DC Potential Drop Calibration in Creep-Fatigue Loading Conditions

The direct current potential drop method (DCPD) is used to detect the initiation of cracks and monitor their growth under the conditions of high gross inelastic deformations (plastic and creep). Testing is conducted on flat SS-304 plates with central circular holes of different diameters under fatigue and creep-fatigue loading at 600°C. The potential drop is measured using two pairs of voltage probes across the hole. The variation of the voltage ratio with the number of cycles is considered in order to eliminate the dependency of the DCPD on the current and material resistivity. The visible crack length is measured by a video camera with image processing to assess the reliability of the DCPD technique. The effects of notch shape and notch plastic zone size prior to initiation are removed by using a special reference voltage ratio. Because of the high inelastic deformations at the crack tip and larger crack opening displacement under fatigue and creep-fatigue conditions at high temperature, the calibration curve developed at room temperature is modified using material property variation with temperature. Furthermore, the variation of notch opening displacement with voltage ratio is employed to correct measured crack length and to determine crack initiation under fatigue with hold time.