Abstract
At present, research on the loosening of bolts under transverse excitation and their fatigue under axial excitation has been relatively mature, but research on the competitive relationship and failure characteristics between loosening and fatigue of bolts under transverse and axial composite excitation is still insufficient. Therefore, a method to accurately determine the failure types of bolts is proposed in this study by conducting a competitive failure test of loosening and fatigue under composite excitation. According to this method, the failure types of bolts can be distinguished. The analysis results reveal that there is an obvious competitive failure relationship between the loosening and fatigue of bolts, and the failure mode is mainly affected by the ratio of the transverse and axial loads (ξ). There is a critical ξ of bolt-loosening or fatigue failure, and the critical ξ is an inherent property of the bolt and is unrelated to the load. The critical ξ of 8.8 grade M8 × 1.25 × 70 high-strength bolts under composite excitation is obtained as 0.075 mm/kN. The failure mode of bolts under composite excitation can be predicted based on the critical ξ.
Highlights
Bolt joints are widely used as fasteners in mechanical equipment, rail vehicles, automobiles, ships, the aerospace industry, and other fields owing to their simple structure, convenient disassembly, high efficiency, low cost, and stable and reliable fastening
It can be seen that the presence of tensile stress under a composite load will restrain the degree of wear from shear stress; that is, the degree of bolt loosening is suppressed by the axial load. is proves the conclusion drawn in Figure 5(b): a small axial load restrains bolt loosening, and there is a critical value of axial load that restrains failure under composite excitation
By conducting a competitive failure test of loosening and fatigue of bolts under composite excitation, the mechanism of the competitive failure of loosening and fatigue, the failure law, and the failure prediction method were revealed. e following conclusions can be drawn based on the results obtained: (1) ere is a severe stress concentration at the transition fillet of the bolt head and the first thread of the bolt working thread under a composite cyclic load, which makes the two positions most likely to fracture
Summary
Bolt joints are widely used as fasteners in mechanical equipment, rail vehicles, automobiles, ships, the aerospace industry, and other fields owing to their simple structure, convenient disassembly, high efficiency, low cost, and stable and reliable fastening. Zhang [10] studied the influence of the pretightening force, transverse amplitude, and vibration frequency on bolt loosening by conducting bolt-loosening tests under transverse loads. Jiang et al [16] studied the influence of displacement, velocity, and acceleration amplitude on bolt-loosening life through bolt transverse vibration tests and found that the transverse displacement amplitude is the main factor affecting bolt loosening. Yang et al [17] studied the mechanism of bolt-loosening accumulation by conducting bolt high-low and low-high transverse variable-amplitude vibration tests and established a linear accumulation model of bolt loosening similar to the material fatigue linear accumulation criterion. Based on the material damage accumulation criterion, Shi [22] studied the influence of the pretightening force and thread structure on the fatigue life of bolts and determined the best pretightening torque range of high-strength bolts and the best root radius of threads. The bolt fracture analysis revealed the competitive failure mechanism of the bolt, providing a useful reference for engineering applications
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