The decreased fatigue capacity of welded components with increasing plate thickness, known as the size effect, is a well-known phenomenon in most fatigue design standards. The size effect links directly to the probabilistic nature of fatigue failure, where the extent of stress concentrations and stress gradients through the thickness govern the fatigue capacity of the joint. Fatigue design standards and recommendations address the thickness effect by reducing the S-N curve for joints thicker than the reference thickness value, where the increase in fatigue strength is often not considered due to the thinness effect.
The probabilistic nature of fatigue failure for welded joints is studied here using the weakest link modelling approach based on the stress distribution within the body of the joint. The modelling approach is evaluated and compared with the effective notch stress method for literature fatigue data on non-load carrying welded joints. The differences and similarities between the probabilistic and effective notch stress methods give a good insight into how the fatigue strength of welded joints is affected by the size effect.