PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 143–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5. Representative fracture surface of LPBF Alloy 400 fatigued at a), b) 400 °C; c), d) 550 °C; e), f) 650 °C; g), h) 750 °C.

3.3. Creep At first, a series of tests at (830 ± 50) °C and constant applied stresses between 50 and 100 MPa on bulk Alloy 400 were carried out. The creep curves are summarized in Fig. 6. The results show typical creep curves with a primary stage where the creep rate gradually decreases, a secondary stage with steady-state creep with strain rates between 10⁻⁵ s⁻¹ and 10⁻³ s⁻¹ and short tertiary stage ending with fracture. The true strains achieved span from 14 % (100 MPa) to 27 % (50 MPa), whereas the time to fracture spans from 2 minutes (100 MPa) to 7 hours (50 MPa).

Fig. 6. Creep curves for Alloy 400 bulk at 50- 100 MPa and 830 ° C.

Two more tests were done with applied stress of 80 MPa at 780 and 880 °C and one more at 60 MPa and 880 °C. These have been used to calculate the activation energy. The steady creep rate can be modelled as: ̇ = exp(− ) (1) where ̇ is the creep rate, A is a calibration constant, σ is the applied stress, n is the stress exponent, Q is the activation energy, R is a gas constant, and T is the temperature in Kelvin. The term corresponds to Norton's law (Kassner and Pérez -Prado, 2004; Norton, 1929) and the exponential term represents Arrhenius's law (Čadek, 1988) .