PSI - Issue 42

Mike Nahbein et al. / Procedia Structural Integrity 42 (2022) 433–440 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. (a) Specimen geometry; (b) location of the potential probes; (c) examined crack initiation sites. The crack initiation at defined positions was forced by secondary notches introduced with a marking laser

For the potential drop measurement, a constant current was conducted through the specimen leading to a potential drop of about U 0 = 0.51 mV at the beginning of the fatigue test. More information about the measurement technology can be found in Campagnolo et al (2019) and Hartweg and Bär (2019). For each cycle the maximum and minimum value of each potential probe was measured. To eliminate the influence of some disturbance values on the potential measurement, e.g. small differences in the length of the cooper wire or in the exact position of the measurement contacts, etc., the absolute potential was normalized by dividing the actual potential difference U i (with i = 1, 2, 3) by the initial voltage U 0,i which is defined as the mean value of the potential of 20 cycles at the beginning of the experiments (see equation 1) � = � � � �,� �,� = � � � ∙ ∑ �,�,� 1(→ 0°), 2(→ 120°), 3(→ 240°) �� ���� (1) For the comparison of the calculated potential drop and the real crack geometries the crack surfaces were analyzed by a 3D laser scanning microscope (3D-LSM) and a field emission scanning electron microscope (FE-SEM). Moreover, the overload marks were colored in the images to enhance the visibility. 3. Results The evaluation of the experiments is performed with the geometrical model by Hartweg and Bär (2019) that is based on a plane in a cylindrical coordinate system, span by three vectors. These vectors are representing the three relative potentials P 1 , P 2 and P 3 with the length of the measured potential. In the case of crack initiation, the normal vector of the plane will tilt because the relative potentials don’t rise with the same level as can be seen in figure 2.