PSI - Issue 79

Daniel Hofferberth et al. / Procedia Structural Integrity 79 (2026) 313–321

318

Fig. 5: Fatigue test results ISO 2740

Fig. 6: Fatigue test results ISO 3928 R09

The resulting slopes of the S-N curves under alternating loading (R = -1) resulted in k = 9.2 for the sharp notched ISO3928 R09 specimens up to a lower gradient of k = 13.2 in case of the ISO3928 R30 specimens with endurable load amplitude at N = 10 6 load cycles between σ a,n = 7.1 MPa and 11.6 MPa. In case of alternating tensile loading (R = 0), the slopes of the S-N curves show comparable values but the endurable load amplitudes drop significantly to lower values between 4.3 MPa and 6.5 MPa. The resulting scatter band, based on the Gaussian log-normal distribution has a large variance between Ts = 1:15 up to Ts = 1:1.50 in case of the sharp notched specimens under alternating loading.

Table 2: Fatigue test results for the individual specimens

Specimen type

ISO 2740

ISO 3928 (R30)

ISO 3928 (R09)

Load Ratio

-1

0

-1

0

-1

0

Slope k

9.5 9.5

10.3

13.2 11.6 1.20

11.7

9.2 7.1

9.4 4.3

Endurable stress amplitude [MPa]

5.8

6.5

Scatter band T  = 1:

1.25

1.15

1.30

1.50

1.30

To determine the influence of mean stresses for bonded specimens, the individual endurable stress amplitudes were transferred into a Haigh-Diagram, Fig. 6 (a). The Haigh-Diagram is designed, that the stress amplitude is drawn on the ordinate (y-axis) and the mean stress is drawn on the abscissa (x-axis). The angle bisection under 45° forms the case of pulsating tensile loading (R = 0), where the mean stress equals the load amplitude. The limitation is given by