High strength bolts can be called high strength bolts, which are made of high strength steel or require a large preload High strength bolts are mostly used for the connection of bridges, rails, high pressure and ultra-high pressure equipment The fracture of this kind of bolt is brittle fracture For high-strength bolts applied to ultra-high pressure equipment, in order to ensure the sealing of the container, large prestress is required. Several concepts about high-strength bolts 1 High strength bolts are those whose performance grade is above 8.8 according to the regulations At present, the national standard only lists M39. For large-size bolts, especially high-strength bolts with a length greater than%10~15 times, domestic production is still short-line high-strength outer hexagon bolts and high-strength T-groove bolts
Difference between high strength bolt and ordinary bolt
High strength bolts can bear a larger load than ordinary bolts of the same specification. Ordinary bolts are made of Q235 (i.e. A3). The material of high-strength bolt is 35\\steel or other high-quality materials. After being made, it is heat treated to improve the strength. The difference between the two is the strength of the material.
From the perspective of raw materials, high-strength bolts are made of high-strength materials. The screw, nut and washer of high-strength bolt are made of high-strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Ordinary bolts are usually made of Q235 (equivalent to A3) steel. From the strength grade, high-strength bolts are increasingly used. Two strength grades of 8.8s and 10.9s are commonly used, of which 10.9 is the majority. The strength grade of ordinary bolts shall be low, generally 4.4, 4.8, 5.6 and 8.8.
From the stress characteristics, high-strength bolts exert pretension and transmit external force by friction. Ordinary bolt connections transfer shear force by the shear resistance of the bolt rod and the pressure on the hole wall. The pretension generated when tightening the nut is very small, and its influence can be ignored. In addition to its high material strength, the high-strength bolt also applies a large pretension to the bolt, resulting in extrusion between the connecting members, so that there is a great friction perpendicular to the screw direction. Moreover, the pretension, the anti slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolt. According to the stress characteristics, it can be divided into pressure bearing type and friction type The two methods are different. The minimum specification of high-strength bolt is M12, commonly used is m16~m30, and the performance of super large bolts is unstable, so they should be used carefully in design. The difference between high-strength bolt friction type connection and pressure bearing type connection: high-strength bolt connection clamps the plates of the connecting plate through a large tightening pretension in the bolt rod, which is enough to generate a large friction force, so as to improve the integrity and stiffness of the connection. When under shear, according to the different design and stress requirements, it can be divided into high-strength bolt friction type connection and high-strength bolt pressure bearing type connection. The essential difference between the two is that the limit state is different, Although it is the same bolt, there are great differences in the calculation method, requirements, scope of application and so on. In the shear design, the high-strength bolt friction type connection is the limit state in which the external shear force reaches the maximum possible friction force provided by the bolt tightening force between the plate contact surfaces, that is, to ensure that the internal and external shear forces of the connection do not exceed the maximum friction force during the whole service period. The plate will not be subject to relative sliding deformation (the original gap between the screw and the hole wall will always be maintained), and the connected plate will be stressed as an elastic whole. In the shear design, the allowable external shear force in the high-strength bolt pressure bearing connection exceeds the maximum friction force. At this time, the relative sliding deformation occurs between the connected plates until the bolt rod contacts the hole wall. After that, the connection relies on the joint force transmission of the bolt rod body shear, the hole wall pressure and the friction force between the plate contact surfaces. Finally, the rod body shear or the hole wall pressure bearing failure is taken as the shear limit state of the connection. In a word, friction high-strength bolt and pressure bearing high-strength bolt are actually the same kind of bolt, but whether to consider sliding in design. Friction type high-strength bolts must not slide. The bolts do not bear shear. Once they slide, the design considers that they reach the failure state, which is technically mature; The pressure bearing high-strength bolt can slide, and the bolt also bears the shear force. The final failure is equivalent to the failure of the ordinary bolt (the bolt is sheared or the steel plate is crushed).
From the point of view of use, the bolt connection of the main components of the building structure generally adopts high-strength bolt connection. Ordinary bolts can be reused, while high-strength bolts cannot be reused. High strength bolts are generally used for permanent connections. The high-strength bolt is a prestressed bolt. The friction type uses a torque wrench to apply the specified prestress, and the pressure bearing type screws off the Torx head. Ordinary bolts have poor shear resistance and can be used in secondary structures. Ordinary bolts only need to be tightened. Ordinary bolts are generally grade 4.4, 4.8, 5.6 and 8.8. High strength bolts are generally grade 8.8 and grade 10.9, of which grade 10.9 is in the majority. Level 8.8 and 8.8s are the same level. The mechanical properties and calculation methods of ordinary bolts and high-strength bolts are different. The high-strength bolt bears external load by applying pretension P inside it, and then generating friction resistance on the contact surface between the connected parts, while the ordinary bolt directly bears external load.
More specifically, high-strength bolt connection has the advantages of simple construction, good mechanical performance, replaceable, fatigue resistance, and no loosening under dynamic load. It is a promising connection method. The high-strength bolt uses a special wrench to tighten the nut, so that the bolt produces a huge and controlled pre tension. Through the nut and base plate, the same pre pressure is generated on the connected parts. Under the preload, a large friction force will be generated along the surface of the connected part. Obviously, as long as the axial force is less than this friction force, the component will not slip and the connection will not be damaged. This is the principle of high-strength bolt connection. High strength bolt connection relies on the friction between the contact surfaces of the connectors to prevent them from sliding. In order to make the contact surfaces have enough friction, it is necessary to improve the clamping force of the components and increase the friction coefficient of the component contact surfaces. The clamping force between components is realized by applying pretension to the bolts, so the bolts must be made of high-strength steel, which is also called high-strength bolt connection. In high strength bolted connections, the friction coefficient has a great influence on the bearing capacity. The test shows that the friction coefficient is mainly affected by the form of the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, the contact surface of components within the connection range shall be treated by sand blasting and wire brush cleaning. In fact, there are two types of high-strength bolts: friction type and pressure bearing type. The criterion of friction type high strength bolt bearing shear force is that the shear force caused by design load does not exceed the friction force. The design criterion of pressure bearing high-strength bolts is that the rod body is not sheared or the plate is not crushed.
