Fatigue Crack Propagation Study of Ω-Shaped Bellows Expansion Joint Based on Fracture Mechanics

This study evaluates the fatigue life of Ω-shaped bellows expansion joints subjected to axial cyclic displacement, in accordance with relevant standards. Using fracture mechanics theory, numerical simulations were conducted to analyze fatigue crack propagation in joints with initial small cracks, and their fatigue life was calculated. The proposed small semi-elliptical surface crack model reveals that stress intensity factors at crack tips escalate sharply with cyclic loading cycles. However, the crack propagates significantly faster in the span direction than in the depth direction, leading to gradual flattening of the crack morphology. For the analyzed Ω-shaped bellows expansion joints and applied loads, the fatigue life of defect-free joints calculated according to standards exceeds the fracture mechanics-predicted fatigue life of joints with initial cracks by approximately 94%. This indicates that the fatigue life of Ω-shaped bellows expansion joints calculated by standards may be overestimated, and design based solely on standards cannot ensure failure-free operation due to crack propagation.