Mechanical Properties Of Bolts

A bolt is a kind of mechanical part, which is widely used in daily life. It is a type of fastener consisting of two parts, the head and the screw (cylinder with external thread), used in conjunction with the nut to fasten two parts with through holes. Some people may be interested in bolt-related knowledge. As a professional fastener manufacturer, Xinch Fastener Company will be glad to introduce the mechanical properties of bolts to you.

The class of bolts:

The strength grades of metric bolts are 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9.

Classification and meaning of high-strength bolts: The bolts of grade 8.8 and above are collectively referred to as high-strength bolts, and the bolts of other grades are called ordinary strength bolts.

The meaning of bolt performance grade label:

The bolt performance grade label is composed of two parts, which respectively represent the nominal tensile strength value and the flexure ratio of the bolt. For example, the meaning of a high-strength bolt with a performance grade of 4.8 is:

1. The nominal tensile strength of bolt material is 400MPa.
2. The bending ratio of bolt material is 0.8.
3. The nominal yield strength of bolt material is 400×0.8= 320MPa class.

The mechanical performance grade of bolts mainly has the following four indexes:

1. Strength index (tensile strength, yield point, yield strength, guarantee stress);
2. Hardness index (Vickers hardness, Brinell hardness, Rockwell hardness, surface hardness);
3. C.plasticity, toughness indicators (elongation, load strength, impact absorption work, head firmness);
4. Index of decarbonized layer (minimum height of undecarbonized layer and the maximum depth of the fully decarbonized layer of thread).

Noun explanation of bolts:

Tensile strength (N/mm): refers to the maximum tensile strength that a product can withstand per unit area. It refers to the maximum stress that a metal material can withstand before it breaks.

The guaranteed load (N/mm) of the bolt: refers to the non-plastic deformation of the bolt under the specified preload, that is, the measurement error of the bolt before and after the test should be within 00125mm.

The yield point: is the point at which the strain increases while the stress does not when the material is stretched. The yield point can be obviously shown in the tensile curve of general low-strength products, and the yield point is the dividing point between elastic deformation and plastic deformation of materials. There is no obvious yield point in the tensile curve of the high-strength product. When the yield point cannot be determined, the yield strength measurement method is allowed instead.

The yield strength: is defined as the yield limit of metal materials when the yield phenomenon occurs, that is, the stress that resists microplastic deformation. For metal without obvious yield, the stress value of 0.2% residual deformation is defined as the yield limit, which is called the conditional yield limit or yield strength. An external force greater than this limit will cause the part to fail permanently and cannot be recovered. For example, the yield limit of low-carbon steel is 207MPa. When the external force is greater than this limit, the parts will be permanently deformed. If it is less than this, the parts will be restored to their original appearance.

Remarks:

• The deformation of materials can be divided into elastic deformation (the original shape can be restored after the external force is withdrawn) and plastic deformation (the original shape can not be restored after the external force is withdrawn, and the shape will change, extend or shorten).
• When the stress exceeds the elastic limit, the deformation increases rapidly after entering the yield stage. In this case, in addition to elastic deformation, some plastic deformation occurs. When the stress reaches point b, the plastic stress will increase sharply. The phenomenon of small fluctuation of stress and strain is called yield. The maximum and minimum of this stage should be called the lower yield point and the upper yield point respectively. As the value of the lower yield point is relatively stable, it is used as an indicator of material resistance, which is called yield point or yield strength (ReL or Rp0.2).

Hardness: The ability of a metallic material to resist being pressed into it by a harder object is called hardness. It is a comprehensive physical quantity of material properties, representing the ability of a metal material to resist elastic deformation, plastic deformation or breaking in this small volume range (Vickers hardness HV30, Brinell hardness HB, Rockwell hardness HRB and HRC, surface hardness HV0.3).

Wedgie load strength: The wedgie load test is performed on the hexagonal head, square head (four corners), hexagonal flange or hexagonal bolt reaction, that is, the tensile strength of the product after adding a wedgie block under the head, the purpose is to get the tensile strength of the product and the rigidity of the head.

Elongation (δ): The elongation of the product is the ratio of the elongation of the product after breaking to the length before breaking.

Head firmness: Load the product into support with an inclined hole, and hit the head of the product with a hammer. For the bolts or screws of the whole teeth, as long as there is no reverse phenomenon, even if there is a crack on the first thread, it should still be regarded as meeting the requirements of this test. For half tooth products, there should be no cracks in the head, support surface and screw transition corner.

Anji Xinchi Fastening Piece Co., Ltd is a one-stop solution fasteners company and is the designated fastener supplier to“STAPLE”. As a trusted fastener supplier, Xinchi can offer a wide range of high-quality fasteners, including tee nuts, bolts, rivets, washers, etc. If you want to know more about fasteners or want to wholesale fasteners, please feel free to contact us. Our experienced and expert service team will reply to you as soon as possible.