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The gel electrode method of detecting and imaging very small fatigue cracks in aluminum alloys provides the basis for a more rapid evaluation of fatigue performance and offers the convenience of periodic inspections during a test to monitor the initiation and growth of cracks. However, repetitive imaging of fatigue cracks can yield images with successively weaker contrast. This problem can be avoided by the correct selection of the operating parameters. This report describes the influence of various parameters on the spatial resolution and repeatability of the image. The spatial resolution is increased by increasing the starch concentration and reducing the charge flow. To obtain repeatable images the pulse duration (and charge) must be adjusted to balance the effects of the processes of passivation and corrosion. Sodium borate, which was previously included as an electrochemical buffer, should be omitted because it promotes too much passivation. Increasing the concentration of potassium iodide deters passivation so that images can be obtained repeatably with a shorter voltage pulse, less charge, and better spatial resolution. A compromise must be made between the spatial resolution and the visibility of the image. For routine inspection of components a highly visible image can be obtained with a gel electrode containing 0.06 M potassium iodide (KI) and 0.2 M starch upon application of a 10 V, 50 ms pulse. This combination yields repeatable images of cracks in aluminum alloys, with a spatial resolution ≈ 6 × 10−2 mm.
Senior Staff Research ScientistMember of ASTM, Physics Department, General Motors Research Laboratories, Warren, MI
Stock #: JTE11244J