1986 Volume 4 Issue 4 Pages 722-728
X-ray stress measurement, calculation of stress concentration factor by FEM, fatigue test, etc. were performed to confirm the effects of weld shape and residual stress on fatigue strength of mild steel non-load-carrying cruciform fillet welded joint.
The results obtained are summarized as follows:
1) A relief of residual stress at the toe by cutting off the large size cruciform fillet welded joint specimen to the small size specimen was about 40%, but the fatigue strengths of both cutting off small size specimen and as welded same size specimen were almost the same. Such a degree of relieving of residual stress by cutting off, therefore, can be considered to have no effect on the fatigue strength.
2) By FEM analysis in the case of axial tension load, the following became clear: Stress did not flow into the rib (cruciform plates) outside fillets, and the stress concentration factor was less than 1.0 constantly at the weld root (tip of slit parallel to the direction of axial load), and on the other hand, was 1.7-2.3 at the weld toe.
3) Using the stress concentration factor in profile of weld toe fatigue fractured pracitcally, and assuming the failure probability, it was found that the fatigue strength of mild steel cruciform fillet welded joint at 2 X 106 cycles could secure about 118MPa {12 kgf/mm2} at failure probability p=5%.
4) It is possible to consider that a fatigue life of peened specimen is improved owing to compressive residual stress and work hardeness (increase of yield point), and decreased owing to increase of surface roughness (5-8 times that of before treatment; Rmax=1518 jem) caused by peening and that the synthetic effect of the former two exceeds the negative effect of the latter, and fatigue ilfe is improved after all.
The effect of compressive residual stress decreased with increasing applied stress, and became uncertain at the applied stress over yield point.
5) By re-peening before or after a initiation of small crack (1, is about 0.5 mm) fatigue strength was improved additionally. We may expect improving fatigue strength in the practical welded structure, therefore, by a practical application of re-peening.
