PSI - Issue 13

Meike Funk et al. / Procedia Structural Integrity 13 (2018) 279–284 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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3.2. Influence of the different experimental conditions on the cyclic lifetime

In Figure 2, the cyclic lifetimes for the different experimental conditions are shown. On all loading levels the experiments undertaken with a constant loading level show the highest cyclic lifetime. The introduction of a single overload after 200 cycles reduces the lifetime significantly. In case of the loading levels 60 and 80 MPa the cyclic lifetime is reduced by 30 %, for the series with a maximum stress of 70 MPa the lifetime is reduced by 10 %. The introduction of laser cuts leads to a higher reduction of 20 % in the cyclic lifetime compared to the corresponding experiments without laser cuts.

60MPa

N=3

60MPa with OL

N=5

70MPa

N=6

70MPa with OL

N=3

70MPa with laser cut

N=8

80MPa

N=6

80MPa with OL

N=4

0

50,000

100,000

150,000

200,000

250,000

300,000

cycle lifetime

Fig. 2. Cyclic lifetimes and its standard derivation for the different experimental conditions. N represents the number of tested specimens.

The formation of cracks by the introduced overload is visible in the potential drop measurements. Figure 3 shows the crack length calculated from the potential drop as a function of the cycle number. The calculation is performed with the Johnson equation (Johnson 1965) with the notch depth a 0 = 1 mm. Due to the overload, a sudden increase of the crack length and therefore the formation of a crack with a length of about 10 µm is visible. By reason that the potential drop method is an integral measuring signal this crack length corresponds to an “over the thickness crack” and no information about the number and the individual size and form of the initiated cracks are provided by the potential drop measurement.

1.02

1.01

1.00 crack length a [mm]

0.99

50

100

150

200

250

300

350

cycles

Fig. 3. Increase of the crack length due to a single overload introduced after 200 cycles.

The differences between the lifetime of the specimen with laser cuts and the specimens loaded with a single overload results from to the plastic zone, which is formed in the front of the crack tip due to the overload. This plastic zone retards the crack propagation leading to an enhanced cyclic lifetime compared to the crack initiation by laser cuts.