PSI - Issue 37

Jürgen Bär et al. / Procedia Structural Integrity 37 (2022) 336–343 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

337

2

Nomenclature U i

measured potential drop with the position of the potential probe i = front, back, narrow relative potential with the position of the potential probe i = front, back, narrow

P i Q i

potential Quotient with i = front, back

1. Introduction Since more than 5 decades the Direct Current Potential Drop Method (DCPDM) is used to measure the crack length in fatigue experiments on specimens (Johnson, 1965; Si et al., 2020) and components (Černý, 2004) . A big advantage of this method is the possibility to determine the crack length during the running experiment automatically which allows for example crack propagation experiments under stress intensity-controlled conditions (Bär and Volpp 2001; Tesch et al. 2007). Although the DCPDM is less sensitive to short cracks, it is also used to detect crack initiation in fatigue experiments (Bauschke and Schwalbe 1985; Meriaux et al. 2010; Bär and Tiedemann, 2017). The measured potential strongly depends on the position of the potential probes on the specimen as well as on the shape of the crack front (Ritchie et al., 1971; Doremus et al., 2005). This is due to the fact that the potential drop measurement bases on a densification of the potential field lines in front of the crack tip (Verpoest, 1981). This effect was used by Enmark et al (1992). They attached four potential probes in a line on the surface of a specimen to determine the shape and the depth of a surface crack. Tada et al (2011) determined the geometry and the location of semi elliptical cracks in a specimen by applying an array of potential probes on the specimen surface. These measurements were undertaken on specimens with a known crack location or on specimen with “cracks” introduced by electrodischarge machining. Investigations by Bär and Tiedemann (2017) on fatigue specimen showed that the shape and the location of the initiated short cracks influences the potential values. Investigations on notched round bars revealed the influence of the crack geometry and position on the measured potential (Campagnolo et al., 2019). Hartweg and Bär (2019) used this effect to detect the crack initiation site by applying three potential probes on the circumference of notched round bars. Using a simple vector-based description it was possible to determine the crack initiation site and time for a single crack initiated on the circumference of the specimen. First experiments on edge notched specimens were undertaken by Wiehler and Bär (2020). They also placed three potential probes on the specimens and calculated quotients between the potentials to determine the crack initiation site. With this method it was possible to detect if the crack was initiated on the back- or frontside of the specimen. In this work a more detailed investigation of crack initiation and propagation is undertaken on single-edge notched specimens. The specimens were equipped with three potential probes – on the frontside (U front ), on the backside (U back ) and on the narrow side (U narrow ) of the specimen. During the fatigue tests the three potentials were measured simultaneously using amplifiers of the control electronics. The crack front was marked on the fracture surface by introducing overloads in defined intervals to allow a direct comparison between the measured potential drop and the real crack location and crack front geometry. 2. Experimental Details The experiments were carried out on single-edge notched specimens made from cladded sheets of the aluminum alloy EN AW 7475 T761 with a thickness of 2.85 mm. In the test section with a width of 20mm a notch with a depth of 1 mm was machined on one narrow side. For the potential drop measurement, copper wires with a diameter of 0.2 mm were laser-welded to the front, back and narrow side of the specimen (Figure 1 (a) and (b)) in a distance of y 0 = 1.5 mm to the notch root (figure 1 (c)). Fatigue tests with a maximum force of F = 9 kN and a load ratio of R = 0 were carried out using a servo-hydraulic testing machine equipped with a DOLI EDC 580 control electronics. Overloads with a maximum force of F OL = 18 kN were introduced in defined intervals to mark the crack front on the fracture surface.