The purpose of this study is to find the reason for decreasing fatigue strength (N=2×10
6) of welded joint of structural steels, especially of high strength steels of which fatigue strength (N=2×10
6) is only a little greater than that mild steel, in spite of the excellence in other mechanical properties.
It is considered that there are the following factors which reduce the fatigue strength (N=2×10
6); (1) notch sensitivity of stress concentration due to reinforcement and welded defects, (2) metallurgical discontinuity and (3) the resistance to crack propagation in case of notched specimens like welded joint.
In order to clarify how these factors affect the fatigue strength of high strength welded joint, fatigue test was made using the specimens notched at base metal, heat affected zone, bonded part and deposit metal of mild steel (SS41) and 60 kg/mm
2 high strength steel (HT60) with various stress concentration factors α (α=1.00, 1.16, 1.57, 1.93, 2.56, 3.90).
In case of sharp notched specimen, non-propagating crack is found at notch root, even though it might be stressed under fatigue limit, and it is possible to distinguish two fatigue limits: one is the stress to limit initiation of crack and the other is the stress to limit propagation of non-propagating crack. Therefore, notch sensitivity for crack initiation and propagation can be discussed of two steels and their welded parts, by comparing these fatigue limits with those of plain specimens.
From these test results, the following charactaristics were known:
I) Notch Sensitivity for Crack Initiation.
1) Base Metal: SS41<HT60
2) Heat Affected Zone: SS41<HT60 (α is small), SS41-HT60 (α is large).
3) Bonded Part: SS41<HT60 (α<2.3), SS41<HT60 (α<2.3)
4) Deposit Metal: SS41<HT60 (α is small), SS41-HT60 (α is large).
II) Notch Sensitivity for Crack Propagation.
Base Metal and Welded Parts: SS41<HT60. Fatigue limits for crack propagation are 12.0 to 13.5 kg/mm
2.
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