Gel electrode imaging of metal fatigue: Part i. cracks in 6061-T6 aluminum
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A simple electrochemical technique is described which produces high-resolution, visible images of fatigue cracks in aluminum alloys, without conventional metallographic surface preparation. The only preparation required is the formation of a thin (14 nm) anodic oxide film on the surface. After fatigue cycling, a semisolid electrolyte is placed in contact with the specimen and a voltage applied. As the current flows to the fatigue cracks a clearly defined image is formed. The capabilities of the technique are demonstrated by measurements on 6061-T6 aluminum, and the images are correlated with scanning electron micrographs of the specimens. The images are reproducible and record features of the cracks which are barely discernible with a scanning electron microscope. Fatigue cracks only - 3 0 / x m long have been detected. Measurement of the charge flow during imaging is a quantitative measure of the crack length. These measurements indicate that considerably smaller cracks should also be detectable.
I.
INTRODUCTION
T o assure adequate fatigue performance, the design engineers must ultimately rely upon testing of actual components. This approach is successful, but time consuming. Therefore, abbreviated analysis and testing procedures could reduce the lead time for the introduction of new designs. One such procedure is based on the assumption that all structures contain flaws, so that fatigue cracks may initiate early in life. The fatigue lifetime is then estimated on the basis of detailed studies of crack propagation rates and the stresses expected in service. However, the starting point for such an estimate is either the length of the largest undetectable crack that could be present, or the detection and measurement of the size of a very small crack produced by a short term test. On carefully prepared and polished surfaces, it is, of course, possible to observe very small fatigue cracks: cracks < 10/zm in length can be observed by direct optical microscopy, while replication techniques for subsequent examination by electron microscopy can reveal cracks < 1 /xm in length. However, these techniques are not appropriate for the examination of real structures, and a need exists for a reliable and simple technique of detecting small fatigue cracks which is applicable without meticulous surface preparation. This paper describes a new and simple method of locating and imaging fatigue cracks in metals which fulfills the above requirement. It is based upon two factors: (i) the rupture of surface oxide films during fatigue of the underlying metal, and (ii) a redox printing technique developed by Klein 1'2 for the study of defective sites of high electrical conductivity in anodic oxide films. In this first report, the technique is illustrated by images of fatigue cracks in 6061-T6 aluminum. It should be noted that the images are deliberately under-exposed to obtain good spatial resolution at the expense of sensitivity, so that the images may be compared directly with optical and scanning electron micrographs of the fatigue cr
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