Effect of testing frequency on the corrosion fatigue of a squeeze-cast aluminum alloy
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I. INTRODUCTION
OVER the past 30 years, several attempts have been made to characterize fatigue crack growth in metals under the combined action of cyclic loading and corrosive environments.[1–6] In aluminum alloys, studies on the 2000 series of aluminum-copper alloys[7–10] revealed that the fatigue threshold stress-intensity-factor range (DKth value) could vary considerably with microstructure and yield strength under different environments. As well, many researchers[11–17] have considered corrosion fatigue of the 7000 series (Al-Zn-Mg). Kirby and Beevers[11] clearly demonstrated that even the seemingly innocuous environment of laboratory air can lead to a marked increase in crack propagation rates, as compared to in a vacuum environment, in the near-threshold fatigue regime of these alloys. Suresh et al.[12] also showed that microstructure-environment interactions at lower stress intensities can be completely different from those at higher levels. Some studies have also been performed on the fatigue behavior of cast aluminum alloys.[18–24] It was found that a squeeze-cast aluminum alloy, which has superior mechanical properties,[19] was also susceptible to environmentally assisted fatigue. Recently, the effect of the precracking environment on the subsequent corrosion fatigue crack growth behavior of a squeeze-cast aluminum alloy has been studied. For long cracks in aggressive environments, the cracking showed a high cracking rate which rapidly decreased in the near-threshold regime. If conditions were such that the cracking did not stop, it subsequently reaccelerated at higher DK values to follow the regular crack growth rate (da/dN ) KAZUAKI SHIOZAWA, Professor, is with the Department of Mechanical Systems Engineering, Toyama University, Toyama 930-8555, Japan. SHUMING SUN, Research Associate, and R.L. EADIE, Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, B, Canada T6G 2G6. Manuscript submitted December 2, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
vs DK curve (Paris law), with rates 1.5 to 5 times higher than those observed in air.[18] Despite the numerous environmental fatigue studies which have been conducted on conventional 2000 and 7000 series aluminum alloys,[7–23] the nature of the underlying interactions between microstructure and environment are still not well understood. The effects of prefatigue environment on the subsequent corrosion fatigue crack growth and associated damage mechanisms have not yet been delineated.[18,19,23] The present study investigates the effect of load cycling frequency on the corrosion fatigue process and evaluates fatigue mechanisms in saline solution (an aggressive corrosive medium). The prefatigue crack–environment effect on the subsequent corrosion fatigue behavior, the crack-environmental interactions, and corrosion product– induced crack closure effects will be described for a squeezecast Al-Si-Mg-Cu alloy (AC8A-T6) with a eutectic microstructure.
II. EXPERIMENTAL PROCEDURES The squeeze-cast Al-Si-Mg-Cu
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