DC-PD technique for crack monitoring in creep crack growth study on sent type of specimen of a cast nickel base super alloy

In this paper, experimental method of dc-pd calibration using single and multiple specimen methods for a SENT type of specimen (CM 247 LC) used in creep crack growth(CCG) studies is presented and discussed. The level of accuracy in the crack length measurement using potential drop technique has been estimated by calibration both at ambient and CCG test temperatures. The accuracy is found to be about 5% in the present investigation. The same has also been confirmed by comparing dc-pd results with the crack length measurements made optically and on fractured specimens in CCG studies on the alloy. Also, by treating SENT specimen as one half of a CCT specimen in terms of electrical potential distribution, a very close agreement between the experimental calibration curves and the theoretical crack length predictions by Johnson's formula has been observed. In addition, it has also been observed that grain size of the specimen, like chemistry and heat treatment has little effect on the calibration curves.