Thickness effect on fatigue strength of welded joint improved by HFMI

Abstract

Fatigue strength decreases by thickness effect, therefore the improvement treatments of the fatigue strength are important as the structure consisting of the thick material. Post weld treatments are one of useful method to improve fatigue strength. However, the thickness effect on welded joint improved by post weld treatments is not clear because the experimental data is insufficient for quantitative evaluation. The object of this study is to evaluate thickness effect on the fatigue strength which is improved by post weld treatments to reflect it in fatigue design guideline; therefore in transverse no-load carrying cruciform joint which is one of the main welded joints of the ship structure, the improvement effect of the fatigue strength by post weld treatments is evaluated. In this report, the result of HFMI (High Frequency Mechanical Impact treatment) and Shot peening are evaluated with the result of the as-welded condition and conventional Burr grinding by nominal stress approach. In addition, the fatigue strength is evaluated using the equivalent stress approach by the modified MIL-HDBK-5 method that both the stress concentration of weld toe and the weld residual stress are separated from nominal stress. Good correlation between the fatigue test results was achieved using modified version of the equivalent stress approach defined in MILHDBK- 5. With the equivalent stress that was adopted in this study, both the stress range and the effect of maximum stress including the residual stress are considered. It is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally.