ABSTRACT

Stress corrosion crack tests were conducted by using SNCM439 steel under various temperature of 3.5%NaCI solution. The crack profiles obtained in these tests were analyzed by fractal geometry. The results obtained were summarized as follows. (1) The higher the temperature of solution is, the faster crack growth rate IS. However the higher the stress intensity factor is, the more poor tendency becomes. (2) The profile of stress corrosion cracks both main crack arid branching crack have the fractal character. Then, the fractal dimension of that main crack is about from 1.05 to 1.10 that branching crack is about 1.15. The result could express quantitatively what phenomena of stress corrosion cracks to region include micro branching. (3) It is able to explain that a dependence of a da/dt upon a T become poor with being high stress intensity factor, by means of measuring crack length using fractal geometry. It was able to explain that a growing crack length per unit time, da, become short. The reason that a ratio of low actual stress intensity factor, K, was large due to the effect of blunt notch the crack length become long with being high K.

INTRODUCTION

Crack growth rate of Stress Corrosion Cracking(SCC) were different on various testing condition. As crack branching phenomena of high strength steel had often been observed, it was characterized by appearance of two types; one is macro branching cracking, the other is micro branching (Tsuda et al, 1992). These type of crack branching were influenced specifically by the environmental condition (Hirose et al., 1978, Tsuda et al.,1988). These main cracking also had existed the irregular waving. As a result of investigating many paper arid book, considerable works on the problem of hydrogen brittlement fracture have been conducted to date (Yamakawa, 1976). Although the stress fields around crack tips had been analyzed well (Ito et al, 1989), no sufficient explanations had been given for the crack branching arid waving phenomena in SCC.

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