In order to allow our customers to have a deep understanding of the fixture problem of tensile testing machines, the Hengyi testing machine has summarized a series of problems related to fixtures based on many years of production experience, hoping to help our customers.
1, the characteristics of the fixture
a. We know that by clamping the specimen (or product) with the fixture to apply force to the specimen, the size of the test force that the fixture can withstand is a very important indicator of the fixture. It determines the size of the jig structure and the labor intensity of the jig operation. The sample material is divided into metal and non-metal, and the shape has the size of the points. The composition of the materials is various, and the test force that the sample can withstand is as small as tens of centimeters (such as spandex for textiles) and as large as several tens of tons (such as ordinary steel ) ; the largest electronic universal test in China Machine test force is 600KN, 0.5 class machine), sample size is as small as φ0.006mm diameter gold wire, and as large as 1m diameter PVC pipe. This requires the design of different fixtures based on different test forces and sample size.
b. 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 steels, alloy high-carbon steels (or low-carbon alloy steels), and cold-working die steels are used. The heat treatment process (quenching and tempering, carburizing and quenching, etc.) increases its strength and wear resistance. Sometimes it also inserts special steel at the jaws, or sprays gold sand on the jaw surface.
2. For fixtures with a small test force, the surface that is in contact with the specimen is made of soft rubber. (Example: clamp surface of sample such as plastic film, filament, etc.)
3Clamps are generally made of high quality medium carbon steel and alloy structural steel, and their mechanical properties are increased through proper heat treatment process. Sometimes non-ferrous metals such as aluminum alloys and special metals are used to reduce weight. Casting structures (cast steel, cast aluminum, etc.) are sometimes used
c. The fixture structure requirements:
1. The design of the fixture is mainly based on the test standard of the material and the shape and material of the sample (specifically, the finished product and semi-finished product). The above-mentioned test standards refer to ISO, ASTM, DIN, GB, BS, JIS, etc., as well as enterprise standards, industry standards, etc. These standards generally have strict regulations for sample preparation and test methods. We can design different fixtures based on different specimens and test methods. For fixtures used for special specimens (finished and semi-finished products), fixtures are mainly designed based on the shape and material of the specimen.
2. The clamp itself has no fixed structure (such as the wire can be clamped by the winding method, and can also be clamped by two flat plates. The metal thin plate sample can be wedge-clamped or clamped). There is a clear difference from the host. Hosts are much the same at home and abroad, and fixtures are different from domestic ones and domestic ones. There are also big differences between different companies. This mainly depends on the overall level of the company and the accumulated experience of designers. Fixtures from abroad, such as INSTRON, MTS, ZWICK, etc., are generally meticulous in workmanship, availability is high, but the price is higher, and they are in the high-end market; while our fixtures have a large market share in the domestic market, To some extent, it can replace some of the fixtures in foreign countries, in the high-end market. However, there are still some gaps between domestic and foreign standards in the use of fixtures for new materials and special materials. The United States TestResources mechanical test system all-electric servo static pull, pressure, torsion testing machine, dynamic & fatigue series testing machine, with a variety of professional fixtures, can also be used for other brand testing machine .
3. The fixture itself is a locking mechanism, we know that the mechanical locking structure is: 缧 lines (ie threads, screws, nuts), bevels, eccentrics, levers, etc. The fixture is a combination of these structures. There is no fixed pattern for the fixture of the test machine. According to the different specimens and test force sizes, the structure is very different. (The test specimen with large test force generally adopts the inclined clamping structure, with the increase of the test force, the clamp With the increase of tightness, the shoulder sample adopts suspension structure, etc.). If the fixture is divided by structure, it can be divided into wedge-shaped fixtures (fixtures with inclined locking principle) and clip -type fixtures (refer to single-sided or Fixtures with a double-sided thread-tight principle structure), winding-type jigs (fixtures in which the sample is locked by a winding method), and eccentric-type jigs are used (fixtures with an eccentric locking mechanism), lever-type jigs (referring to leverage (Amplification of the principle of the structure of the fixture), shoulder type fixture (refers to the fixture for the shoulder specimen), bolt type fixture (refers to the bolt, screw, stud strength test fixture, etc.), 90 ° peel type fixture ( Refers to fixtures that are suitable for vertical and double peeling of two specimens). Each of these fixtures has its own advantages and disadvantages. For example, a wedge-shaped fixture has a small initial clamping force and increases with the test force. The clamping force increases. For clip fixtures, the initial clamping force is large and increases with the test force. The clamping force decreases.
2. Fixture status and development trend
a. The development direction of the testing machine is to develop the testing direction of the product (ie, the finished product and the semi-finished product) from the sample preparation. This requires the corresponding fixture to be used from the fixture used for the standard specimen test to the fixture used for the finished product testing. development of.
b. The use of fixtures has evolved in the direction of high efficiency and low labor intensity. In the past, the fixtures were generally mechanically locked, which was time-consuming, labor-intensive, labor-intensive, and inefficient. With the improvement of the working environment and the need for large-scale testing (random sampling of the production line), the clamping method of the fixture is developed from the original mechanical clamping to the pneumatic clamping and hydraulic clamping.
c. Full-automatic fixture: From the measurement of the specimen size to the fixture, to the start of the test, the test report is completed at one time. This type of fixture is very costly and is only suitable for testing and inspection of large batches of the same sample or finished product.
d. Increased number of environmental tests (high and low temperature tests), increased use of high and low temperature fixtures. The increase in environmental tests (high and low temperature boxes) has increased the difficulty of designing fixtures. We know that the national standards for high temperature tensile tests are stipulated in the following: Circular sample threads, and holes in plate samples. Because the connection is fixed, the fixture design is relatively simple. However, the high-low temperature test is different, it is generally tested in high and low temperature box, its sample generally short gauge (usually normal temperature sample). In this case, the fixture must be installed in the high and low temperature box. The high and low temperature test is generally limited due to the stroke of the tester (the tester travels when installing the standard fixture). This requires the fixture to have a small size, but also to meet the test force, but also resistant to High temperature and low temperature are generally more difficult to design.
e. Increased number of continuous test fixtures. Since in the past it was generally a sample preparation test, the tensile and compression of the sample were performed separately (ie, the tension and compression were performed using different jigs). Now that more and more finished products are being tested, the sample is in the same test. It must be stretched, but it must be compressed, and it must have high efficiency. It can only be used for stretching and compression with the same fixture.
f. Increased number of 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 fixtures, non-magnetic, corrosion resistance (testing in solution) and so on.
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