On 01/18/2022, the XXX rail transit special bridge boom broke.

Failure analysis process:

Evaluation of product quality to determine whether the person responsible for the accident was the manufacturer of the product: analysis of chemical elements, analysis of mechanical properties, measurement of geometrical and dimensional parameters;

Study the failure mechanism, simulation and analysis of the stress situation, to determine whether the person responsible for the accident is the design unit: macro-micro-morphological analysis, microstructure analysis, focus on the fracture morphology characteristics, thread root stress-strain field analysis.

According to the test results of the chemical composition analysis of the material, the material meets the requirements of the specification GB/T 3077-2015.

According to the mechanical properties test results, the tensile properties of the sample sent for inspection meet the requirements of the specification "YB/T 036.7-1992", and the impact properties and hardness meet the requirements of the specification "YB/T 036.7-1992".

According to the results of metallographic organization analysis, the fork lug anchor cup intergranular for precipitated ferrite, the other for tempered quartzite, not quite normal. Fork ear cracks at the microscopic metallographic tempered quartzite, the organization state is normal.

According to the results of thread size and surface roughness analysis, the thread size and surface roughness are in accordance with the standard, but the surface roughness is high, and the preload conversion and connection effect are affected.

The results of fracture morphology analysis and dislocation grouping analysis are consistent with the characteristics of micro-motion fatigue fracture and unconventional fatigue fracture.

The maximum equivalent force is located in the subsurface area of the root of the first turn of the engaged thread (crack initiation location).

Main findings

The screw fracture mechanism is (tension bending) micro-motion fatigue fracture.

The screw is subjected to abnormally large additional bending moments and vertical loads, and the large thread root stress and its high amplitude are the main causes of fatigue fracture of the boom.

It is recommended to optimize the screw design (improve the design criteria) to reduce the constraint of the short screw to a greater extent and to reduce the additional bending moment on the screw. Increase the fatigue life of the screw by modifying the thread type, optimizing the thread geometry (e.g. increasing the diameter of the transition arc) and surface treatment to reduce the contact stresses at the root of the thread. Increase screw surface roughness design requirements.