Theoretical Development of Lower Bound S-N Fatigue Curves

摘要

Fatigue strength of welded structures is typically estimated through several approaches, including experimental S-N curves, the hot spot stress approach, and fracture mechanics. Recent work on the local stress field singularity of the weld toe has provided analytical expressions for the stress intensity factor of a crack emanating from the weld toe. Invoking the Paris law for fatigue crack growth together with the stress intensity factor expressions allows fatigue life to be evaluated in closed form. The fracture mechanics solution takes the same power-law form as the S-N curves and produces accurate lower bound estimates to the experimental fatigue data of stiffeners and attachments. The results show a strong relationship between the fracture mechanics, S-N curve, and hot spot approaches to fatigue that is based on two or more parameters from the local weld toe stress field. The two parameters may be tabulated as functions of weldment geometry and loading conditions for improved fatigue design and assessment calculations.