A total of 4 pieces of bolt fracture specimens collected at the macroscopic analysis of bolt fractures have obvious fatigue failure characteristics. After direct observation and scanning electron microscopy low magnification, and sawing the metallographic sample from the broken bolt, after grinding and polishing, it was analyzed under the metallographic microscope: A region is the crack source region, and the light is flat. The macro field of B is a crack extension area, the structure is fine and flat, with a small reflective surface, and the section has a clear beach corrugated pattern. The C region is the final fracture zone with rough texture and tearing. It can be seen that the broken section of the bolt broken specimen has obvious macroscopic characteristics of fatigue fracture. Microscopic analysis of bolt fractures The bolt fracture samples 1, 2, 3, and 4 were further observed by scanning microscopy. A region (fatigue crack source region): the cross-section pattern was basically polished due to repeated friction of the section; B region (fatigue) Crack propagation zone): The cross-section exhibits quasi-cleavage morphology, and obvious fatigue fringes are observed. In addition, obvious extrusion marks are observed in this area, which indicates that the bolt has undergone multiple extrusions before fracture. Area C (final fracture zone): The fracture morphology is a quasi-cleavage dimple, and the bolt fracture sample has the microscopic characteristics of fatigue fracture. It can be seen from the fracture analysis that the fatigue area of ​​the bolt fracture is small, the bolt strength is insufficient, and the bolt fracture belongs to the fatigue fracture of the pull and bend combination.
Vibration test of the sidewalk system In order to find out the cause of the breakage of the U-bolt, a dynamic test was carried out on the 1.5 m wide sidewalk system to record the acceleration change on the sidewalk when the train crosses the bridge. From the corresponding relationship of the acceleration waveforms, the power amplification factor varies from several times to ten times. It can be seen that in the U-bolt design, the static strength design method is used for stress verification, and the power amplification effect of the bracket itself is neglected. It is a problematic calculation method and one of the main causes of the bolt breakage on site. Due to the complicated coupling condition of the bracket at the bolt, the bolt is sometimes loosely connected, and the rigidity of the bracket angle steel is small, which may cause the bolt to withstand the tensile force and the shear force, and also bear the extremely dangerous bending effect. This is extremely unfavorable for the force of the bolt, and may be an important cause of forming a bolt crack source and causing the bolt to break. Fracture analysis of U-bolt The railway bridge pedestrian walkway system exhibits obvious dynamic response characteristics under the dynamic load excitation of the bridge train, and has a large power amplification effect. Fatigue fracture is the main form of failure of U-bolts under alternating loads. The fatigue failure of bolts under cyclic alternating stress is essentially different from the strength failure under static stress. The static strength failure is caused by excessive residual deformation in the dangerous section of the bolt. Fatigue failure is due to the existence of local high stress regions in the bolts. The weaker grains form microcracks under the action of alternating stress, and then develop into macroscopic cracks, and the cracks continue to expand and eventually become brittle. In the calculation of static strength, the material strength indicators used are the yield limit and the strength limit.
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