The mechanical mechanism of anti-loosening failure in hexagon flange bolts under vibration is essentially a process of dynamic loads disrupting the preload maintenance capability of the threaded connection system. The core contradiction lies in the transmission of vibration energy through the structure to the bolted connection, causing periodic fluctuations in the normal pressure between the threads, ultimately leading to the failure of the frictional anti-loosening mechanism. This process involves the intersection of materials mechanics, tribology, and vibration theory, and can be analyzed from three dimensions: energy input, stress redistribution, and contact condition deterioration.
Vibration energy input is the initial trigger for anti-loosening failure. When the equipment is running, the alternating load generated by the vibration source is transmitted to the bolt through the connected parts, forming a dynamic shear force between the threads. This force, together with the bolt's axial preload, forms a composite stress field, subjecting the thread contact surface to periodic tensile and compressive forces. Compared to static loads, vibration loads have the characteristics of high frequency and low amplitude, continuously consuming the connection system's energy reserves below the material fatigue limit, accumulating damage for subsequent failure processes.
Stress redistribution leading to preload attenuation is the key pathway for anti-loosening failure. Under vibration, the difference in elastic modulus between the bolt and the connected parts can cause localized stress concentration. As the primary load-bearing area, the stress distribution on the flange face in contact with the connected parts exhibits dynamic changes: in the initial stage of vibration, the edge areas of the contact surface undergo micro-plastic deformation due to stress concentration; as vibration continues, the deformation area expands towards the center, leading to a reduction in the actual effective contact area. This stress redistribution directly weakens the flange face's lateral restraint capability on the bolts, causing the preload to gradually decrease under the combined action of axial and lateral components.
Deterioration of the contact condition is a direct manifestation of anti-loosening failure. Vibration-induced fretting wear alters the surface morphology of the threaded pair. Under alternating loads, micro-protrusions on the thread tooth surface fracture and detach, forming abrasive particles. These particles accumulate in the thread clearance, changing the friction coefficient of the contact surface, transforming static friction into dynamic friction; furthermore, they act as a "third body" in the friction process, accelerating the wear rate of the thread teeth. As the surface roughness decreases, the actual contact area further decreases, forming a vicious cycle of "wear - reduced contact area - decreased friction."
The unique structure of the flange face exacerbates the failure process of anti-loosening mechanisms. Compared to ordinary hexagonal bolts, hexagonal flange bolts improve static anti-loosening capabilities by increasing the diameter of the bearing surface. However, under vibration conditions, this design becomes a weak point for failure. A larger flange face diameter means a longer lever arm; under lateral vibration loads, the bending moment generated at the flange edge is significantly increased, making the contact surface edge area more susceptible to plastic deformation. Furthermore, the micro-gap between the flange face and the connected components creates a "pumping effect" during vibration, accelerating the intrusion of corrosive media and further weakening the frictional properties of the contact surface.
The coupling effect of material creep and thermal effects accelerates the failure process. Under continuous vibration, the bolt material undergoes dynamic creep, especially in high-temperature operating environments, where this effect is more pronounced. Material creep leads to increased bolt elongation, directly reducing the preload level. Simultaneously, the frictional heat generated by vibration raises the contact surface temperature, reducing material hardness and accelerating the wear rate. This thermo-mechanical coupling effect accelerates the failure of the bolted connection system, ultimately leading to the complete failure of the anti-loosening mechanism.
The failure of hexagon flange bolts under vibration is the result of multiple mechanical mechanisms. From energy input to stress redistribution and contact condition deterioration, each stage presents a critical failure trigger point. To improve their anti-loosening performance, systematic optimization is needed in various aspects, including material selection, structural design, and surface treatment, to construct an anti-loosening system that adapts to dynamic loads.
