PSI - Issue 57

Grégoire Brot et al. / Procedia Structural Integrity 57 (2024) 53–60 G. Brot et al. / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 2. Evolution of the recorded mean specimen temperature T ( t ). a) Temperature variation at the beginning of the step at σ a = 400 MPa during the test of P 1 920 ◦ Cgrade at f load = 15 . 67 Hz. b) Time zooming of a) showing the sampling of T ( t ). c) Temperature variation during all loading steps on P 1 920 ◦ Cgrade at f load = 800Hz.

amplitudes higher than fatigue limit before specimen failure. As a consequence, thermal state is not steady at the end of loading step (Figure 2.c). Due to the high loading frequency, temperature T ( t ) cannot be demodulated and only the temperature increase ∆ T end and S heat , i are determined. Temperature is recorded at f IR = 10 Hz during the whole loading steps. Thermoelastic coupling source is not visible on Figure 2.c. S heat , i is estimated on the first 8000 cycles ( ≡ 10 s) of each step. Within this time, changes of heat sources in the hydraulic actuator should not a ff ect the temperature in the gauge of specimen.

3. Results

Self-heating curves obtained with f load = 15 . 67 Hz are presented in Figure 3. Evolution of temperature increment is not represented as it is almost identical to the one of S heat , i . Figure 3.a present the evolution of the initial heating speed S heat , i with the stress amplitude. A second regime is visible for wrought, P 0 920 ◦ Cand P 1 1020 ◦ C specimens. The second regime of wrought Ti-6Al-4V is much stronger than others. Using the two line method of Luong (1995), σ SH , lim is 650 MPa, 650 MPa and 525 MPa respectively for wrought, P 0 920 ◦ Cand P 1 1020 ◦ C specimens. It should be noted that no pore nor defect were detected on the fracture surface of these specimens. For P 1 920 ◦ C, P 2 920 ◦ C and P 1 650 ◦ C specimens, SH curves present only one regime and fatigue crack initiation was porosity-induced. For specimen P 2 920 ◦ C, S heat , i increased a lot only at the last step when specimen failed, it does not constitute a second SH regime. The first SH regime is common for all specimens. Although signal-noise ratio is low, S heat , i clearly increases with σ a in the first regime. Variation of the first harmonic A f is linear with the stress amplitude for all tested stress amplitudes and material grades. For all material grades, the second harmonic A 2 f strongly increases with stress amplitude σ a so that: A 2 f ∝ σ 2 a (Figure 3.b). However, these curves do not present a transition from one self-heating regime to another and do not discriminate specimens from di ff erent material grades. A 2 f is therefore not suited to