Effects of stress concentration factors on corrosion fatigue strength of a steel plate for ship structures

This paper presents the results of corrosion fatigue tests on notched specimens of a steel plate for ship structures. Stress concentration factors, Kt, are 1.2, 2.0, 2.93 and 3.77. The corrosion fatigue tests were carried out at a cycling rate of 0.17Hz under a natural corrosion condition in synthetic seawater. A corrosion fatigue strength was estimated from the S-N curves based on crack initiation life. The S-N curves were expressed by an equation LogN=A+B Delta S with experimental constants A and B. The corrosion fatigue strength decreased with the increase in the stress concentration factor. The S-N curves can be expressed as straight lines for a stress range of more than 100 MPa. The fatigue lives have a tendency to increase fora stress range of less than 100 MPa. In the lower stress range, a crack initiation life should be examined in connection with corrosion wastage and corrosion pits. For Kt less than or equal to 3.1 in air or in seawater, the linear relations were observed between the Kt and the constant A or B. Therefore, a slope of the S-N curve can be estimated for arbitrary Kt values. A fatigue limit in air also can be estimated from the linear relationships for Kt<3.1. It gives a constant value for Kt>3.1, In the equivalent stress in consideration of the maximum stress at the notch root, the fatigue crack initiation life can be collected onto each different one line in air and in seawater, respectively.