PSI - Issue 37

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

340

5

2.6

1,06

P front P back P narrow

Q front Q back

2.4

1,05

2.2

1,02 Potential quotient Q i 1,03 1,04

1.4 relative Potential P i 1.6 1.8 2.0

1,01

1.2

1,00

1.0

0

10.000 20.000 30.000 40.000 50.000

0

10,000 20,000 30,000 40,000 50,000

cycle number

cycle number

(a)

(b)

(c)

Fig. 4. (a) crack surface of a specimen with a secondary center notch, the crack is propagating nearly perfect symmetrical; (b) Run of the three relative potentials, P front and P back are nearly perfect congruent; (c) Quotients Q front and Q back are also nearly perfect congruent. Figure 5a shows the crack surface of another specimen with a centered secondary notch. In contrast to the specimen shown in figure 4 in this case the crack front is not perfect symmetrical. Especially the crack front marked by the second overload at 20,000 cycles shows that the crack has a greater extension on the backside of the specimen. This can also be seen in the run of the relative potentials (figure 5b) where P back shows higher values than P front . In figure 5c a clear separation of the two potential quotients is visible. The both quotients begin to differ after about 6,000 cycles indicating that a crack has been initiated at the secondary notch.

2.2

1,07

P front P back P narrow

Q front Q back

1,06

2.0

1,05

1,02 Potential quotient Q i 1,03 1,04

1.4 relative Potential P i 1.6 1.8

1,01

1.2

1,00

1.0

0

10.000 20.000 30.000 40.000 50.000

0

10,000 20,000 30,000 40,000 50,000

cycle number

cycle number

(a)

(b)

(c)

Fig. 5. (a) crack surface of a specimen with a secondary center notch, the crack is propagating not perfect symmetrical; (b) Run of the three relative potentials, P front and P back are separated due to the asymmetric crack-front; (c) Due to the different crack length on the front and backside the Quotients Q front and Q back are clearly separated.

3.3. Crack at a quarter of the thickness As expected, the initiation of a fatigue crack at a quarter of the specimen thickness results in an asymmetrical crack propagation as in the case of an edge crack shown in figure 3. Figure 6a shows the corresponding crack surface with the colored overload marks which were introduced in intervals of 7,500 cycles. The relative potential P front exhibits higher values than P back (figure 6b) and the potential quotients show a clear separation of the curves even before the first overload. With increasing crack length, the differences in crack length between the front- and backside are decreasing resulting in a decreasing difference in the run of the potential quotients (figure 6c).