Crack Detection and Localization with Multiple Potential Drop Measurements

Abstract

The direct current potential drop method (DCPD) is commonly used to measure the crack length in fatigue tests. Due to a propagating crack the cross section of the specimen is reduced leading to a densification of potential field lines, whereby the measured potential difference increases. First experiments on notched round bars have shown that the location of a crack can be determined with a multiple potential drop measurement (Hartweg and Bär, 2019).In this study single-edge notched specimens were equipped with three potential probes – on the frontside (Uf), on the backside (Ub) and on the narrow side (Un) of the specimen. During the fatigue test the three potentials were measured simultaneously using amplifiers of the control electronics. To enable a direct comparison between the potential drop measurement and the real crack geometry, the crack front was marked on the fracture surface by introducing overloads in defined intervals.The measured potential values were normalized on the starting potential and quotients of the potentials were formed (Pf/Pn and Pb/Pn). By plotting the potential quotients over the cycle number, the time as well as the location of the crack initiation site can be identified. In addition, it was possible to identify subsequent cracks and to determine the shape of the crack front of long fatigue cracks based on the measured potential data.