Pitting Corrosion and Its Influence on Local Strength of Hull Structural Members

The objective of this study is to obtain basic data for discussing the structural integrity of aged ships, especially hold frames of aged bulk carriers. Firstly, shapes of corrosion pits observed on hold frames of bulk carriers have been investigated. It was shown that the shape of the corrosion pits is a circular cone and the ratio of the diameter to the depth is in the range between 8 to 1 and 10 to 1. Secondly, a series of tensile tests has been conducted to investigate the effect of pitting corrosion on tensile strength. It was pointed out that the tensile strength decreases gradually and the total elongation decreases drastically with the increase of thickness loss due to pitting corrosion. Thirdly, a series of 4-point bend tests with structural models which consist of shell, web and face plates simulating hold frames of bulk carriers has been carried out to investigate the effect of pitting corrosion on collapse behavior and lateral-distortional buckling behavior. Following the experiment, a series of non-linear FE-analyses has been also made. In the case where tensile load acted on the face plate, cracks were initiated at the bottom of the pits when pitting concentrated on the web near the face plate. On the other hand in the case where compression load acted on the face plate, lateral-distortional buckling has been observed and the ultimate load of the structural models where pitting developed regularly on the web was found to be almost the same as that of the structural models where the web has uniform corrosion corresponding to the average thickness loss.