MAG welding was applied to the butt welded joint for a 500 MPa class TMCP steel. Fatigue tests were performed under a constant-amplitude load with the stress ratio of 0.1 in air and in synthetic seawater. In particular, corrosion fatigue tests were carried out under a natural corrosion condition with the cycling rate of 0.17 Hz at 25°C. The fatigue crack initiation life, Nc, was evaluated from a crack length measured by a DC electric potential difference method.
In this paper, the S/N relations, macroscopic stress concentration factors and crack initiation behaviors at a toe of weld are discussed. The results obtained are as follows:
(1) It was found that there exists a linear relation between ΔSn and Nc when tested in the seawater, where the ΔSn is a nominal stress range. The Nc-value tested under a higher applied stress range in the air agrees well with that observed in the seawater. It may be, therefore, quite reasonable to apply the ΔSn/Nc relation obtained in the seawater as the most safety crack initiation life in the air.
(2) There is an applied stress range in which no fatigue failure arises even after 1×107 cycles in spite of the existence of a crack at the toe of weld when tested in the air. On the other hand, the crack at the toe of weld grows in the seawater due to corrosion dissolution of the crack tip.
(3) The macro-stress concentration factors at the toe of weld, Kt2, obtained from strain gages were smaller than the localized stress concentration factor, Kt, , estimated from two-dimensional cross section at the crack initiation point. The scattering of the Kt2 was also small. The ΔSt in the ΔSt/Nc relation evaluated at the toe of weld, where bending stress components caused by the angular deformation as well as tensile stress components of the axial force exist, can be estimated as ΔSt=Kt2*ΔSn.
(4) The ratio of crack initiation life to failure life, Nc/Nf, was about 0.84 in the air, and about 0.91 in the seawater. This difference may be caused by an environmental effect on crack growth rates.
(5) The fatigue crack always initiates at the toes of weld, and propagates in a direction perpendicular to the loading direction.