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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surfaces show a single initiation on the surface followed by a propagation step. The self -heating curves are plotted in Figure 3, showing an excellent repeatability between the three bare 300M F50 coupons. Figure 2 – θ evolution during each loading block of a self-heating test on F50 (from σ a = 100 MPa to σ a = 975 MPa), f r = 135 Hz

Figure 3 – Self-heating curve of three F50 bare 300M steel, R = -1

To overcome machine limitations, the same protocol has been applied on three F25 type coupons. Indeed, thanks to a smaller cross-section, higher stress amplitudes can be applied. Also, as the ratio of heated volume to exchange surface decreases, τ eq decreases too. Moreover, as the Amsler Vibrophore works as a mass-spring system, with the coupon playing the spring role, decreasing the cross-section also decreases the loading frequency (f r = 118 Hz). As the number of cycles for a loading block is 5f r τ eq , the required number of cycles per loading block is reduced and more loading blocks can be performed before rupture. Rupture was reached for σ a = 1 150 MPa loading block on first bare 300M F25 coupon. We assumed that the dissipation is constant per cycle, simplifying the heat equation resolution and summarizing the temperature rise as a saturating exponential. For lower loading amplitudes, the temperature rise actually follows well the described evolution. However, for higher loading amplitudes the temperature rise evolution diverges slowly from that saturating exponential shape. In Figure 2, for the greatest temperature rise evolution plotted, we can notice that the temperature does not completely stabilize but continues to slowly increase. For the first tested F25 coupon, the two last loading blocks ( σ a = 1 100 MPa & 1 150 MPa) clearly indicate that the temperature is not yet stabilized. This is why the last block considered for the self-heating analysis is σ a = 1 075 MPa, as the difference between the