PSI - Issue 57

Pierrick Lepitre et al. / Procedia Structural Integrity 57 (2024) 395–403 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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experimental temperature rise and the fitted saturating exponential remains low. The rupture for the two other F25 coupons occurred at σ a = 1 075 MPa and σ a = 950 MPa. The self-heating curves are plotted in Figure 4, together with the F50 coupon curves. There is no significant difference between the curves, showing that the representative elementary volume is smaller than the F25 tested volume.

3.2. Self-heating model and links to the high cycle fatigue properties

Previous studies have identified, modeled and simulated the usual dissipation mechanisms of steel under macroscopic elastic loadings (Doudard et al. (2005) and Munier et al. (2014)). Munier et al. (2014) developed a probabilistic two mechanism self-heating model. It predicts two regimes: • first regime: predominant at low loading amplitudes, where Munier et al. (2014) observed intragranular disorientation evolution, • second regime: predominant for higher loading amplitudes and related to localized micro-plasticity, manifested by slip bands at the surface. The total dissipation can be expressed as ( ) = ∗ ( S 0 ) + ∗ ( S 0 ) +2 , (6) with α a coefficient representing the first regime and β a coefficient representing the second regime process intensities. The slopes of the regimes are represented by p for the first regime and m+2 for the second. S 0 represents a normalizing stress. Munier et al. (2014) showed that for various steel grades, p is usually close to 2 and m is much higher. It has been assumed and experimentally tested that the first mechanism (associated to the first regime) does not participate in the material fatigue process. So only the second regime is associated to mechanisms with an effect on material high cycle fatigue behavior. The σ AE stress at transition between the two regimes is then considered as an estimation of the mean fatigue limit σ D . Moreover, Doudard et al. (2005) showed that the relative standard deviation of the fatigue limit can be computed as = √Γ (1 + 2 ) − Γ 2 (1+ 1 ) Γ (1 + 1 ) , (7) w ith Γ the gamma function and m the slope of the second regime. The self-heating model (6) has been identified on the six bare 300M self-heating curves. The experimental values can be well described by the model and identified parameters are consistent with previous studies on steels. The identified parameters are reported in Table 1 and the model with its two regimes is plotted in Figure 4.

Table 1 – Self-heating model parameters for 300M bare steel Parameters Unity Value S 0 MPa 1 000 α µJ.mm -3 .cycle -1 2.87 p - 2.1 β µJ.mm -3 .cycle -1 0.65 m - 28