Bolts made of high-strength steel or requiring significant pre tension can be referred to as high-strength bolts. High strength bolts are commonly used for connecting bridges, rails, high-pressure and ultra high pressure equipment. The fracture of these bolts is mostly brittle. High strength bolts used in ultra high pressure equipment require significant pre tension to ensure container sealing. Several concepts about high-strength bolts: 1. According to regulations, bolts with a performance level of 8.8 or above are called high-strength bolts. Currently, national standards only list M39. For large-sized specifications, especially high-strength bolts with lengths greater than 10~15 times, domestically produced high-strength bolts are still short-term high-strength hexagonal bolts and high-strength T-groove bolts
From the perspective of raw materials, high-strength bolts are made of high-strength materials. The screws, nuts, and washers of high-strength bolts are made of high-strength steel, commonly including 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Ordinary bolts are commonly made of Q235 (equivalent to A3 in the past) steel. From the perspective of strength grade, high-strength bolts are becoming increasingly widely used. There are two commonly used strength levels, 8.8s and 10.9s, with 10.9 being the most common. Ordinary bolts have lower strength grades, generally 4.4, 4.8, 5.6, and 8.8.
From the perspective of force characteristics, high-strength bolts apply pre tension and transmit external forces through friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure on the hole wall to transmit shear force. When tightening the nut, the pre tension generated is very small, and its effect can be ignored. On the other hand, high-strength bolts not only have high material strength, but also apply a large pre tension to the bolt, causing compression force between the connecting components, resulting in a large frictional force perpendicular to the screw direction. Moreover, the pre tension, anti slip coefficient, and steel type all directly affect the bearing capacity of high-strength bolts. According to the characteristics of the force, it can be divided into pressure type and friction type. The calculation methods for the two are different. The minimum specification for high-strength bolts is M12, commonly M16~M30. Bolts with oversized specifications have unstable performance and should be used with caution in design. The difference between high-strength bolt friction type and pressure type connections: High strength bolt connections clamp the plates of the connecting plate through a large pre tightening force inside the bolt rod, which is sufficient to generate a large frictional force, thereby improving the overall integrity and stiffness of the connection. When subjected to shear force, according to different design and stress requirements, they can be divided into two types: high-strength bolt friction type connections and high-strength bolt pressure type connections. The essential difference between the two is the limit state. Although they are the same type of bolt, there are significant differences in calculation methods, requirements, and applicable scope. In shear design, high-strength bolt friction type connections reach the maximum possible frictional force provided by the tightening force of the bolt between the contact surfaces of the plate when the external shear force reaches the limit state, which ensures that the internal and external shear forces of the connection do not exceed the maximum frictional force throughout the entire use period. The plate will not undergo relative sliding deformation (the original gap between the screw and the hole wall will always be maintained), and the connected plate will be subjected to elastic force as a 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 slip deformation occurs between the connected plates until the bolt rod contacts the hole wall. After that, the connection is transmitted by the bolt body shear, the hole wall pressure bearing and the friction force between the plate contact surfaces. Finally, the shear failure of the rod body or the hole wall pressure bearing is taken as the ultimate shear state of the connection. In short, friction type high-strength bolts and pressure type high-strength bolts are actually the same type of bolt, it's just a matter of whether slip is considered in the design. Friction type high-strength bolts must never slide, as they do not bear shear forces. Once they slip, the design considers them to have reached a state of failure, which is technically mature; Pressure bearing high-strength bolts can slide and withstand shear forces, resulting in a failure equivalent to that of ordinary bolts (bolt shear failure or steel plate compression failure).
From a usage perspective, high-strength bolts are generally used to connect the main components of building structures. Ordinary bolts can be reused, while high-strength bolts cannot be reused. High strength bolts are generally used for permanent connections. High strength bolts are prestressed bolts. Friction type bolts apply the specified prestress with a torque wrench, while pressure type bolts unscrew the plum blossom head. Ordinary bolts have poor shear resistance and can be used in secondary structural areas. 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 classified as 8.8 grade and 10.9 grade, with 10.9 grade being the most common. 8.8 and 8.8S are of the same level. The stress performance and calculation methods of ordinary bolts and high-strength bolts are different. The force on high-strength bolts is first applied by applying a pre tension force P inside them, and then generating frictional resistance on the contact surface between the connected parts to bear external loads, while ordinary bolts directly bear external loads.
More specifically, high-strength bolt connections have the advantages of simple construction, good stress performance, replaceability, fatigue resistance, and no loosening under dynamic loads, making them a promising connection method. High strength bolts are tightened with a specially designed wrench to generate a huge and controlled pre tension on the bolt. Through the nut and gasket, the same amount of pre pressure is also applied to the connected parts. Under the action of preloading, a large frictional force will be generated along the surface of the connected component. Obviously, as long as the axial force is less than this frictional 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 connections rely on the frictional force between the contact surfaces of the connecting components to prevent mutual sliding. In order to provide sufficient frictional force on the contact surfaces, it is necessary to increase the clamping force of the components and increase the friction coefficient of the component contact surfaces. The clamping force between components is achieved by applying pre tension to the bolts, so the bolts must be made of high-strength steel, which is why it is called high-strength bolt connection. In high-strength bolted connections, the magnitude of the friction coefficient has a significant impact on the bearing capacity. Experiments have shown that the friction coefficient is mainly influenced by the form of the contact surface and the material of the component. In order to increase the friction coefficient of the contact surface, methods such as sandblasting and cleaning with a wire brush are often used during construction to treat the contact surface of components within the connection range. There are actually two types of high-strength bolts: friction type and pressure type. The criterion for friction type high-strength bolts to withstand shear force is that the shear force caused by the design load does not exceed the frictional force.