Fatigue Strength of Butt Welded Joints Having Angular Distortion

Abstract

In structures subjected to cyclic loading, fatigue damage is usually concentrated at location with geometric discontinuity such as butt welded joints, where stress and strain concentrations are present.In this paper, experimental results of fatigue tests on butt welded specimen are discussed and also a method to estimate the fatigue strength of butt welded joints with arbitrary amount of angular distortion is proposed, which showed a good agreement with the test results.The tests were performed on butt welded joint specimens prepared from two types of high tensile strength steels, namely WES HW 50 and HW 70. Butt welded joints with various amount of angular distortion were prepared to investigate its effect on fatigue strength. Effects of span and overstressing prior to fatigue were also studied. From these investigations it was found that, regardless of the amount of angular distortion, span or overstressing, the strain range measured at a point 5 mm from the toe of the weld, denoted εR has a good correlation with the failure life Nf.Stress and strain relation at a joint with angular distortion was previously proposed by Yada et al. This method was modified here to obtain strain behavior for repeated loading. The calculated strain range and the experimentally obtained strain range εR were in good agreement for any of the test conditions in specimens with angular distortion.It was concluded that, knowing the εR-Nf curve for butt weld joints of a given steel, fatigue life for joints with arbitrary amount of angular distortion can be obtained through calculation.Overstressing was effective in increasing the fatigue strength of butt welded joints with angular distortion above a certain stress level, but no apparent difference was seen for all stress levels in practically straight butt welded joints. The effect of overstressing was quantitatively explained, which is in accordance with the experimental results.It is pointed out a typical expression in present standards for allowable amount of angular distortion which is in terms of “initial amount of distortion over a certain span” may require reconsideration.