PSI - Issue 71

Haru Fujishima et al. / Procedia Structural Integrity 71 (2025) 18–25

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f = 1 Hz. This result suggests that, in the absence of the test frequency effect, the impact of χ is minor in a range χ = 0 0.4 mm -1 . On the other hand, when the test frequency is high, the inertia force suppressing the change in deflection of the specimen becomes large. Consequently, a smaller stress than the calculated stress ( σ = 210 MPa) is applied to a crack in a specimen. As a result, as shown in Fig. 7, the crack growth decelerates, exhibiting a longer fatigue life. In addition to the effect of large f , if the stress gradient χ is simultaneously large, the crack becomes more difficult to propagate. The crack can finally cease propagating in the extreme case, as shown in Fig. 5 and 8. When the defect shape is symmetrical concerning the specimen’s axis, such as circumferential notches, the depth of a crack initiated at the notch root would uniformly increase, maintaining axial symmetry. In this case, the inertia force does not act on the specimen, and a still deflection should be kept irrespective of test frequency. From this perspective, it is a natural consequence that the results of RB fatigue tests for circumferential notches agree well with the predicted line, as shown in Fig. 2.

Fig. 5 Experimental results and prediction of S-N curve for specimens containing the identical drilled holes subjected to ɱ = 210 MPa

Fig. 6 Crack growth curves for = 0.2 mm -1 and 0.4 mm -1 at the same f = 1 Hz.

Fig. 7 Crack growth curves for the same = 0.2 mm -1 at f = 1 Hz and 67 Hz.

Fig. 8 Crack growth curves for the same = 0.4 mm -1 at f = 1 Hz and 67 Hz.