Tetsuro Nose

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

Numerical Study on the Fatigue Crack Propagation Behavior in Flattened Martensite Dual Phase Steel

The relation between the fatigue crack propagation behavior and the microscopic characteristics (the morphology, distribution, hardness of the 2nd phase) of Ferrite / Martensite Dual Phase (FMDP) steel is investigated numerically using Crystalline-Elastic-Plastic F.E. (CPFE) analysis. The calculation results lead us to a prediction that the crack growth rate of FMDP steels with polygonal and banded M-phase becomes much higher than that with flattened and banded M-phase because cracks can slip through M-phase when there exist narrow F-phase slits. The experimental results agree approximately with this prediction. This demonstrates the validity of the CPFE theory employed in this research.