FCP Report No. 19

Estimation of the Fatigue Crack Initiation Life in Welds Using Low Cycle Fatigue Concepts


R. J. Mattos
F. V. Lawrence
Department of Metallurgical Engineering


An analytical model for calculating the fatigue crack initiation life of welds has been developed. In order to use the model, the stresses and strains at the critical location (toe of the weld) and a mechanics analysis to relate these local stresses and strains to the remotely applied stresses and strains are needed. Computation of the local stresses and strains was performed using Neuber’s equation, and the fatigue behavior of the material at the critical location was computer simulated using a rheological model. Verification of the model was accomplished by comparing fatigue crack initiation life predictions with data obtained by testing simulated weld specimens and by utilizing data from the literature for several notch geometries, materials and fatigue loading conditions. The results of these comparisons indicate that a very good estimate of the fatigue crack initiation life can be obtained if the material properties at the critical location are well known.

Based on the analytical model developed, an analysis of the factors influencing the fatigue crack initiation life of welds was made. The results of this analysis indicate that the fatigue crack initiation life of welds is influenced by the micro- and macrogeometry of the weld reinforcement, the residual stresses, and very markedly by the material properties. It also indicates that the fraction of the fatigue life spent in fatigue crack initiation is dependent on the material and on the fatigue regime under consideration.

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