As a Railway Fastening System Supplier, share with you. There are many
reasons for the fracture of fastener bolts. In summary, the damage of general
bolts is caused by stress factor, fatigue, corrosion and hydrogen
1. Stress factor
Exceeding normal stress (super stress) is caused by any one or a combination of shear, stretching, bending, and compression.
Most designers first consider the combination of tensile load, pre-tightening force and additional practical load. The pre-tightening force is basically internal and static, and it compresses the joint components. The practical load is external, generally the cyclic (reciprocating) force exerted on the fastener.
The tensile load attempts to resist opening of the joined components. When these loads exceed the yield limit of the bolt, the bolt changes from elastic deformation to a plastic zone, resulting in permanent deformation of the bolt. Therefore, the original state cannot be restored when the external load is removed. For similar reasons, if the external load on the bolt exceeds its ultimate tensile strength, the bolt will break.
Bolt tightening is achieved by twisting with pre-tightening force. During installation, excessive torque leads to over-torque, and at the same time, the fastener is over-stressed and the axial tensile strength of the fastener is reduced. That is, the bolt that is continuously twisted and the same bolt that is directly stretched under tension are phased out. Than, the yield value is relatively low. In this way, the bolt may yield when the minimum tensile strength of the corresponding standard is not reached. A large torsion moment can increase the bolt pre-tightening force and reduce the joint slack. In order to increase the locking force, the pre-tightening force generally takes an upper limit. In this way, unless the number of differences between the yield strength and the ultimate tensile strength is small, the bolt will not yield due to torsion.
The shear load exerts a vertical force on the longitudinal axis of the bolt. Shear stress is divided into single shear stress and double shear stress. From empirical data, the ultimate single shear stress is about 65% of the ultimate tensile stress. Many designers prefer the shear load because it takes advantage of the tensile and shear strength of the bolt. It mainly acts like a pin and makes the sheared fasteners form a relatively simple connection. The disadvantage is the range of application of shear connections. Small and shear joints cannot be used often because they require more materials and space. We] know that the composition and accuracy of materials are also decisive. However, the material data for converting tensile stress into shear load is often not available.
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