Fatigue properties of 9% Ni steel and its welded joint were investigated at room and liquid nitrogen temperatures using notched and unnotched specimens.
The fatigue strengths of unnotched base metal and welded joint specimens at liquid nitrogen temperature were higher than those at room temperature except for the welded joint specimens broken at the weld bond region, while the notched strengths at liquid nitrogen temperature coincided well with those at room temperature for both the base metal and the weld bond specimens. The extended life of the unnotched specimens at low temperatures may be caused by retarded crack initiation. In the notched specimens, however, such extension of the crack initiation life seems to be cancelled by acceleration in crack propagation rate, so that the final fracture lives at both temperatures appear to be the same.
The effect of mean stress superposition was more distinct in the notched specimens than in the unnotched ones at both room and liquid nitrogen temperatures. This phenomenon may be explained by the fact that the mean stress increased crack propagation rate but gave no effect on crack initiation life.
The fatigue life was not affected much by stress variation in all the cases tested, and the results of the varying stress amplitude tests agreed rather well with those of constant amplitude tests.
The clearcut effect of temperature change during the test was observed. The change from low to high temperatures gave the cumulative cyclic ratio larger than unity, though the reversed temperature sequence gave the ratio of less than unity. This tendency is attributable to late crack initiation and increased crack propagation rate at lower temperatures, together with the crack propagation behavior following the crack propagation curve at the actual temperature.