Improvement Effect of Fatigue Strength by HFMI Treatment under the Negative Stress Ratio Condition

Abstract

Fatigue tests under the condition of stress ratio ＝ -0.3 are conducted for three types of welded joints (as-weld, grinding treatment, and HFMI (High Frequency Mechanical Impact) treatment.), and the fatigue strength of these welded joints are obtained experimentally. By comparing the present results with the results by Iwata et al., the fatigue strength improvement effect of HFMI treatment under the condition of negative stress ratio is discussed. The stress concentration factor of the HFMI-treated welded joint increases as the plate thickness increases, and the weld toe may be yielded in compression when the plate thickness exceeds 40 mm. The fatigue strength of the HFMI-treated welded joint decreases when the plate thickness exceeds 40 mm. However, even when the weld toe of the HFMItreated welded joints is compressively yielded, a significant decrease in fatigue strength is not observed. In addition, the modified MILHDBK- 5 method is applied to all experimental data, which includes differences in stress ratio. By the equivalent stress based on the modified MIL-HDBK-5 method, a good correlation between the fatigue test results is achieved regardless of joint type, plate thickness, and stress ratio.