APPLICATION OF A NEW MODEL FOR FATIGUE THRESHOLD IN A STRUCTURAL-STEEL WELDMENT

A new model for fatigue threshold has been proposed. The model takes i nto consideration the influence of material strength. grain size, and load ratio on fatigue threshold. Fatigue crack growth behavior of a st ructural steel weldment (ASTM Grade A 514) was investigated to examine the applicability of this model. The investigation also examined the influence of welding procedure [submerged arc welding (SMAW) versus me tal inert gas welding (MIG)] on the resultant microstructure and the n ear threshold fatigue crack growth behavior of the material in room te mperature ambient atmosphere. Compact tension specimens were prepared from ASTM Grade A 514 structural steel weldment in such a way that cra cks propagated through the heat affected zone (HAZ), base metal and we ldments. This arrangement permitted the influence of HAZ on fatigue cr ack growth rate to be determined in both linear and threshold regions of the Paris curve in room temperature ambient atmosphere. Welding was accomplished by two different techniques, i.e. SMAW and MIG processes . The results of the present investigation demonstrated that the near threshold fatigue crack growth rate was lowest and fatigue threshold w as highest when the crack propagated through the weldment. The fatigue threshold values predicted by the model were in good agreement with t he experimentally determined values. The MIG welding process was found to be beneficial compared to the SMAW process as far as the fatigue t hreshold is concerned for weldment.