Characteristics of the universal material testing machine fixture and requirements for material structure 万能材料试验机夹具特点和材料结构要求

The fixture is an indispensable component in the universal material testing machine. The fixture applies force to the sample by clamping it. The magnitude of the test force that the fixture can withstand is an important indicator of the fixture. The structure of the universal testing machine fixture does not have a fixed pattern. Depending on different samples and the magnitude of the test force, the structure varies greatly, and the quality of the product also varies. 

According to the different testing methods, fixtures can be roughly classified into: tensile fixtures, compression fixtures, bending fixtures, peel fixtures, and shear fixtures, among which tensile fixtures account for approximately 80% of the total fixtures.

We know that when applying force to a sample (or product) using a fixture, the magnitude 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 level of labor intensity required for fixture operation. Samples can be made of either metal or non-metal materials, and their shapes can vary in size. The composition of the material can be diverse. The test force that a sample can withstand ranges from as little as several tens of centinewtons (such as spandex fibers used in textiles) to as much as several tons (such as ordinary steel; the largest electronic universal testing machine in China has a test force of 600KN and a 0.5-level machine), and the sample size can range from a diameter of φ0.006mm gold wire to a PVC pipe with a diameter of 1m, etc. This requires the selection of different fixtures based on different test forces and the shapes and sizes of the samples. 

Requirements for the material of the fixture 

①. For general metal and non-metal specimens, the jaws of the fixture directly come into contact with the specimens. Generally, high-quality alloy structural steel, alloy high-carbon steel (or low-carbon alloy steel), cold-forming die steel, etc. are selected. Through appropriate heat treatment processes (quenching and tempering, carburizing and quenching, etc.), their strength and wear resistance are enhanced. Sometimes, special steel is embedded in the jaws, or the surface of the jaws is sprayed with diamond sand, etc. 

②. For some fixtures with small test forces, the surfaces in contact with the specimens should be covered with soft adhesive rubber sheets, etc. (For example: the holding surfaces of fixtures for specimens such as plastic films and fiber threads.) 

③ The clamping elements are generally made of high-quality medium-carbon steel or alloy structural steel. Through appropriate heat treatment processes, their mechanical properties are enhanced. Sometimes, to reduce weight, aluminum alloys and other non-ferrous metals and special metals are also used. Sometimes, casting structures (such as cast steel, cast aluminum, etc.) are adopted. 

Requirements for the fixture structure 

The design of the fixture is mainly based on the test standards of the materials and the shapes and materials of the specimens (specifically referring to finished products and semi-finished products). The aforementioned test standards include ISO, ASTM, DIN, GB, BS, JIS, etc., as well as enterprise standards and industry standards, etc. Generally, these standards have strict regulations on specimen preparation and test methods. We can design different fixtures according to the different specimens and test methods. For special specimens (finished products and semi-finished products), the fixtures used are mainly designed based on the shapes and materials of the specimens. 

②. The fixture itself does not have a fixed structure (for example, metal wires can be clamped by winding, or by using two flat plates, and metal thin plates can be clamped using a wedge-shaped method or a double-clamping method). This is significantly different from the main machine. The main machine is largely similar both domestically and internationally, but the fixtures have very different characteristics abroad and domestically, and there are also significant differences among different companies. This mainly depends on the overall level of the company and the accumulation of the design personnel's experience. In some cases where new materials or special materials are used for fixtures, there is still a certain gap in the level between domestic and foreign. 

③. The fixture itself is a locking mechanism. We know that there are various locking structures in mechanical systems, such as threads (i.e., screws, nuts, and washers), inclined planes, eccentric wheels, levers, etc. The fixture is a combination of these structures. The fixtures used in the testing machine do not have a fixed structure pattern. Depending on different specimens and the magnitude of the test force, the structures vary greatly. (For specimens with large test forces, an inclined plane locking structure is generally used. As the test force increases, the locking force also increases. For shoulder specimens, a suspension structure is used, etc.). If the fixtures are classified by structure, they can be divided into wedge-type fixtures (referring to fixtures with a locking structure based on the inclined plane principle), clamp-type fixtures (referring to fixtures with a locking principle based on single or double-sided threads), winding-type fixtures (referring to fixtures that lock the specimen through winding), eccentric-type fixtures (referring to fixtures with an eccentric locking principle), lever-type fixtures (referring to fixtures with a force amplification principle structure), shoulder-type fixtures (referring to fixtures suitable for shoulder specimens), bolt-type fixtures (referring to fixtures suitable for testing the strength of screws, bolts, and studs), and 90° peel-type fixtures (referring to fixtures suitable for vertical and direct peeling of two specimens). Each of these fixtures has its own advantages and disadvantages in structure.

夹具是万能材料试验机中不可以缺少的一个零件。夹具通过夹持试样对试样进行加力，夹具所能承受的试验力的大小是夹具的一个很重要的指标。万能试验机夹具在结构上没有固定的模式，根据不同的试样及试验力大小，在结构上差别很大，产品质量好坏也不一。

夹具根据试验方法不同，大致可分为：拉伸类夹具、压缩类夹具、弯曲类夹具、剥离类夹具、剪切类夹具等，其中拉伸类夹具约占夹具总量的80％左右。

我们知道通过夹具夹持试样(或产品)对试样进行加力，夹具所能承受的试验力的大小是夹具的一个很重要的指标。它决定了夹具结构的大小及夹具操作的劳动强度的大小，试样材质有金属和非金属之分，形状有大小之分。材料的成分组成各种各样，试样所能承受的试验力小到几十厘牛(如纺织用氨纶丝)，大到几十吨(如普通钢材等；国内最大的电子式万能试验机试验力为600KN,0.5级机)，试样尺寸小到直径φ0.006mm的金丝，大到直径1m的PVC管材等。这就要求根据不同的试验力、试样的形状大小选择设计不同的夹具。

对夹具材料的要求

①.对一般的金属及非金属试样,夹具的钳口直接与试样接触，一般都选用优质合金结构钢，合金高碳钢(或低碳合金钢)、冷作模具钢等，通过适当的热处理工艺(淬回火、渗碳淬火等)增加其强度、耐磨性.有时也在钳口处镶装特种钢材,或在钳口表面喷涂金钢砂等。

②.对一些小试验力的夹具，与试样接触的表面采用粘软质胶皮等。(例如：塑料薄膜、纤维丝等试样的夹具夹持面。)

③夹具体一般采用优质中碳钢、合金结构钢，通过适当的热处理工艺增加其力学性能。有时为了减轻重量也采用铝合金等有色金属及特种金属。有时也采用铸造结构(铸钢,铸铝等)。

对夹具结构的要求

①.夹具的设计主要依据材料的试验标准及试样(特指成品及半成品)的型状及材质。以上所说的试验标准是指ISO、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(成品及半成品的)使用的夹具，主要根据试样的型状及材质设计夹具。

②.夹具本身没有固定的结构(如金属丝可采用缠绕方式夹紧,也可采用两个平板夹紧，金属薄板试样可采用楔形夹紧方式，也可采用对夹夹紧方式)，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。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。

③.夹具本身就是一个锁紧机构，我们知道机械上的锁紧结构有：缧纹(即螺纹，螺钉，螺母)、斜面、偏心轮、杠杆等，夹具就是这些结构的组合体。试验机用夹具在结构上没有固定的模式,根据不同的试样及试验力大小,在结构上差别很大。(大试验力的试样一般采用斜面夹紧结构,随试验力的增加,夹紧力随之增加，台肩试样采用悬挂结构等)，如果夹具按结构划分，可分为楔形类夹具(指采用斜面锁紧原理结构的夹具)、对夹类夹具(指采用单面或双面螺纹顶紧原理结构的夹具)、缠绕类夹具(指试样通过缠绕方式锁紧的夹具)、偏心类夹具指采用(偏心锁紧原理结构的夹具)、杠杆类夹具(指采用杠杆力放大原理结构的夹具)、台肩类夹具(指适用于台肩试样的夹具)、螺栓类夹具(指适用于螺栓、螺钉、螺柱等测试螺纹强度的夹具)、90°剥离类夹具(指适用于两试样进行垂具，直剥离的夹具)等。这些夹具的结构各有各的优缺点。