Effects of frequency and temperature on short fatigue crack growth in aqueous environments

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I.

INTRODUCTION

THE rate

of corrosion fatigue crack growth, or environmentally assisted fatigue crack growth, is controlled by a combination of the local stress-strain field and the local electrochemical (or chemical) environment at the crack tip. The stress-strain field at the crack tip has been analyzed by fracture mechanics, and the stress intensity factor range (AK), or its effective component (AKaf), can be used to characterize the driving force for fatigue crack growth in inert environments. The local chemical environment at the crack tip (e.g., electrode potential, solution pH, concentration of ionic species, etc. ), on the other hand, cannot be easily defined. Because the rate of growth of long fatigue cracks (with length greater than about 10 mm) appeared to be well characterized by the stress intensity factor (AK or AK,ff) even in aqueous environments, 1 the local environment at the crack tip is presumed to be well defined for a given AK o r AKeff. Fracture mechanics and surface chemistry studies of environmentally assisted crack growth have shown that crack growth in some material-environment systems may be controlled by the rate of surface reactions at the crack tip, and in others, by the rate of transport of the deleterious environment to the crack tip. Wei and his co-workers have proposed models for transport and reaction controlled fatigue crack growth which allowed comparisons to be made between the surface chemistry and crack growth data. 2'3 The concept has been extended to the consideration of corrosion fatigue in aqueous environments, and the data on HY130 steel tended to support electrochemical reaction control of corrosion fatigue crack growth in aqueous environments. 4 Recently, Gangloff reported that the growth of short fatigue cracks in aqueous environments is much faster (by

Y. NAKAI is with the Department of Mechanical Engineering, Faculty of Engineering, Osaka University, 2-1, Yamadaoka Suita, Osaka 565, Japan. A. ALAVI is with The Sherwin-Williams Company, 10909 South Cottage Grove Avenue, Chicago, IL 60628. R.P. WEI is Professor of Mechanics, Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015. Manuscript submitted January 28, 1987. METALLURGICAL TRANSACTIONS A

upward of hundreds of times) than of long fatigue cracks. 5'6 Tanaka and Wei7 and Nakai et al. 8 showed the same trend, although the increases were limited to less than a factor of 10 in crack growth rates. The difference in growth rates between long and short cracks is believed to result from differences in the chemistry of the crack tip environment between long and short cracks. These differences in chemistry, in turn, reflect the complex interactions among the transport and reaction processes within the crack and at the crack tip. The transport processes p e r se are expected to be dependent on the stress intensity factor through its influence on crack opening and on fluid flow (advection). 9 As such, the concept of reaction controlled crack growth for long cracks needs to be re-exa