Failure Prevention Chapter 8A MECH 5602 Weld Fatigue Life Improvement Techniques _____________________________________________________________________________________________ Chapter 8A - WLIT.doc Page 1 of 34 2006 CHAPTER 8WELD FATIGUE LIFE IMPROVEMENT TECHNIQUES BackgroundThe total fatigue life of a component subjected to cyclic loading can be described by two phases known as initiation life and propagation life. For smooth machined components the init iation phase is the most important because the majority of the life is spent in the initiation of small cracks and in relation to total life the propagation p hase is very short. However, in the case of welded components it is generally accepted that crack like defects exist at the welds from the welding process and therefore the initiation phase is insignifican t. As a result the bulk of th e fatigue life in welded comp onents is spent in propagation of thes e crack-life defects until final fail ure occurs. Experimental result s on joints with full penetration transvers e welds have shown that abo ut 70 % of the fatigue life is sp ent in propagation of weld toe cracks from an initial s ize of 0.5 mm to final failure, Bell et al (1987), Vosikovsky et al (198 5) and Yee et al, (1990). To illustrate the effect of welding on the fatigue life of structures the co mparison of the total life, (ie, initiation + propagation) of steel plate in the plain, notched and welded condition is shown in Figure 8.1. The large decrease in fatigue life is attributable to welding and the resulting inclusions and high stress concentration at the weld toe region. Various types of weld defects are shown in Figure 8.2. In welded components fatigue micro-cracks initiate from the defects along the weld toe where they progressively, coales ce as they develop into fewer cracks un til a dominate crack grows un der cyclic loading to final failure. The fatigue crack initiates at the weld toe from defects such as slag inclusions, weld toe undercuts, weld run ends, and sharp weld toe angles. In addition the welding process result s in high levels of tensile residual stresses in the weld region. This also contributes to the reduction in fatigue life of welded components and structures. Figure 8.1 Comparison Fatigue Strength of Plain Steel Plate, Notched Plate and Plate with a welded Attachment, (Haagenson 1985)Figure 8.2 Effect of Tensile Strength on Fatigue Strength of Steel, (Haagenson 1985)