Calibration of the potential drop method for monitoring small crack growth from surface anomalies - Crack front marking technique and finite element simulations

The Direct Current Potential Drop method is one of the possibilities to measure the crack growth during fatigue tests without optical access to the sample. The accuracy of this technique applied to short cracks mainly depends on the calibration curve. In the present work experimental and numerical approaches are proposed to calibrate the potential drop measurement. An optimization of the calibration procedure is supported by finite element calculations. The crack front shape and the location of the potential probes are found to be of great influence. An extensive study is conducted to identify all the mechanisms controlling the potential drop measurement. Plastic deformation remains the last parameter which is not directly considered in this study; therefore a calibration strategy is suggested to take into account this effect and achieve a calibration curve with high accuracy. This strategy shows a very good agreement between the obtained calibration curve and the experimental calibration.