Gel electrode imaging of metal fatigue: Part ii. deformation in 1100 Aluminum
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BAXTER
During the early stages of metal fatigue, the accumulation of damage produces microcracks in the surface oxide film. This process was first measured quantitatively by the exoelectron method. This paper describes a new and much simpler electrochemical technique, which can also detect microcracks in surface oxide films and provide quantitative information on the distribution and severity of fatigue damage. In these experiments the specimens are coated initially with thin (14 nm) anodic oxide films. After fatigue cycling, a semisolid electrolyte is placed in contact with the specimen, and the flow of current to the microcracks in the anodic oxide is measured. The distribution of fatigue deformation which accumulates prior to the appearance of a fatigue crack is easily measured, and in this regard the sensitivity of the technique is shown to exceed that of a scanning electron microscope. Fatigue deformation in 1100 aluminum is detected as early as 1 pct of the fatigue life, and the electrochemical current flow increases systematically with fatigue cycling as the density of microcracks in the oxide increases. The charge flow can therefore be used to predict the remaining fatigue life.
I.
INTRODUCTION
A
quantitative measure of the development of fatigue damage in metals could provide the basis of an abbreviated testing technique for the prediction of fatigue life. Such a tool would permit rapid design iteration, with the conventional and time-consuming procedure of testing to failure being reserved for final validation. An electrochemical method, based upon measurements of oxidation currents, was described in an earlier report. ~ The basic procedure involved developing an adherent surface oxide film on the metal prior to the application of the fatigue loading. During a fatigue test this oxide film develops microcracks, which can be detected by measuring the flow of electric charge as they are 'healed' during subsequent electrochemical reoxidation. It was demonstrated that fatigue cracks as short as 100/xm could be detected in aluminum in this way, 1 while more recently cracks -> 1 mm long have been detected in steel. 2 Concurrent fundamental studies in the photoelectron microscope3 have shown that the microcracks in the oxide film not only identify fatigue cracks in the underlying metal, but also occur at the slip bands that precede the appearance of a fatigue crack. Indeed, more recent measurements of reoxidation currents have shown that it is just possible to detect some areas of very severe fatigue deformation in aluminum. 3 But in general, the reoxidation currents have been too small to detect the early stages of fatigue. This report is the second describing a new technique, which appears to take full advantage of the sensitivity to fatigue damage offered by the microcracks in a surface oxide film. As outlined in Report I of the series, 4 the technique is a modified version of a redox printing technique developed by Klein 5 for the study of defective sites of high electrical conductivity in anodic oxide f
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