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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2

Nomenclature a f,k

area of the secondary notch

n r P i

radius-coordinate of the normal vector

relative potential with the position of the potential probe i = 1, 2, 3

q variable of the Tiedemann-Formula which correlates with the inclination of the fitted curve t variable of the Tiedemann-Formula which describes the curvature of the fitted curve U i measured potential drop with the position of the potential probe i = 1, 2, 3 W f total cross-sectional area y 0 half spacing between the potential probes  crack angle 1. Introduction The DCPDM is often used to investigate crack propagation in fatigue experiments (Johnson (1965), van Stone et al (1985), ASTM (2013), Si et al. (2020)). At this method, the potential drop depends on the densification of the potential lines due to the crack propagation and is not proportional to the cracked area and the resulting increase of the electrical resistivity – this is only an insufficient approximation. Therefore, the position of the potential probes on the specimen surface as well as the crack geometry are influencing the measured potential (Ritchie et al (1971), Verpoest et al (1981), Doremus et al (2015)). Especially the form and position of the crack as well as the location of the measuring contacts have a great influence on the measured potential for short cracks at notches (Bär and Tiedemann (2017), Campagnolo et al (2019), Hartweg and Bär (2019)). In this study a more detailed investigation of crack initiation and propagation is undertaken on notched steel bars to evolve the geometrical model by Hartweg and Bär (2019). 2. Experimental Details The experiments were carried out on notched round bars of 1.4301 (AISI 304L) austenitic steel. The specimen geometry is displayed in figure 1a. For the potential drop measurement, copper wires with a diameter of 0.22 mm were laser-welded directly on the notch flank in a distance of 2y 0 = 8.08 mm with an angle of 120° in between as displayed in figure 1b. Force-controlled fatigue experiments with a maximum force of F = 30 kN and a load ratio of R = -1 at a frequency of f = 20 Hz were carried out using a servo-hydraulic testing machine equipped with a DOLI EDC580V control electronics. Overloads with an amplitude of 60 kN were introduced every 10,000 or 15,000 cycles to mark the crack front on the fracture surface. To force a crack initiation at defined positions, secondary notches with a width between 0.659 and 2.636 mm and a depth of about 150 µm were produced in the notch root by a marking laser at 0°, 15°, 30° and 60° relative to the first potential probe position as shown in figure 1c.