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An electrochemical technique is described that images and quantitatively measures the distribution and severity of fatigue damage. The technique is based upon (1) the creation of microcracks in a surface anodic oxide film during fatigue of the underlying metal, and (2) the detection of these microcracks by contacting the surface with a gel electrode. When a voltage pulse is applied, current passes through the fatigue-induced microcracks in the oxide film. An image of the sites of current flow is retained in the surface of the gel, while the total charge flow is a measure of the extent of rupture of the oxide film. The capabilities of the technique are illustrated by measurements on aluminum alloys. The distribution of surface fatigue damage may be mapped as early as 1% of the fatigue life, while fatigue cracks ≥ 10 µm long are easily imaged.
metal fatigue, microcracks, oxide films, gel electrode
Senior Staff Research Scientist, General Motors Research Laboratories, Warren, Mich.