According to the studies on the ductile strength of cruciform welded joints, it is known that the fracture strength of the joints with comparatively small amount of gaps, does not decrease in comparison with that of the joints with no gaps, because of the increase of penetration, which is roughly proportional to the increase of amount of gap.
However, such joints are frequently subjected to fatigue failure, therefore the effect of root gaps on low cycle fatigue strength of cruciform joints is investigated.
The main conclusions obtained are as follows.
(1) At the roots of the joints, initial microcracks are observed occasionally. The average length and the occurrence percentage of these cracks become minimum when amount of initial gaps before welding is about 2 mm.
(2) As for the fillet welded joints used in this study, the penetrations are approximately equal to the amount of initial gaps before welding.
(3) It is shown that the crack initiation lives can be neglected, therefore, the fatigue lives of the joints are almost regarded as the crack propagation lives.
(4) It is confirmed that the well known relationship between the crack propagation rate, dl/dN and the stress intensity factor range, Δk; dl/dN=c(Δk)
m are applied well to the fatigue cracks advanced toward the right angle to the loading direction.
(5) The fatigue strength of the joints with actual gaps below 3 mm after welding does not decrease in comparison with that of the joints with no gaps.
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