ICF100805OR CORROSION-PIT-GROWTH BEHAVIOUR DURING THE CORROSION FATIGUE PROCESS IN ALUMINUM S. Ishihara1, T. Goshima1, A. J. McEvily2, S. Sunada1and S. Nomata1 1Department of Mechanical Engineering, Toyama University, Toyama, Japan 2Department of Metallurgy, University of Connecticut, Storrs, CT, USA ABSTRACT The fatigue strength of machines and structures operating under corrosive environment falls off remarkably in comparison with the fatigue strength in the ambient atmosphere. This decrease in fatigue strength is due to the easy initiation of fatigue cracks at corrosion pits in the early stage of the corrosion fatigue process. In order to make predictions of the lifetime of the machines and structures operating under corrosive environment, it is therefore important to understand the initiation as well as the growth characteristics of corrosion pits. However such kinds of research are few and limited, in contrast to the many studies of crack growth behavior during the corrosion fatigue process, which have been conducted. Furthermore, the existing data on corrosion pit growth are not always consistent, particularly with respect to the role of stress amplitude. In this study, plane bending fatigue tests of commercially pure aluminum were carried out in 3% NaCl solution, and the effects of stress amplitude and test frequency on the initiation and growth characteristics of corrosion pits were studied in detail. In addition the critical condition for the nucleation of a fatigue crack at a corrosion pit was considered. KEYWORDS Corrosion Pit, Pit Growth, Stress Amplitude, Cyclic Frequency, Crack Initiation, Aluminum. INTRODUCTION The fatigue strength of machines and structures exposed to corrosive environments is considerably reduced in comparison with that in the ambient atmosphere, and a fatigue limit may not exist. In the design of components for applications in corrosive environments allowance should be made for this degradation in fatigue properties. In the corrosion fatigue process it is known that corrosion pits arise in the initial stages of the fatigue process and that fatigue cracks develop at these pits. Therefore, it is important to clarify the generation and growth characteristics of corrosion pits as well as the process by which the cracks are generated in order to make reliable predictions of the corrosion fatigue lifetimes. Although there has been much research [1] on the generation and growth behavior of cracks during the corrosion fatigue process, relatively little work has been done on the generation and growth behavior of corrosion pits. It has been reported that the following power law equation describes the growth law of a corrosion pit [2]. (1) β a At = where A and βare experimental constants. However, whether or not these experimental constants are dependent on stress amplitude and cyclic frequency has not been made clear. The present study was carried out using commercially pure aluminum, which was cyclically loaded in plane-bending in a dripping 3% salt solution. Observations were made of the initiation and growth
RkJQdWJsaXNoZXIy MjM0NDE=