Investigation of stress corrosion crack growth in Mg alloys using J-integral estimations
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I.
INTRODUCTION
STRESS corrosion cracking often shows some of the features associated with stable mechanical crack growth. For instance, non-propagating cracks are observed in magnesium and other alloys loaded in aqueous solutions. The cracks are observed to nucleate at loads below the threshold stress intensity but are seen to arrest after they have grown a short distance.~ This is reminiscent of events in stable mechanical crack growth where crack growth resistance is due to the reduced strain concentration which develops at the crack tip as it grows beyond its original position. 2 More generally, recent advances in the theory of crack growth in elastic/plastic materials 2'3'4 opens up the question of what small scale events near to the crack tip control the process of stress corrosion cracking (SCC). 5'6 It is the purpose of this paper to present the results of experiments on two magnesium alloys which were undertaken with the aim of considering SCC in terms of the ideas which have been applied to mechanical crack growth. The essence of the method is the application of a constant rate of displacement to precracked specimens. At the same time the necessary measurements were made to give a result showing G (Energy Release Rate) or J (J-contour Integral) vs crack extension. The approach is much as is conventionally made in the slow strain rate method of detecting susceptibility to SCC 7 and similar to the tests designed to investigate stable crack growth. 8'9 The main difference is that rate effects are very important in SCC, while time does not enter directly into a consideration of quasi-static crack growth by mechanical means. Magnesium alloys were used in the work because of the high crack growth rates which are associated with SCC in the alloy.
II.
EXPERIMENTAL M E T H O D
Precracked specimens of the alloy were loaded at constant displacement rates in a specially designed horizontally aligned testing machine. In addition to load, a record of load-line crack opening displacement was made during the test together with the potential drop produced by a constant G. ABRAMSON, formerly with The University of Newcastle upon Tyne, is now with International Computers Ltd., Wakefield, England. J.T. EVANS, Lecturer, and R.N. PARKINS, Professor and Head, are with the Department of Metallurgy and Engineering Materials, The University, Haymarket Lane, Newcastle upon Tyne, NE1 7RU, England. Manuscript submitted January 9, 1984. METALLURGICALTRANSACTIONS A
current flow of 17 amps through the specimen. From these measurements G, J, and Aa were calculated at various stages during the test. Tests were done with the specimens in air and in solution with the specimen immersed up to the level of the notch. Two magnesium alloys were used. The first, Mg-7 pct AI, was available only in the form of extruded bar (10 • 26 mm) from which double cantilever beam specimens were machined. The second alloy, MgHK31A (3Th, 0.7Zr, bal Mg), was available in larger plate from which 18 mm thick compact tension specimens were machined. The speci
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