Calibration of DC Potential Technique Using an Optical Image Processing System in LCF Testing

The direct current electrical potential drop (DCPD) technique is applied to monitor fatigue crack growth in plate specimens with central circular holes at room and high temperatures.The application of the calibration curves obtained from replica specimens to both room and high-temperature fatigue testing conditions is studied. The actual fatigue crack length is measured using a video-camera with image processing to assess the reliability of the DCPD technique. The use of two pairs of DCPD probes across the hole and the introduction of a special reference voltage have eliminated the dependence of the potential drop on the hole size and shape as well as the material resistivity and the notch plastic zone size prior to crack initiation. At high temperature, however, the calibration curve should be corrected to take into account the shifting in the potential ratio due to the increases in crack tip plastic zone and crack opening displacement. The effect of the temperature difference between the different potential probes is also examined. By analyzing the results given by the DCPD in connection with the image processing technique, a criterion is suggested to define the interface between the crack initiation and propagation periods.