Experimental crack length measurement under variable temperature using a thin film AC potential difference technique

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Roberge, Douglas M.
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An alternating current (AC) potential drop (PD) method is developed for measuring crack length under combined thermal and mechanical loading. An insulated aluminum film deposited on the surface of center cracked 6061-T6 aluminum alloy test specimens provides a medium for the measurement of PD data used in calculating crack length. Experimental PD data is collected over a range of crack lengths and temperatures and compared against theoretical and numerical models. Potential difference data is found to be linear over a wide range of crack lengths. An analytical relationship is derived based on a theoretical model to provide a predicted crack length given potential difference and specimen temperature. Calculated crack lengths are compared to optically measured crack lengths for cracks 0 to 30 mm in length at temperatures up to 300$\sp\circ$C. Calculated values were found to be within $\pm$0.3 mm of measurement results over nearly all temperatures and crack lengths. Improvement of the accuracy of calibration equations and the sensitivity of the foil deposit is achieved through a numerical study of the film potential field in various configurations. Application of the technique to studies of creep and fatigue induced crack growth under varied thermomechanical loading suggested a resolution of 0.02 mm and accuracy within $\pm$0.09 mm using an excitation current of only 1.98 mA.