To fully consider intact initial imperfections, including welding initial deformation and residual stress, it is necessary to conduct a study on the plastic deformation distribution and ultimate strength of a box girder under different cyclic bending moments.

First, an isotropic material model and the Chaboche material model are chosen. Using the Python language, programs are developed to directly apply the initial deformation in finite element software. Subsequently, ABAQUS software is employed to conduct nonlinear finite element numerical simulations of the ultimate strength of a box girder under different cyclic bending moment patterns. The analysis considers the influence of welding initial deformation, residual stress, material hardening and the Bauschinger effect.

The results show that under unidirectional cyclic bending moments, the bending stiffness and ultimate strength of the box girder decrease continuously as the number of cycles increases. The plastic deformation extends from the upper deck of the box girder to the side plate region. The plastic distribution region of the box girder with welding initial deformation and residual stress is wider in the ultimate state. Compared to a single loading and considering only the initial deformation, after three bidirectional cycles, the ultimate strength of the box girder decreases by 10.44% to 15.15%. Considering intact initial imperfections, the ultimate strength decreases by 8.41% to 14.50%.

The ultimate strength of a box girder considering intact initial imperfections under different cyclic bending moments decreases more moderately. The obtained results can serve as a reference for the study of the ultimate strength of box girders under different cyclic bending moments.

In order to improve the accuracy of nonlinear numerical simulation of the ultimate load-bearing behavior of a hull stiffened plate, the effects of ideal elastoplastic, isotropic hardening and cyclic plastic Chaboche material models on the plastic yield distribution, compression and tensile ultimate strength of stiffened plates in their ultimate state are studied.

For a stiffened plate of the same size, ANSYS software is used to carried out non-linear finite element numerical simulation of ultimate bearing performance under axial cyclic compression and cyclic compression-tension loads.

The results show that different material properties have a significant impact on the ultimate bearing capacity of stiffened plates and the plastic yield distribution in the ultimate state. When carrying out nonlinear finite element numerical simulation of the ultimate bearing behavior of a hull stiffened plate, it is necessary to select the appropriate material model according to different load forms.

The results of this study can provide valuable references for further research on the ultimate strength characteristics and cumulative plastic failure mechanisms of hull structures under cyclic loading.