Fatigue Strength Evaluation Method Using Local and Structural Stress Concentration Factors for Weld Toe of Welded Joint

Abstract

Stress concentration at weld toe is one of the major factors influencing fatigue strength of welded structures. The stress concentration factor (SCF) Kt is expressed by Kt =Kw·Ks, where Kw is the local SCF depending upon the local geometry of a weld, and Ks is the structural SCF determined by the joint type of a structural member. The structural SCF is currently taken into account by some of fatigue design guidances. While the local SCF and Kt are considered as unknown factors and to be involved in the basic S-N curves. For a consistent fatigue evaluation of various types of joints, it is necessary to establish a unified method dealing with both local and structural SCFs quantitatively.In this study, a fatigue evaluation method has been studied based on the following phenomenon ;1. Since local stress concentration is principally caused by a very small radius of weld toe, a high stress region is quite limited in a small area. Therefore, the local SCF affects fatigue crack initiation and growth of a very small crack. This means that even if Kw is as large as infinity, the fatigue strength is still greater than that for a crack propagation life. Namely, as Kw becomes large, the fatigue strength decreases from that for a small Kw to that for crack propagation life.2. Whereas the affecting area of structural stress concentration is relatively large, the structural SCF influences not only crack initiation but also crack propagation until the crack becomes large. Thus, the Ks has the same effect as an increase in nominal applied stress.Eventually, a unified equation in which the local and structural SCFs are integrated has been proposed for fatigue strength estimation of welded structural members.