Quantitative Measurement and Imaging of Metal Fatigue

Abstract

A simple electrochemical technique is described, which images and quantitatively measures the distribution and severity of fatigue damage in aluminum alloys. The technique is based upon (i) the creation of microcracks in a surface anodic oxide film during fatigue of the underlying metal, and (ii) 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, and an image of the sites of current flow is retained in the surface of the gel. The capabilities of the technique are illustrated by measurements on 6061-T6, 7075-T6 and 2024-T4 aluminum. The electrochemically formed images correlated directly with scanning electron micrographs of the specimens. Hairline fatigue cracks ~10 ~ long are easily imaged, while the charge flow during the formation of the image is a quantitative measure of the crack length. The accumulation of fatigue deformation prior to the appearance of a fatigue crack is also detected, and in this regard the sensitivity of the gel electrode exceeds that of a scanning electron microscope. The distribution of fatigue deformation may be mapped as early as 1% of the fatigue life, and the charge flow to the regions of most severe damage increases systematically with fatigue cycling as the density of microcracks in the oxide increases.The simplicity of this electrochemical gel electrode method renders it directly applicable to field investigations, and provides a new tool for quantitatively assessing the distribution of fatigue damage. 1039 1040 W.J. BAXTER