Fatigue failures in weldments are always initiated at notches or stress raisers. Notches include changes in section due to weld geometry or reinforcement, surface ripples, undercuts, and lack of penetration. In addition, welds may be subject to inclusions, porosity, shrinkage cracks, and lack of fusion. The fatigue life of a weldment consists of three phases: (1) development of a macroscopic crack (called crack initiation for brevity), (2) propagation of the crack to a critical size, and (3) exceeding of the residual strength of the cracked weldment. The relative magnitudes of the three phases depend on notch acuity, material, stress level, structural stiffness, and environment. Various fatigue life assessment models have been developed to describe one or more of these phases. The component test model and three analytical models -- stress-life, local strain, and linear elastic fracture mechanics -- are described in this paper. Also reviewed are the uses of these models as bases for typical fatigue design criteria and the extent to which they are or can be used to evaluate the effects of weld discontinuities.
"State of the Art of Design and Analytical Models for Fatigue of Weldments" Proceedings of the International Conference on Fitness for Purpose in Welded Construction, May 18-20, 1982, American Welding Society, 1982, pp. 41-55.