Research on Fatigue Degradation Mechanism and Service Life of Orthotropic Steel Box Girder Bridge Decks with Open Ribs in Corrosive Environments

To investigate the fatigue performance of orthotropic steel bridge decks with open ribs in coastal areas， fracture mechanics analysis and finite element simulation were employed. Focusing on the deck-T rib weld detail， this study adopted a simplified corrosion model. The ABAQUS and FRANC3D software tools were combined to examine the distribution patterns of stress intensity factor amplitudes throughout the entire crack propagation process， including crack initiation within the corrosion pit， crossing over the pit edge， and propagation outside the pit. This study simulated the propagation paths， morphologies， and rates of initial cracks under different corrosion states. Furthermore， it analyzed the influence of varying deck thicknesses and corrosion pit sizes on the fatigue performance of the deck-T rib details in a corrosive environment. The results indicated that the influence of corrosion pits on the stress intensity factor amplitude at the crack tip exhibited distinct stages. The effect was most severe when the crack was inside the pit or had just crossed over the pit edge. During this brief period， the stress intensity factor amplitude at the ends of the crack front increased sharply to 2.96 times the original amplitude. Once the crack moved away from the pit， the influence rapidly diminished. The fatigue life of the corroded model was significantly lower than that of the uncorroded model. When the crack propagated to the critical size （half the deck thickness）， the fatigue life of the 50-year corroded model （3.403 million cycles） was only about 51% of the uncorroded model’s life （6.660 million cycles）. The life of the corroded model （3.689 million cycles） when the crack reached a depth of 8 mm was also significantly lower than that of the uncorroded model when it reached the critical depth， indicating that corrosion markedly accelerated crack growth rates. Increasing the deck thickness substantially improved fatigue performance. When the deck thickness increased from 12 mm to 20 mm， the fatigue life to reach the critical crack size increased dramatically by 1866.5%， and the average crack propagation life increased by 968.6%. This strongly demonstrated that increasing deck thickness effectively reduced stress levels at the weld and was the most effective structural measure for enhancing fatigue resistance. The detrimental effect of corrosion on fatigue life was primarily concentrated in the initial corrosion stage. When the corrosion duration increased from 0 to 30 years， the critical point fatigue life decreased sharply by 47.1%， and the average crack propagation life decreased by 40.9%. In contrast， when the corrosion duration increased from 30 to 100 years， the critical point fatigue life decreased by only an additional 4.0%， and the average crack propagation life showed a slight increase （+16.5%， which is possibly related to a weakening of stress concentration at the pit in later stages）. This indicated that the presence or absence of corrosion had a significant impact on the crack propagation life of the deck-rib fatigue details， while the duration of corrosion had a relatively minor effect on it.