The number on the bolt refers to the strength level. For example, the number 4.8 means that the strength level of the bolt is 4.8, and its shear stress is 4.8 GPa. According to regulations, bolts with a strength grade of 8.8 or higher are collectively called high-strength bolts, and the rest are collectively called ordinary bolts. General bolts use "XY" to indicate the strength, X*100=the tensile strength of this bolt, X*100*(Y/10)=the yield strength of this bolt (because according to the label: yield strength/tensile strength =Y/10). For example, grade 4.8, the tensile strength of this bolt is 400MPa; the yield strength is 400*8/10=320MPa.
Bolts that are made of high-strength steel or require a larger pre-tightening force can be called high-strength bolts. 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 mostly brittle fracture. High-strength bolts applied to ultra-high pressure equipment need to be prestressed in order to ensure the tightness of the container.
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