PSI - Issue 17

Moritz Hartweg et al. / Procedia Structural Integrity 17 (2019) 254–261 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The potential drop method (PDM) is widely used to measure the crack length in fatigue crack propagation experiments (Johnson (1965), van Stone et al (1985) ASTM (2013)). The PDM bases in a first approach on the reduction of the cross sectional area by a propagating crack and the resulting increase in the electrical resistivity of the specimen. In fact, the measured potential depends on a densification of the potential field lines and therefore the position of the measuring contacts as well as the crack geometry play an important role (Verpoest et al (1981), Doremus et al (2015)). Especially for short cracks at notches, the form and position of the crack has a great influence on the measured potential drop (Bär and Tiedemann (2017)). This knowledge enables the development of a method for the detection and localization of cracks in fatigue experiments based on a multiple PDM measurement. Campagnolo et al (2019) have already shown that the position of the measuring contacts play an important role for the measured potential drop in notched bars. Their work was mainly focused on the calibration of the PDM using FEM but the results have shown that the crack location and form influences the measured potential drop. In the present work, the possibilities arising from the multiple PDM measurements will be discussed. The investigations are focused on the determination of the crack location and the early detection of small fatigue cracks. The experiments were undertaken on notched round bars of AISI 303 austenitic steel. The specimen geometry is shown in figure 1a. For the PDM measurements, copper wires with a diameter of 0.2 mm were laser-welded 0.5 mm above and below the notch flanks with an angle of 120° in between as shown in figure 1b. Force-controlled fatigue tests were performed under fully reversed loading conditions (R = -1) at a frequency of 20 Hz with an amplitude of 35 kN on a servo hydraulic testing machine equipped with a DOLI EDC 580V controller. To mark the crack front in some specimens, overloads with an amplitude of 70 kN were introduced in defined intervals. To force a crack initiation at defined points related to the position of the potential probes, in some specimens small cuts in the notch root were produced by a marking laser. 2. Experimental Details

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Fig. 1. (a) Specimen geometry, (b) location of the potential probes on the circumfere, (c) mounted specimen with 3 potential probes.

For the potential drop measurement, a constant current was conducted through the specimens. The clamped specimen with the 3 potential probes is shown in figure 1c. A detailed description of the testing equipment is given by Campagnolo et al (2019). The 3 potential drops were measured using an amplifier of the control electronics. For each cycle the maximum and minimum value of each potential probe were stored.