Failure mechanism of unequal parameters metal bellows under repeated bending process

The metal bellows used for flexible connection of oxygen lance in metallurgical converter compensate the displacement of pipeline by its bending deformation. Due to the waveform structure, the adjacent waves at the fixed end of the metal bellows are squeezed together, leading to premature fracture. In order to explore the influence of waveform structure parameters on the failure mechanism of the metal bellows, a U-shaped metal bellows with different waveform structure parameters was designed in this paper. The ultimate repeated bending process of the metal bellows was numerically simulated by ABAQUS finite element software. The influence law of waveform structure parameters on equivalent stress, equivalent strain and ultimate bending angular displacement of the metal bellows were obtained. Then, the repeated bending fatigue life of the bellows was tested on the fatigue testing machine, and the influence law of the waveform structure parameters on the fatigue life of the bellows under ultimate bending angular displacement amplitudes was obtained. The experimental results verify the accuracy of the finite element simulation. Finally, X-ray diffraction peaks confirmed the proved that the plasticity accumulation at the wave trough of the equal parameters metal bellows (EPMB) is large, so the hardness value is large. The micro morphology of the fracture and the surrounding area was analyzed by scanning electron microscope (SEM), and the crack propagation and the ultimate repeated bending failure mechanism of the bellows with different waveform structures was revealed. The initial cracks appear in the inner and outer surfaces of the wave trough, distribute along the circumferential direction, and propagate from the inner and outer surfaces to the central layer. Among them, the fracture morphology of the EPMB is mainly composed of fatigue fracture and plastic fracture. However, the fracture morphology of the unequal parameters metal bellows (Un-EPMB) is dominated by fatigue fracture. The results show that the fatigue life and bending fracture resistance of metal bellows can be improved by designing waveform structure with unequal parameters.