PSI - Issue 5

Jürgen Bär et al. / Procedia Structural Integrity 5 (2017) 793–800 Jürgen Bär et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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specimen shown in figure 2 the crack initiation lifetime using a technical initial crack length is reduced from about 71,600 cycles (about 63 % of the total lifetime) to about 3,200 cycles (about 3% of the total lifetime) using the minimum criterion.

0.210

crack length a = 250 µm

0.208

0.206

0.204

0.202 Potential [mV]

0.200

start of crack propagation

0.198

0

20,000

40,000 Cycles

60,000

80,000

Fig. 2. LOESS smoothed potential data of a fatigue experiment. The inception of the crack propagation can be set in the minimum of the curve.

The differences in the crack initiation lifetime between the 250 µm and the minimum criterion was investigated on SEN-specimen with notch depths of 1, 2 and 3 mm loaded on different stress levels. In figure 3 the ratio between the crack initiation and the total lifetime is shown in relation to the different notch depths and loading levels. In case of the 250 µm criterion, the portion of the initiation lifetime decreases with increasing notch depth and increasing loading amplitude. Relating to the specimen with a notch depth of 1 mm, more than 60 % of the cyclic lifetime is covered by the propagation of short fatigue cracks, i.e. until a crack length of 250 µm is reached. For notches with a depth of 2 and 3 mm, this portion is reduced, but still the short crack propagation covers a big part of the cyclic lifetime. These results clearly indicate that in order to get more reliable lifetime predictions the short crack propagation has to be considered.

1.0

a = 250 µm

minimum

a k = 1 mm a k = 2 mm a k = 3 mm

0.8

0.6

0.4 N Initiation /N failure

0.2

0.0

50

60

70

80

Stress [MPa]

Fig. 3. Percentage of the crack initiation lifetime for different stress levels and notch depths.