PSI - Issue 28

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

929

5

Figure 4 shows the fracture surface of specimen 1 with a fatigue crack emanating near the backside of the specimen. The crack expansion was marked by introducing overloads every 15,000 cycles. By counting back the overload marks, the crack initiation can be assigned to the first overload. 3.3. Centered crack In case of specimen 2, a crack can be recognized after 11,250 cycles (fig. 5). At this point, both quotients are decreasing and below Q i = 1. In this case the potential P narrow exceeds the other two potentials indicating the initiation of a crack in the center of the notch. After 15,000 cycles the decrease of Q front is steeper compared to Q back indicating the initiation of a secondary crack near the backside of the specimen.

crack

1.001

1.000

0.999

Q front or Q back

0.998

0.997

Q front Q back

0.996

0

5,000

10,000 15,000 20,000 25,000 30,000

cycle number

Fig. 5. Run of the quotients Q front and Q back , specimen 2.

Fig. 6. Fracture surface with overload marks and potential probes, specimen 2.