Special section on testing machine fixtures

1、 The testing machine must have three elements:
There is a force device: there is a fixture; Powerful value display device and recording. The importance of fixtures in testing machines can be seen. We use fixtures to hold samples (or products), and use force devices, force value display devices, and records to determine whether materials (or finished products) are qualified and meet predetermined performance indicators. Without reliable fixtures, these cannot be judged. Fixture is a frequently changing part of the 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. Reasonable and correct use of fixtures is conducive to the smooth progress of the test. With the development of science and technology, various industries have increasingly high requirements for materials, leading to the continuous emergence of new materials,Higher requirements have been put forward for the design of fixtures.

twoAccording to different testing methods, fixtures can be roughly divided into:Stretching fixtures, compression fixtures, bending fixtures, peeling fixtures, shearing fixtures, etc. Among them, stretching fixtures account for about the total number of fixtures80％左右。

3、 Basic performance of fixtures

1.Fixtures require strength:
By clamping the sample (or product) with a fixture and applying force to the sample, the size of the test force that the fixture can withstand is an important indicator of the fixture. It determines the size of the fixture structure and the labor intensity of the fixture operation. The material of the sample can be classified into metal and non-metal, and the shape can be classified into size,The composition of materials varies, and the test force that the sample can withstand ranges from a few tens of centimeter newtons (such as spandex yarn used in textiles) to tens of tons (such as ordinary steel). The test force of electronic testing machines is600KN,0.5Grade machine, sample size as small as diameterφ0.006mmGold wire, large to the diameter1mofPVCPipes and other materials require the selection and design of different fixtures based on different test forces and sample shapes and sizes.

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 and are generally made of alloy structural steel, alloy high carbon steel or low carbon alloy steel, cold work die steel, etc. The strength and wear resistance are increased through appropriate heat treatment processes such as quenching and tempering, carburizing and quenching. Sometimes special steel is also embedded at the jaws,Or spray steel sand on the surface of the jaws, etc.
②. For some fixtures with small testing forces, the surface in contact with the specimen should be covered with soft adhesive tape. For example, the clamping surface of the fixture for samples such as plastic film and fiber thread,）
③. The clamp is generally made of medium carbon steel or alloy structural steel, and its mechanical properties are increased through appropriate heat treatment processes. Sometimes, in order to reduce weight, non-ferrous metals such as aluminum alloys and special metals are also used. Sometimes cast steel structures are also used,Cast aluminum, etc

3. Requirements for fixture structure. The design of fixtures is mainly based on the testing standards of materials and the shape and material of samples (especially finished and semi-finished products). The above mentioned test standards refer toISOTheASTMTheDINTheGBTheBSTheJIS…In addition, there are also enterprise standards, industry standards, etc. These standards generally have strict regulations on sample preparation and testing methods. We can design different fixtures according to the different samples and testing methods.

For fixtures used for special samples (finished and semi-finished products), the fixture design is mainly based on the shape and material of the sample.

4、 Structure of fixtures
The fixture itself does not have a fixed structure (such as metal wire can be clamped by winding),Two flat plates can also be used for clamping, and wedge-shaped clamping or double clamping can be used for metal thin plate samples. This is significantly different from the main machine, which is similar in both domestic and foreign markets. However, there are significant differences in fixtures between foreign and domestic markets, as well as between different companies. This mainly depends on the overall level of the company and the accumulation of experience by the designers. Foreign fixtures, such asINSTRONTheMTSTheZWICKThe fixtures of the company are usually meticulously crafted,High reliability, but high price, in the market, while domestically, such asSANSDue to its wide involvement in various industries and large market share in China, the fixture can to some extent replace foreign fixtures,Located in the middle market,But in some new materials,There is still a certain gap between the domestic and foreign levels in the use of fixtures for special materials.

The fixture itself is a locking mechanism. We know that mechanical locking structures include threads (i.e. threads, screws, nuts), inclined surfaces, eccentric wheels, levers, etc. The fixture is a combination of these structures. There is no fixed pattern in the structure of the fixture used in the testing machine,According to different samples and test force sizes,There is a significant difference in structure.The specimen with high test force generally adopts a slanted clamping structure,With the increase of experimental force,The clamping force increases accordingly, and the shoulder specimen adopts a suspension structure. If the fixture is divided by structure, it can be divided into wedge-shaped fixtures (referring to fixtures that use the inclined plane locking principle structure), clamping fixtures (referring to fixtures that use single-sided or double-sided thread tightening principle structure), winding fixtures (referring to fixtures that lock the specimen by winding), eccentric fixtures (referring to fixtures that use the eccentric locking principle structure), lever fixtures (referring to fixtures that use the lever force amplification principle structure), shoulder fixtures (referring to fixtures that are suitable for shoulder specimens), bolt fixtures (referring to fixtures that are suitable for testing thread strength of bolts, screws, studs, etc.)90°Stripping fixtures (referring to fixtures suitable for hanging and directly peeling two samples), etc. The structures of these fixtures each have their own advantages and disadvantages, such as wedge-shaped fixtures, which have a small initial clamping force and increase with the test force. The clamping force increases accordingly. For clamping fixtures, the initial clamping force is high and increases with the test force. The clamping force decreases accordingly

5、 Criteria for determining the applicability of fixtures

It is difficult to define the applicability of fixtures due to the special structure of fixtures,For a fixture,Sometimes it's difficult to determine which sample is more suitable for it, but it's not impossible. Here are a few points for reference:

◆ Is the fixture easy and safe to use.
◆ Whether the clamping is reliable and there should be no slipping phenomenon.
◆ During the experiment, the breakpoint of the sample was good. The data has low dispersion. (i.e. the sample is clamped continuously, within the jaws, parallel segments, or outside the gauge)

6、 Current situation and development trend of fixtures

◆ The development direction of testing machines is from sample testing to product (i.e. finished and semi-finished) testing, which requires the corresponding fixtures to be developed from fixtures originally used for standard sample testing to fixtures used for finished product testing.

◆ The usage rate of fixtures is developing towards low labor intensity. In the past, fixtures generally used mechanical locking, which was time-consuming and labor-intensive,The labor intensity is high and the efficiency is low. With the improvement of the working environment and the need for random sampling on large-scale experimental production lines, the clamping method of fixtures has evolved from mechanical clamping to pneumatic clamping, hydraulic clamping, and other directions.

◆ Fully automatic fixture

From measuring the sample size to clamping, and then starting the test, the final test report is completed in one go. This type of fixture has a high cost and is only suitable for testing and inspecting large quantities of identical samples or finished products.

◆ An increase in environmental testing (high and low temperature testing),The increase in fixtures used for high and low temperatures, as well as the increase in environmental testing (high and low temperature chambers), has made fixture design more difficult. We know that national standards for high temperature tensile testing have regulations, with threaded circular specimens and holes on plate specimens. Due to the fixed connection method,So the design of the fixture is relatively simple,But the high and low temperature tests are different, they are usually conducted in high and low temperature chambers,Its samples generally have a short gauge length (usually at room temperature). In this way, the fixture must be installed in a high and low temperature box. High and low temperature tests are generally limited by the travel of the testing machine (the travel of the testing machine when installing standard fixtures), which requires the fixture to have a small volume, meet the testing force, and withstand high and low temperatures,It is generally difficult to design.

◆ Increase in continuous testing fixtures

Due to the past practice of sample testing, the stretching and compression of samples were carried out separately (i.e. using different fixtures for stretching and compression). However, with the increasing use of finished product testing, samples are subjected to both stretching and compression in the same test, while maintaining high efficiency. Therefore, only the same fixture can be used for both stretching and compression.

◆ An increase in test fixtures for special industries

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

7、 Difficulties in fixture design

◆ Due to the high hardness of the specimens, relatively loose internal structure, uneven stress during tensile testing, and easy wear of the clamping jaws that hold the specimens, the fixtures for steel wires and steel strands have not been well solved.
◆ Nylon ropes with high testing force and large diameter are difficult to clamp due to excessive deformation, and the design of fixtures is also a challenge.
Due to the special nature of the use of testing machine fixtures,And the continuous emergence of new materials,The design of fixtures has always been in a passive position,We encounter new materials every day, need to design new fixtures, and summarize past successful experiences.