Corrosion fatigue strength of ship structural steel butt welded joints in synthetic sea-water

Abstract

Summary This paper describes an investigation into the corrosion fatigue strength of a MAG-welded 50 MPa class TMCP steel in fatigue tests carried out under a constant-amplitude load at a stress ratio of 0.1 in air and synthetic sea-water. The tests took place under natural corrosion conditions at 25°C at a cycling rate in air and sea water of 3–12 and 0.17 Hz respectively. The fatigue corrosion initiation life Nc was evaluated from the crack length measured by the DC electric potential difference method. This paper describes the S/N relationships, the macroscopic stress concentration factors and the corrosion initiation behaviour at the toes of butt welded joints. The results obtained may be summarised as follows. 1. The ΔSn/Nc relationship between the nominal stress range and the corrosion initiation life as determined in the tests made in synthetic sea-water is represented by a straight line. In the high applied stress range, the corrosion initiation life as determined in the tests made in air is virtually equal to that determined in the tests made in synthetic sea water. The ΔSn/Nc relationship found in the tests made in synthetic sea-water can be applied as the corrosion initiation life for purposes of ensuring the greatest margin of safety in tests made in air. 2. Even when a weld toe crack is present in the specimens tested in air, there is an applied stress range leading to no fatigue failure after fatigue cycling through 1 × 107 cycles. As distinct from the crack arrested at the weld toe in the tests made in air, the crack found in the specimens tested in synthetic sea-water propagates through corrosion dissolution of the crack tip. 3. The stress concentration factor at the weld toe Kt2 determined from strain gauge data is smaller than the local stress concentration factor Kt1 calculated from the two-dimensional cross section at the corrosion initiation point. Its scatter is also smaller. The ΔSt/Nc relationship of the weld toe in which the bending stress of welding angular distortion is superposed on the tensile stress of the axial force is therefore determined as ΔSt = Kt* 2ΔSn. 4. The corrosion initiation life to failure life Nc/Nf has a value of around 0.84 and 0.91 in air and synthetic sea-water, respectively. This difference is due to the effect of the environment on the crack propagation rate. 5. Fatigue cracks are always initiated in the weld toe bond and propagate at right-angles to the loading direction.