Investigation of Crack Formation and Propagation in AA7475 using Multiple Potential Drop Measurement

The direct current potential drop method (DCPD) is a well-known method for crack detection and crack length measurement in fatigue experiments. Recent Investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in round bars (Hartweg and Bär, 2019) and single edge notched specimens (Wiehler and Bär, 2020). In this work a more detailed investigation of crack initiation and propagation is undertaken on single-edge notched specimens. The specimens were equipped with three potential probes – on the frontside (Ufront), on the backside (Uback) and on the narrow side (Unarrow) of the specimen. During the fatigue tests all three potentials were measured simultaneously using amplifiers of the control electronics. The crack front was marked on the fracture surface by introducing overloads in defined intervals to allow a direct comparison between the measured potential drop and the real crack location and crack front geometry. In a first step relative Potentials Pi (i= front, back and narrow) were calculated by normalizing the actual potential on the potential of the crack-free specimen. To localize the crack initiation site and to eliminate temperature effects quotients of the potentials were formed by dividing Pfront and Pback by Pnarrow, respectively. By comparing the runs of the potential quotients plotted over the cycle number, the time of crack initiation and the location of the crack initiation site can be determined. In addition, the run of the quotients gives information about the shape of the crack front of long fatigue cracks.