1. A test machine must be equipped with a urging device, a fixture, a display device with a strong value, and a recorded display. The fixture is very important in the testing machine. During the test, we need a fixture to hold the test material (product), and the force device, the lychee display device and the record to determine whether the material (product) in the test is qualified or whether the predetermined performance is achieved.
First, the testing machine must have three elements:
A test machine must be equipped with a force device: a clamp; a force display device and a record. Visible the importance of the fixture in the testing machine, we clamp the specimen (or product) through the fixture, through the force device, the force value display device and the record to judge whether the material (or finished product) is qualified and reach the predetermined performance index, no Reliable fixtures, these can not be judged. The fixture is a frequently changing part of the test machine 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. Reasonable and correct use of the clamp is beneficial. With the smooth progress of the experiment, with the development of science and technology, the requirements for materials in all walks of life are getting higher and higher, resulting in the continuous emergence of new materials, and put forward higher requirements for the design of fixtures.
According to different test methods, the fixture can be roughly divided into: stretching type fixture, compression type fixture, bending type fixture, peeling type fixture, shear type fixture, etc., wherein the stretching type fixture accounts for about 80% of the total amount of the fixture.
Second, the basic performance of the fixture
1. Fixture strength requirements:
The sample is clamped by the clamp (or product), and the test force that the clamp 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 has the size and the composition of the material is various. The test force that the sample can withstand is as small as several tens of centimeters (such as textile spandex). To several tens of tons of ordinary electronic universal testing machines such as ordinary steel, the test force is 600KN, 0.5-class machine, the sample size is as small as the diameter of φ0.006mm gold wire, as large as 1m diameter PVC pipe, etc. According to different test forces, the shape and size of the sample, choose different fixtures.
2. Requirements for fixture materials.
a. For general metal and non-metal specimens, the jaws of the fixture are directly in 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 used, and appropriate heat treatment is adopted. Process quenching and tempering, carburizing and quenching, etc. increase its strength and wear resistance. Sometimes it is also equipped with special steel at the jaws, or gold steel sand on the surface of the jaws.
b. For some small test force fixtures, the surface in contact with the sample is made of soft rubber. (For example, the clamping surface of a sample such as a plastic film or a fiber yarn, etc.)
c. The clamp is generally made of high-quality medium carbon steel and alloy structural steel, and its mechanical properties are increased by a suitable heat treatment process. In order to reduce the weight, non-ferrous metals such as aluminum alloys and special metals are sometimes used. Casting steel, cast aluminum, etc.
3. Requirements for the structure of the fixture. The design of the fixture is mainly based on the test standards of the materials and the shape and material of the samples (specifically, finished and semi-finished products). The test standards mentioned above 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 test methods. We can design different fixtures depending on the sample and test method.
For fixtures used for special specimens (finished and semi-finished), the fixture is designed mainly according to the shape and material of the specimen.
Third, the structure of the fixture The fixture itself has no fixed structure (such as the wire can be clamped by winding, can also be clamped by two plates, the metal sheet can be clamped by wedge clamp, or clamped clamp ), this is obviously different from the host. The host is similar in domestic and foreign countries, and the difference between the foreign and domestic fixtures is very large, and there are big differences between different companies. This depends mainly on the overall level of the company and the accumulation of experience of the designer. Foreign fixtures, such as INSTRON, MTS, ZWICK and other companies' fixtures are generally meticulously crafted, with high reliability, but at a higher price, in the high-end market, while domestic, such as SANS fixtures, are involved in the industry, The market share is large, and it can replace foreign fixtures to a certain extent, and it is in the middle and high-end market. However, there are still some gaps between domestic and foreign standards in some new materials and fixtures for special materials.
The fixture itself is a locking mechanism. We know that the mechanical locking structure is: crepe (ie thread, screw, nut), bevel, eccentric, lever, etc., the fixture is a combination of these structures. There is no fixed mode in the structure of the test machine fixture. According to different samples and test force, the structure is very different. The test piece with large test force generally adopts the inclined surface clamping structure, and the clamping force increases with the increase of the test force. The force is increased accordingly, the shoulder sample is suspended structure, etc. If the clamp is divided according to the structure, it can be divided into wedge-shaped clamps (referring to the clamps with the principle of inclined locking principle), and clamp clamps (refer to single or double sides) The clamp of the thread tightness principle structure, the winding type clamp (refers to the clamp that the sample is locked by the winding method), the eccentric type clamp refers to the (the fixture of the eccentric locking principle structure), the lever type clamp (refers to the principle of lever force amplification) Structural fixture), shoulder clamps (referring to fixtures for shoulder specimens), bolt clamps (referring to fixtures for bolt strengths such as bolts, screws, studs, etc.), 90° peeling fixtures (refers to For the two specimens, the hanging device, the straight peeling fixture, etc. The structure of these clamps has its own advantages and disadvantages, such as: wedge clamps, the initial clamping force is small, and the test force increases. The clamping force increases. For the clamp fixture, the initial clamping force is large and increases with the test force. The clamping force is then reduced.
Fourth, the judgment criteria of the applicability of the fixture is difficult to define the applicability of the fixture. Due to the particularity of the fixture structure, sometimes it is difficult for a fixture to determine whether it is more suitable for the specimen, but it cannot be said that there is no way. The following points are for reference:
1 Whether the fixture is convenient and safe to use.
2 Whether the clamping is reliable and there is no slippage.
3 During the test, the sample breakpoint is good. Data dispersion is small. (ie, the sample is constantly clamped, inside the jaws, parallel segments or outside the gauge length)
V. Status and development trend of fixtures
1 The development direction of the testing machine is to develop from the inspection of the sample to the inspection of the product (ie, finished product, semi-finished product), which requires the corresponding fixture to be developed from the fixture originally used for the test of the standard sample to the fixture for the inspection of the finished product. .
2 The use of fixtures is developing in the direction of high efficiency and low labor intensity. The past fixtures generally use mechanical locking, which is time-consuming and labor-intensive, labor-intensive, and inefficient. With the improvement of the working environment, and random sampling of the production line of large-scale test production. Needed, the clamping mode of the clamp is developed from the original mechanical clamping to the air pressure clamping and hydraulic clamping.
3 The automatic clamp is measured from the sample size to the clamping, and then the test is started. Finally, the test report is completed once. Such fixtures are costly and are only suitable for testing and inspection of large quantities of the same sample or finished product.
4 The increase of environmental test (high and low temperature test), the increase of fixtures used in high and low temperature, and the increase of environmental test (high and low temperature box) have made it difficult to design the fixture. We know that the national standards for high temperature tensile test have regulations. The sample is threaded and the plate sample has a hole. 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 chamber, and its sample generally has a short gauge length (generally a normal temperature sample). In this way, the fixture must be installed in the high and low temperature chamber. The high and low temperature test is generally limited due to the stroke of the testing machine (the stroke of the testing machine when the standard fixture is installed), which requires the fixture to be small in size, and must meet the test force and high temperature resistance. Low temperature is generally difficult to design.
5 The number of continuous test fixtures is increased. In the past, the sample preparation was generally performed. The stretching and compression of the specimens were carried out separately (that is, the stretching and compression were carried out with different fixtures), and now more and more finished products are being tested. In the same test, it has to be stretched, compressed, and highly efficient. It can only be stretched and compressed with the same type of fixture.
6 Increased 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.
Sixth, the difficulties in the design of fixtures
7 steel wire, steel strand due to the high hardness of the sample, the internal structure is relatively loose, the force is not uniform during the tensile test, the jaws of the clamped sample are easy to wear, etc., the fixture has not been solved.
The 8 large test force and large diameter nylon ropes are difficult to clamp due to excessive deformation, and the design of the fixture is also a difficult point.
Due to the special use of the test machine fixtures and the continuous emergence of new materials, the design of the fixtures has been in a passive situation. We encounter new materials every day, need to design new fixtures, and summarize the past successful experience.
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