Abstract: Different from the lab-scale fatigue coupon, the welded structure has its unique fatigue characteristics. The fatigue life of a welded structure is dominated by the fatigue crack propagation life, while the fatigue initiation period is relatively short. Furthermore, the size of a welded structure also has some influence on its fatigue life, and compared with other factors, the applied stress range serves as the major driven force for fatigue failure. The finite element-based mesh-insensitive traction structural stress can efficiently capture the stress state at the weld location. The family of S-N curves documented in the international standard can be unified into a single Master S-N curve once the equivalent traction stress parameter is used, which is calculated based on traction stress and taking account of the fatigue crack growth behavior. The Master S-N curve can be utilized to capture the fatigue behavior of joints with different geometry and thickness. The method won the 2003 SAE "Fatigue Challenge" competition and has been adopted in the ASME FFS-1 standard. The structural strain method is developed upon the traction structural stress method, which further correlated the high- and low-cycle fatigue data of weldment made from different base metal. The method has been widely used for fatigue design and optimization, fatigue life prediction for the complex welded structure in railway transportation, orthotropic bridge, and heavy equipment, etc. The method can also quantitively analyze the effect of weld distortion, weld size, weld penetration of fatigue life.

Key words: welded structure, fatigue design, master S/E-N curve, traction structural stress/strain