Analysis of crack geometry and location in notched bars by means of a three-probe potential drop technique

It is often necessary to define the crack initiation life of a fatigue tested component, generally at a given (short) crack length. The size of an initiated crack can be estimated by employing different experimental methods, one of which is the direct current potential drop (DCPD) technique. In the case of notched bars subjected to fatigue loadings, the crack configuration (i.e. circumferential or semi-elliptical) and location cannot be singled out by means of the potential drop method (PDM) operating with a single potential probe. In the present contribution, three potential probes are adopted to overcome this issue. The calibration curves reporting the three potential drops as a function of the crack size are derived by means of 3- dimensional electrical FE analyses. Two different crack configurations are analyzed: (i) circumferential and (ii) semi-elliptical surface cracks. The calibration curves have been validated by systematic comparison with experimental results, generated by fatigue testing of sharp as well as blunt notched specimens made of steel and a titanium alloy under pure axial loading. Finally, a procedure to assess the area, the configuration and the location of the initiated fatigue crack starting from the experimentally measured potential drops is discussed.