Interactions between mechanical and environmental variables for short fatigue cracks in a 2024-T3 aluminum alloy in 0.5M
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I. INTRODUCTION
THE problem of environmentally enhanced fatigue, or corrosion fatigue, is recognized as an important factor in the design of structural components subjected to dynamic loading in corrosive environments. Corrosion fatigue of materials in gaseous environments has been extensively studied in an attempt to develop a mechanistic understanding of the influences of mechanical, environmental and metallurgical variables.[1–6] The electrochemical conditions at a crack tip, however, complicate the study of corrosion fatigue in aqueous environments. The interactions between crack geometry, mechanical loading, and species in the solution govern the cracking kinetics and, therefore, require further analysis. It is well accepted that the steady-state fatigue-crack growth rates (FCGRs) of long cracks depend uniquely on DK for a fixed load ratio (R) and test environment. The crack growth rates of short cracks, on the other hand, are crack-size dependent. Anomalous growth behavior of short cracks in aqueous environments has been reported,[7–10] which challenges the validity of using only long-crack results in evaluating the service life of a structure and argues for the need for considering the effects of crack size. Moreover, since the early stage of crack growth accounts for a high percentage of total life, the characterization of short-crack K.-C. WAN, Project Engineer, is with the Applied Mechanics Group, Product Development Center, Outboard Marine Corp., Waukegen, IL 60085. G.S. CHEN is with Akrion, Allentown, PA 18106. M. GAO, Engineering Advisor, is with the Materials, Corrosion & Inspection Group, Mobil Exploration & Producing Center, Farmers Branch, TX 75381-9047. R.P. WEI is with the Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015. Manuscript submitted May 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
behavior is necessary to better predict the service lives and inspection periods of engineering components and structures. Short cracks have been categorized into three groups:[9] microstructurally short, mechanically short, and chemically short cracks. Microstructurally short cracks are small relative to the grain size of the material, for which continuum mechanics is not applicable. Mechanically short cracks are those that are too small for linear elastic fracture mechanics to apply or are associated with the changing effects of crack closure. Chemically short cracks are microstructurally and mechanically long, but still exhibit short-crack behavior due to different electrochemical conditions at the crack tip. In this article, only chemically short cracks are considered. Although several steels in salt water have exhibited shortcrack behavior,[7–10] the behavior of chemically short fatigue cracks has not been reported for nonferrous alloys. Similarly, a recent study found no pronounced crack-size effects for a 2024-T3 alloy exposed to deaerated 1 pct NaCl solution.[11] Since short-crack effects are very sensitive to the conditions of the material, environme
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