5. Conclusions
The process by which a box-type steel member subjected to strong and repeated loading undergoes failure and the factors relevant to damage were used to develop a new damage assessment method using a nonlinear analysis. The major conclusions of this study are as follows.
1. By using the dimensionless local stress-ratio–strain-ratio envelope curve, a new damage index was proposed. This damage index is an area ratio of the stress ratio and the strain ratio corresponding to the cumulative plastic strain of each load step and the ultimate load state, and the cumulative damage to the member can be taken into account when evaluating the damage capability.
2. The results of a damage assessment using the damage index for a box-type steel member indicated that the position where local buckling occurred was the most severely damaged. This indicates that damage is closely related to the local buckling and plastic strain.
3. The boundary conditions of the model in this study were set as one end fixed and one hinged end. For this reason, local buckling, when occurring concentrically, occurred at lower positions when the strength of the steel member is higher. On the other hand, when repetitive loading is applied, buckling occurred around the edge where stress could be easily concentrated owing to cumulative plastic deformation, regardless of the strength of the steel member.
4. Although the strength of Posten 80, which is the highly tensile, is 1.5 times higher than that of SM 570, the strain restoration of Posten 80 was not lower than that of SM 570.
