PSI - Issue 76

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

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(a)

(b)

Fig. 3. Notch produced by (a) EDM; (b) an engraving laser, the notch tip of which shows a radius of less than 3 µm.

The determination of the fatigue limit is generally carried out using the staircase method in accordance with DIN 50100 (DIN 50100, 2022). To validate the KT diagram, this would have to be carried out in a similar manner with notched samples. However, this test method requires many identical samples for each notch size. In addition, depending on the selected limit number of loading cycles, one single test series can take several weeks. This makes the process expensive and extremely time-consuming. Since fatigue limit poses a crack arrest problem, an alternative method can be used for its determination. In the method presented here, the load is increased block wise until the failure of the specimen. However, the length of the individual load blocks must be selected so that crack arrest is reliably achieved. In the method presented here, the length of the load blocks is controlled by determining the crack arrest using a Direct Current (DC) potential probe. If a crack no longer propagates, the measured potential remains constant. To enable this experimentally, the procedure shown in Fig. 4 was implemented in the test program.

X cycles

X cycles

Cycle number N

N actual - x

N actual - 2x

N actual

M 1 -M 2

Mean value M 1

Mean Value M 2

Potential drop

Fig. 4. Flowchart illustrating the test mode.

To determine the crack arrest, the mean values of the potential M 1 and M 2 were calculated in consecutive intervals with a length of X cycles, as shown in Fig. 4. When the difference between M 1 and M 2 was smaller than a predefined value  Pot , crack arrest was reached, the force automatically switches to the next loading. This method eliminates the influence of scatter in the potential drop data, allows a reproducible determination of crack arrest during the tests and thus leads to a significant reduction in the test duration.