The AC potential drop method

Indirect techniques are useful for measuring small surface crack length during fatigue, as they can be applied in situ and are amenable to automatization. For computer-controlled experiments such as constant ΔK testing, accurate determination of crack length is essential. This is particularly important for corrosion-fatigue, where time-dependent interactions between crack growth and oxide layer build-up with hydrogen embrittlement are to be studied. The AC potential drop method allows the detection of crack initiation and growth in cylindrical specimens with an artificially-induced crack for crack lengths between 10 μm and 1.6 mm ± 10 μm. (orig.)