Electrochemical detection of fatigue damage
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INTRODUCTION
EXPERIMENTAL
describes a new method of detecting fatigue damage in metals, based upon the measurement of electrochemical oxidation currents. The basic principle is to form electrochemically a layer (>20 nm) of strongly adherent surface oxide on the metal prior to the application of the cyclic fatigue loading. During the fatigue test, microcracks are created in the oxide wherever deformation develops in the underlying metal. The fresh metal surfaces revealed by these microcracks are rapidly reoxidized under normal atmospheric conditions, but only a very thin layer ( - 4 nm) of " n a t u r a l " oxide is reformed. Thus these microcracks may be detected by measuring the flow of electric charge as they are " h e a l e d " during subsequent electrochemical reoxidation. This initial demonstration of the technique is illustrated by experiments on 1100-0 aluminum. Since it is impossible to produce two fatigue specimens with equal amounts of " d a m a g e , " some preliminary experiments were performed on tensile specimens wherein the amount of deformation was well controlled. These measurements established that the reoxidation currents provide a quantitative measure of the total area of the microcracks in the anodic oxide film, and the flow of charge agrees with a simple model calculation. In the second series of experiments, the incorporation of a simple scanning procedure is shown to be an important refinement for the successful detection of fatigue damage, and the magnitude of the oxidation current is correlated with observations of the surface fatigue damage by scanning electron microscopy.
The specimens were fabricated from strips of 1100-0 aluminum, 1.5 mm thick. The initial surface oxide film was formed by anodization in a solution of tartaric acid (3 pct by wt) adjusted to pH = 5 with ammonium hydroxide. Another piece of aluminum served as the cathode. The applied voltage was gradually increased to the desired value, so that during anodization the current density did not exceed 10 m A / c m 2. The anodization was judged to be complete when the anodization current had decayed to a level o f - 10 -6 A / c m 2. The thickness of the oxide produced by this procedurO is approximately 1.4 n m / V . After anodization, the tensile samples were strained by known amounts in an Instron tensile machine. The tensile strain was measured by the movement of the cross head, which had been previously calibrated with an extensometer using identical tensile samples. (The clip-on extensometer was not attached to the actual test samples because it could scratch the surface.) The ends of the specimen were of course heavily deformed by the grips of the Instron machine, so it was necessary first of all to reanodize the bottom end at the original voltage. The gage section of the sample was then reanodized at a lower voltage (10 V). At this reduced voltage, reanodization only occurs where the initial thick oxide has actually ruptured and been partially healed by the very thin ( - 4 nm) natural oxide. The fatigue specimens were
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