Fatigue Strength of Side Frame Based on Plate Thickness Reduction Due to Corrosion

Abstract

Where the ship structural members are designed, it is one of the important items to consider the decrease of steel thickness due to corrosion. The authors introduced the procedure of fatigue fractures reliability analysis where the decrease of thickness was gradually progressed by time passing and this decrease caused the gradual increase of characteristic extreme value stress under the condition of fluctuating wave. Using above procedure and the results of fatigue model tests, the probability of fatigue crack initiation was calculated and compared on the four types of lower ends of side frames which were used in multi-purpose cargo ships; and sensitivity analysis on random variables was also performed. As the results, the following conclusions were obtained. (l) In reliability analysis of ships structure, the decrease of plate thickness due to corrosion has a great effect on the magnitude of the probability of fatigue crack initiation. (2) The probability of crack initiation is not always high when the expected corroded thickness is taken off from the original thickness for stress aanlysis in stead of consideration that the decrease of plate thickness due to corrosion is gradually progressed by time passing. (3) Under selected ranges of random variables, the mean values of coefficient of S-N curve have the most important effect upon the fatigue reliability.