Crack propagation-based assessment of the endurance limits of welded joints

Abstract: The welded joint and heat-affected zone are characterized by considerable heterogeneity of mechanical properties, diameter of grain, as well as by the existence of different stress concentrators which provoke the appearance of fatigue cracks. It is known, that the main factor that influences on the possibility of the appearance and the propagation rate of microcracks is the level of the stress condition, as well as the structural and mechanical discontinuity of the metal of welded joints. The level of the stress condition largely depends on residual welding stress and local physical stress concentrators.

The article presents the authors’ model and approach to the assessment of the effective coefficients of the stress concentrations for welded joints having the defects like undercuts, reeds, and other stress concentrators caused by propagating cracks that are generated by the stationary variable load. The proposed model is based on Kirsch solution for circular hole in the conditions of plane stress state under the influence of uniaxial load. The results of the model have been checked experimentally on carbon and ferrite-pearlite steels. The values of the endurance limits, the propagation of cracks depending on the size of the defect being the criterion, have been obtained. The sizes of the cracks that appear in the area of stress concentration and do not propagate in relation to the diameter of the circular hole in the plate have been calculated. Unlike the model presented in G.M. Charzynski’s monograph this one gives an asymptotic approximation of the endurance limit to the value  with the macroconcentrator stress increase in the shape of a hole. The model is built for infinite plate and it takes no account of change in the stress field conditioned by limited sizes. The presented model does not take into consideration the correction of Kirsch solution as a necessity under changes of the stress area caused by the appearance of cracks in the area of maximum stress concentration. The model is based on the consideration of the area with higher stresses adjacent the edge of the hole as the area gaining the most intense damages near the stress concentrator under the action of cyclic loads.