample.

Different materials require different fixtures. It is an important factor for the smooth progress of the test and the accuracy of the test results. The correct use of the fixture is beneficial to The experiment proceeded smoothly. With the development of science and technology, all walks of life have higher and higher requirements for materials, resulting in the continuous emergence of new materials, and higher requirements for the design of fixtures.

According to different test methods, fixtures can be roughly divided into: tensile fixtures, compression fixtures, bending fixtures, peeling fixtures, shearing fixtures, etc. The tensile fixtures account for about 80% of the total fixtures.

Second, the basic performance of the fixture

1. The strength requirements of the fixture:

The sample (or product) is clamped by the fixture to apply force to the sample. The test force that the fixture can withstand is a very important indicator of the fixture, which determines the size of the fixture structure and the labor intensity of the fixture operation , The sample material is divided into metal and non-metal, the shape is divided into size, the composition of the material is various, the test force that the sample can withstand is as small as tens of centimeters (such as spandex yarn for textile), To dozens of tons, such as ordinary steel, the largest domestic electronic universal testing machine with a test force of 600KN, 0.5 grade machine, sample size as small as φ0.006mm gold wire, as large as 1m diameter PVC pipe, etc., this requires Choose and design different fixtures according to different test forces and sample shapes.

2. Requirements for fixture materials.

①. For general metal and non-metal specimens, the jaws of the fixture are in direct contact with the specimen. Generally, high-quality alloy structural steel, alloy high-carbon steel or low-carbon alloy steel, cold-work die steel, etc., are selected through appropriate heat treatment Process quenching and tempering, carburizing and quenching, etc. increase its strength and wear resistance. Sometimes special steel is installed at the jaws, or gold steel grit is sprayed on the surface of the jaws.

②. For some fixtures with small test force, the surface in contact with the sample is made of viscous rubber. (For example: the clamping surface of plastic film, fiber yarn and other samples,)

③. The clamp is generally made of high-quality medium carbon steel and alloy structural steel, and its mechanical properties are increased through appropriate heat treatment processes. Sometimes non-ferrous metals such as aluminum alloys and special metals are also used to reduce weight. Sometimes cast steel, cast aluminum, etc. are also used

3. Requirements for fixture structure. The design of the fixture is mainly based on the test standard of the material and the shape and material of the sample (especially the finished product and semi-finished product). The test standards mentioned above refer to ISO, ASTM, DIN, GB, BS, JIS, etc., as well as corporate standards, industry standards, etc. These standards generally have strict regulations on sample preparation and test methods. We can design different fixtures according to different samples and test methods.

For the fixtures used for special samples (finished and semi-finished products), the fixtures are mainly designed according to the shape and material of the sample.

Three, the structure of the fixture

The fixture itself does not have a fixed structure (for example, the metal wire can be clamped by winding method, or two flat plates can be used for clamping, and the metal sheet sample can be clamped by wedge or by the clamping method). There are obvious differences. The domestic and foreign mainframes are similar, but the fixtures are very different from abroad and domestic, and there are also big differences between different companies.

This mainly depends on the overall level of the company and the accumulation of experience of the designers. Foreign fixtures, such as those from INSTRON, MTS, ZWICK, etc., are generally meticulous in workmanship, high in reliability, but at a higher price, and are in the high-end market, while domestic fixtures, such as SANS, are widely used in the domestic market. The market share is large, which can replace foreign fixtures to a certain extent, and is in the mid- to high-end market. However, there is still a certain gap between domestic and foreign standards in some new materials and special materials.

The clamp itself is a locking mechanism. We know that the mechanical locking structures include: ridges (threads, screws, nuts), inclined surfaces, eccentric wheels, levers, etc. The clamp is a combination of these structures. There is no fixed pattern in the structure of the fixture for the testing machine. According to different samples and the size of the test force, the structure is very different.

The specimens with large test force generally adopt the inclined surface clamping structure. With the increase of the test force, the clamping The force increases, the shoulder specimen adopts a suspension structure, etc. If the clamps are divided according to the structure, they can be divided into wedge-shaped clamps (referring to the clamps with the inclined locking principle structure), and the clamp type clamps (referring to the single-sided or double-sided Thread clamping principle structure fixture), winding type fixture (referring to the fixture that the sample is locked by winding method), eccentric fixture refers to the use of (eccentric locking principle structure fixture), lever type fixture (refers to the lever force amplification principle Structural fixtures), shoulder fixtures (referring to fixtures suitable for shoulder specimens), bolt fixtures (referring to fixtures suitable for testing thread strength of bolts, screws, studs, etc.), 90° peeling fixtures (referring to applicable Perform a hanging tool on the two samples, a straight peeling jig), etc.

The structure of these fixtures has its own advantages and disadvantages. For example, the wedge-shaped fixture has a small initial clamping force and increases with the test force. The clamping force increases accordingly. For clamping fixtures, the initial clamping force is large and increases with the test force. The clamping force decreases accordingly.

4. Judgment criteria for applicability of fixture

It is difficult to determine the applicability of the fixture. Due to the particularity of the fixture structure, it is sometimes difficult for a fixture to be more suitable for the sample, but it cannot be said that there is no way. The following points are for reference:

◆ Whether the fixture is convenient and safe to use.

◆ Whether the clamping is reliable and there should be no slipping.

◆ During the test, the sample breaks well. Data dispersion is small. (I.e. the sample is constantly jaw, inside jaw, parallel section or outside gauge length)

Five, fixture status and development trend

◆ The development direction of the testing machine is from sample preparation to product (ie, finished, semi-finished) testing, which requires the development of the corresponding fixture from the fixture used for standard sample testing to the fixture used for product testing .

◆ The use of fixtures is developing in the direction of high efficiency and low labor intensity. In the past, fixtures generally used mechanical locking, which was time-consuming, labor-intensive, labor-intensive, and low-efficiency. With the improvement of the working environment, and the mass test production line random sampling If necessary, the clamping method of the clamp has been developed from the original mechanical clamping to pneumatic clamping and hydraulic clamping.

◆ Automatic fixture

From the sample size measurement to the clamping, to the start of the test, and finally the test report is completed in one go. This type of fixture is very costly and is only suitable for testing and inspection of large quantities of the same sample or finished product.

◆ The increase in environmental tests (high and low temperature tests), the increase in fixtures used in high and low temperatures, and the increase in environmental tests (high and low temperature boxes) have increased the difficulty of fixture design. We know that the national standards for high temperature tensile testing have regulations. The sample is threaded, and the plate sample has holes. Because the connection method is fixed, the design of the fixture is relatively simple, but the high and low temperature test is different.

It is generally tested in a high and low temperature box, and its sample is generally short in gauge length (usually a normal temperature sample). In this way, the fixture must be installed in the high and low temperature box. The high and low temperature test is generally limited by the stroke of the testing machine (the stroke of the testing machine is when the standard fixture is installed). This requires the fixture to be small, meet the test force, and be resistant to high temperature. , Low temperature, generally more difficult to design.

◆ More continuous test fixtures

In the past, sample preparation testing was generally performed, and the tensile and compression of the sample were carried out separately (that is, the tensile and compression were carried out with different fixtures), but now more and more finished products are tested, and the sample is in the same test It needs to be stretched, compressed, and high efficiency. Only the same clamp can be used for both stretching and compression.

◆ More test fixtures for special industries

With the development of science and technology, some emerging industries have put forward new requirements for test fixtures, such as requiring small fixture structure, non-magnetic, corrosion resistance (testing in solution) and so on.