Trial Manufacture and Performance Evaluation of Fatigue Crack Growth Restraining Sheets Using a High Water Content Gel

A simple technique of restraining fatigue crack growth utilizing wedge effects of corrosion products was newly proposed and experimentally tested. Crack growth restraining sheets with a high water content gel were manufactured by way of trial and applied to notched plate specimens of steel, and the efficacy of the sheets to restrain crack growth was evaluated by fatigue tests. In the fatigue tests, the crack growth restraining sheet was renewed at proper timing to maximize its performance. In order to find a local humid (or wet) corrosive environment, which is most suitable for the crack growth restraint by wedge effects of corrosion products on the crack surfaces, water components absorbed in the gel and effects of a cathode layer as promoting factors of the corrosive reaction were comparatively examined. As a result, it was found that all the proposed crack growth restraining sheets were effective and extend the failure life in the range of 2.1 ~ 9.6 times as compared with the bare base metal specimen. Especially, the efficacy of the gel sheets with 3% salt water and synthetic sea water was remarkable (4.1~9.6 times extension in failure life), and the cathode layer (copper-leaf tape) produced certain additional positive effects in those gel sheets. From fracture mechanical analyses with FE analyses, it was found that the crack restraining effect of the gel sheet greatly varies depending on the water gel content and the existence of cathode layer. In situ observations around the growing cracks and macro- and microscopic observations of the fracture surfaces were also performed for a comparative study between the different types of crack growth restraining sheets.