PSI - Issue 57

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

57

G. Brot et al. / Structural Integrity Procedia 00 (2023) 000–000

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discriminate self-heating regimes for Ti-6Al-4V. In all tests, A 2 f slightly drops for σ a = 200 MPa. It seems to be related to a numerical artifact not identified yet but not to SH behavior.

Fig. 3. Results of the self-heating tests at f load = 15 . 67 Hz. a) Initial heating speed S heat , i as a function of stress amplitude. b) Amplitude of the second harmonic of temperature A 2 f as a function of stress amplitude.

Figure 4 present the e ff ect of the loading frequency on the initial heating speed S heat , i . Only one SH regime is visible for the three specimens tested at f load = 800 Hz. Self-heating is more intense when f load = 800 Hz leading to a much better signal-noise ratio than when f load = 15 . 67 Hz. It means that dissipation was higher at 800 Hz as thermal boundary conditions and geometry at the gauge of specimens are similar for the two tested frequencies. Moreover, for both frequencies in the first regime: S heat , i ∝ σ 2 a . The higher dissipation should therefore be caused by an anelastic mechanism (i.e. thermodynamically irreversible but recoverable strain) as defined in Mareau et al. (2012) and not to microplasticity. Due to the high number of cycle per load step (100 000 cycles / step), specimens failed at stress amplitudes lower than the one for which a second regime is observed for grade P 1 1020 ◦ Cwhen f load = 15 . 67Hz. Fatigue crack initiation is porosity-induced in specimens P 1 920 ◦ C and P 1 650 ◦ C and microstructure-induced in specimen P 1 1020 ◦ C.

Fig. 4. E ff ect of the loading frequency on the initial heating speed S heat , i as a function of stress amplitude.

Figure 5 present the e ff ect of a plastic deformation prior to SH testing. After a tensile prestrain ( ε pl ≈ 4%), SH curves present two regimes with an intense second regime appearing at low stress amplitudes. SH curves of prestrained specimens are similar whatever the porosity level or the microstructure.