PSI - Issue 76

Jürgen Bär et al. / Procedia Structural Integrity 76 (2026) 27–34

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3.2. Specimen and fatigue tests The tests were performed on flat specimens of the two steels with a length of 80 mm and a cross section in the testing area of 8 x 3 mm. Laser notches with rectangular geometry and sizes shown in Table 3 were machined in the center of the specimen by a Trumpf TruMark 5000 engraving laser with a laser spot size of 30 µm. Since this system cannot produce notches with a precisely defined depth, the size of all notches was measured after the fatigue tests on the fracture surface. For the potential drop measurement, wires with a diameter of 0.22 mm were welded in a distance of 1 mm above and below the notch using a laser welding system.

Table 3. Notch sizes. a [mm]

2 c [mm] √ [mm]

a / c

Notch 1 Notch 2 Notch 3 Notch 4

0.05

0.2 0.4 0.8 1.6

0.1 0.2 0.4 0.4

0.5 0.5 0.5

0.1 0.2 0.1

0.125

The fatigue tests were performed with a servo hydraulic testing machine under fully reversed loading conditions ( R = -1) at a frequency of 40 Hz. Guided clamps were used to minimize bending forces. The potential drop was measured using an amplifier of the control electronics (DOLI EDC 580V). The above-described testing method was implemented in the testing software ( “ test&motion+ ” ). The mean values of the potential drop M 1 and M 2 were calculated in intervals of X = 100,000 cycles. When the difference between the two mean values M 1 and M 2 was smaller than  Pot = 0.01 µV,crack arrest was achieved and the test was switched to the next loading block. 4. Results Fig. 5 shows a typical curve of applied force and measured potential drop during a test. The different lengths of the loading blocks, which are caused by the earlier or later occurrence of crack arrest, are clearly visible. In the last loading block, the fatigue limit of the specimen is exceeded, and the crack propagates until fracture. This allows the fatigue limit ∆ σ th to be narrowed down to the range between the penultimate and last loading block.

0 1 2 3 4 5 6 7

0.50

0.45

0.40

0.35

0.30

−7 −6 −5 −4 −3 −2 −1

0.25

Force [kN]

0.20

Potential drop [mV]

0.15

0.10

0

1x10 6

2x10 6

3x10 6

cycle number

Fig. 5. Force (blue) and the potential drop (red) history in a test.

For each material and notch type, three experiments have been performed. For the KT diagram, the mean value of the notch depth and the stress range at crack arrest and failure were calculated from these three experiments. In the