高張力鋼溶接継手の余盛形状および組織と疲労強度

抄録

The present study was undertaken to determine the fatigue strength of weld joint of high tensile steel of 80 kg/mm² (≈ 110 ksi) level. The endurance limit of this steel was 46 kg/mm² obtained from repeated bending fatigue test.
The endurance limit was remarkably decreased to only about 13 kg/mm² due to welding.
The metallurgical structural change caused by welding seems not to be a dominant factor for the decrease in the fatigue strength. Because the endurance limit increased to 40 kg/mm² when the reinforcement was completely machind off after welding.
When the reinforcement was almost completely dressed, leaving the micro-defects on the surface of the plate, the endurance limit was again 40 kg/mm². Therefore, the micro-defects appearing at the toe of the reinforcement have little effect upon the endurance limit, although such defects were always the sites from which fatigue crack was initiated.
Macroscopic stress concentration due to reinforcement may be a major factor for the reduction of fatigue strength of the joint. However, fatigue test results on the simulated weld specimens, machined from the base metal, showed that the fatigue strength reduction factor was never higher than 3.0 in the present investigation.
It is concluded that the drastic reduction in fatigue strength of weld joint can not be completely explained by any one of factors alone but the strength reduction by the reinforcement must be intensified by the other factors such as micro-defects and metallurgical structural changes.
To determine the endurance limit of joint, an apparent fatigue strength reduction factor, Kf^w, is proposed as follows,
Kf^w=Kf⋅Cu⋅Cn⋅Cs⋅Cx
where Kf is the fatigue reduction factor due to reinforcement, Cu, Cn and Cs correction factors for micro-defects, notch sensitivity of material and structural changes, respectively. And Cx is also a correction factor for other unknown effects. The value of each factor except Cx was determined from the result of present investigation and the calculation of endurance limit was made. A good agreement was obtained between the calculated and observed endurance limits of weld joint.