1971 Volume 40 Issue 8 Pages 723-735
Low alloy high tensile steels are of limited use in welded structures under alternating load, because welded joints in those steels have not greater fatigue strength than similar joints in mild steels. It has been believed that a metallurgical factor is responsible for the low fatigue strength of the welded joint.
Nevertheless it has been made clear recently that all metallurgical microstructures in the heat-affected zone (HAZ) of welds are not weak to fatigue and not very notch-sensitive. Only in the welded joint of larger heat-input than the proper welded condition for the steel, the softening zone of HAZ may decrease the fatigue strength by a few kg/mm2.
In order to examine the effect of the external geometry of welded joints, investigations were carried out using simulated weld reinforcement specimens. In these previous papers, the external geometry of reinforcement was not carefully observed, so that the effect of a very small toe radius upon the fatigue strength has been ignored. A reduction in the fatigue strength as large as 70% remained unclarified.
The present research was devoted to studying the effect of the external geometry of the reinforcement on fatigue strength. For this purpose, a careful observation of the external geometry of the toe of the reinforcement was performed. It was found that the acute flank angle θ ranged from about 20 to 60 deg and the radius of curvature R at the toe of the reinforcement ranged from about 10 to 0.02 mm. Fatigue tests under pulsating tension were conducted on simulated butt-welded specimens of weldable high tensile steel. The profile of the weld reinforcement was controlled by varying θ and R. How the geometry of the reinforcement toe influences the fatigue strength of the welded joint has been revealed. The stress concentration factor at the toe of the reinforcement with 60 deg flank angle and 0.02 mm toe radius, affecting the fatigue crack initiation, was deduced from photo-eastic experiments and determined to be 3.5 by the finite element method. The fatigue notch factor of the mild steel was 1.9 and that of 80 kg/mm2 class high tensile steel was 3.2 for the stress concentration factor of 3.5. From these values, the butt-welded joint in these steels can be explained well.
The main reason for the very poor fatigue strength of the welded joint is the stress concentration associated with the severe discontinuity of the external geometry at the toe of the weld reinforcement.