PSI - Issue 28

Levke Wiehler et al. / Procedia Structural Integrity 28 (2020) 925–932 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The direct current potential drop method (DCPD) is widely used to measure the crack length in fatigue crack propagation experiments (Johnson (1965), ASTM (2013)). In this method a time-stable current is conducted through the specimen during the test. As soon as the crack grows, the measured potential difference increases because the potential field lines are densified in front of the crack tip. The measured potential is significantly influenced by the position of the potential grips (Ritchie et al (1971), Doremus et al (2015)). Bär and Tiedemann (2017) have shown that the measured values depend on the shape and the position of the crack. Therefore, the measurement with multiple potential probes can be used to detect and localize cracks in fatigue experiments (Hartweg and Bär (2019)). The investigations of the present study concentrate on the determination of the crack location, the early detection of small fatigue cracks, as well as the determination of the shape of the crack front in single-edge notched specimens. 2. Experimental Details The experiments were carried out on single-edge notched specimens. The geometry and dimensions of the specimen are displayed in figure 1. The specimens were made from sheets of high-strength aluminum alloy with the material designation EN AW 7475 T761. Secondary notches were lasered in the notch root with a length of 250 μm to force crack initiation at a defined position. For the potential drop measurement, copper wires with a diameter of 0.2 mm were laser-welded to the specimen. As shown in figure 1 (b), three potential probes were placed on the front-, back- and narrow side of the specimen. The distance between the two contact points of a potential probe was 3 mm. Fatigue tests were carried out using a servo-hydraulic testing machine equipped with a DOLI EDC 580 control electronics. The test parameters are listed in table 1.

Fig. 1. (a) Specimen geometry [mm]; (b) location of the potential probes as top view.

Table 1. Test parameters.

base load

overload

stress

frequency

R

stress

frequency

interval 15.000

70 [MPa]

20 [Hz]

-1

175 [MPa]

1 [Hz]

A DC source conducted a constant current through the specimen leading to a potential at the beginning of about U 0 = 0.2 mV. A detailed description of the measurement technology is given by Campagnolo et al (2019) and Hartweg and Bär (2019). In each cycle, the maximum of the potential was measured. Because the tests differ, e.g. due to the quality and the exact location of the welding spot, length of the copper wire, laboratory environment etc., the absolute measurement results vary and are therefore not comparable with one another. To remedy this, the measurement results were standardized by dividing the potential difference U i by the initial voltage U i,0 , defined as the mean value of the potential of 200 cycles at the beginning of the experiment (see equation 1). � � � � � ��� ��� � � � �� ∙ ∑ � � ��� �� �� (1)