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I.

INTRODUCTION

IT is

generally agreed that tensile stress is a necessary condition of stress corrosion cracking (SCC). 1 If the SCC mechanism is due to hydrogen induced cracking, it is widely demonstrated that the hydrogen induced cracking occurs only when hydrogen evolved from cathodic reaction is enriched to a critical value. 2,3,4 The enrichment of hydrogen cannot take place in the region of compressive stress; thus it may be deduced that compressive stress would not induce this type of SCC. If SCC mechanism is due to anodic path dissolution, the function of the stress is to induce slip or creep and film rupture at a crack or notch tip so that the crack can propagate via the anodic dissolution of fresh metal. Since compressive stress can also cause the slip or creep and then film rupture, the anodic dissolution process under the compressive stress, in principle, may occur. Previous work showed that macroscopic compressive stress was able to induce SCC of Type 304 stainless steel in a boiling 42 wt pct MgCI2 solution. 5 The incubation period of the SCC under compressive stress, however, was one to two orders of magnitude longer than that under tensile stress. 5 Further work indicated that SCC of mild steels in a boiling nitrate solution under compressive stress could occur, and the incubation period of SCC under compressive stress was also ten to a hundred times longer than that under tensile stress. 6 Since the incubation period of SCC under compressive stress is one to two orders of magnitude longer than that under tensile stress, the SCC under tensile stress will lead to failure of a specimen or structure before SCC under compressive stress occurs if there exists simultaneously tensile stress. Besides, the crack under the compressive stress, even if it occurs, will not lead to failure of the specimen or the structure. Perhaps, this is one of the

WU-YANG CHU, Associate Professor, and CHI-MEI HSIAO, Professor, are with the Department of Metal Physics, Beijing University of Iron and Steel Technology, China. JUN-WEN WANG, formerly Student at Beijing University of Iron and Steel Technology, is now with Taiyuan Steel Company. Manuscript submitted September 6, 1984. METALLURGICALTRANSACTIONS A

reasons why SCC under compressive stress was omitted and a wrong impression, that compressive stress wilI not induce SCC, was obtained. The present work is intended to clarify whether the SCC under compressive stress can also occur in other stress corrosion systems, e.g., an aluminum alloy in an aqueous solution. Besides, previous works investigated only qualitatively the possibility of the SCC under compressive stress, and there were not any quantitative data about the threshold value of SCC caused by the compressive stress. Therefore, the other purpose of the work is to measure the threshold stress intensity factor for the SCC under compressive stress and to compare with that under tensile stress. When SCC of Type 304 stainless steel in the boiling MgC12 solution at 143 °C under compressive stress was investigated using