Abstract: The influence of hot spot stress gradient on fatigue assessment of welded joints is investigated. Based on the critical distance theory proposed by Taylor, and which is combined with finite element calculation, the fatigue notch coefficients of both cruciform joints and T-joints are obtained. They are used to predict the variance of fatigue strengths of main plate subjected to tension-compression loading and bending loading respectively, and which agrees with the test result. The effect of hot-spot stress gradient on fatigue behavior is considered by introducing effective hot-spot stress, which is determined with the reduction of the component of bending stress involved. Cruciform joints and right-angle shaped joints of specified geometrical size are examined. It is shown that fatigue notch coefficients of these joints are irrespective to both joint type and loading mode in main plate if the control stress is effective hot-spot stress, also the thickness effect of main plate can be estimated by existing power law type relations. In terms of the concept of effective hot-spot stress, the re-analysis of S-N curves for several types of welded joints is carried out. It is found that an identical S-N curve can be drawn to deal relevant fatigue test data with the scattering band becoming narrow and standard deviation getting smaller.

Key words: Fatigue assessment, Fatigue notch coefficient, Hot-spot stress, Stress gradient, Welded joint