PSI - Issue 28

L.R. Botvina et al. / Procedia Structural Integrity 28 (2020) 1686–1693 L.R. Botvina et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 6. (a) Three-parameter Weibull cumulative distributions for SUJ2 data (Sakai et al., 2001, Harlow, 2006); (b) cumulative distributions in inverse axes obtained from literature data.

Lifetime cumulative distribution functions were also described as exponential functions with slope parameter γ. It is shown, that γ-σ dependence changes its slope after stress amplitude σ=1200 MPa (fig.7, a). The change in γ parameter was compared to fatigue lifetime curve (fig.7 b) , and it can be noticed that in vicinity of σ =1250-1300 MPa there is a discontinuity of S-N curve, showing the change of fracture mechanism due to transition to gigacycle fatigue. It was established by Sakai, Tanaka et al. (2001) that fracture initiation changes from surface to “fish-eye” type. The γ-parameter is sensitive to such discontinuity of S-N curve, proving that it can serve as informative parameter in fracture mechanism identification. The change in the mechanism of fatigue fracture approximately coincides with the beginning of a decrease in the γ-parameter with an increase in the test stress, which is indicated by the red line in fig. 7 (a).

Fig. 7. (a) Correlation between γ-parameter and stress amplitude σ; (b) S-N diagram with fracture initiation type (T. Sakai, N. Tanaka et. al, 2001). We also decided to apply Basquin’s equation (1910), typically used for S-N curves description (eq.6), for describing parameter dependence on number of cycles (eq. 7):