Fatigue-Strength Distribution of a Cathodic-Protection Steel and Stainless Steels in Seawater.

Abstract

The influence of a corrosive environment on fatigue-strength distribution was studied for a slightly notched specimen (stress concentration factor α=1.03) and a sharply notched specimen (α=4.60) of S35C carbon steel. Fatigue strength is markedly lower and the standard deviation is also lower for S35C steel in synthetic seawater than those in laboratory air, because many cracks initiate on the specimen surface. Cathodic protection restores the fatigue strength and increases the standard deviation to those obtained in laboratory air. Since many cracks initiate in the sharply notched specimen in laboratory air, the standard deviation of the fatigue strength is lowered to one-third that of slightly notched specimen. Cathodic protection in seawater reduces the number of cracks and increases the standard deviation to a value twice as large as in the air. The influence was also studied for three kinds of stainless steels with slight notch (SUS304, SUS316 and SUS403). Among stainless steels, low-corrosion-resistance SUS403 in seawater exhibits a large decrease in fatigue strength compared with that in air. However, the standard deviation is not lowered by the kind of stainless steel, because the number of cracks is extremely low compared with the carbon steel. The dispersion of the fatigue life has a good correlation with that of the fatigue strength in terms of standard deviation. Therefore, the standard deviation is a significant parameter for the corrosion fatigue. The standard deviation of the fatigue strength has a good correlation with the number of cracks which led to the final fracture.