PSI - Issue 52

Tomáš Vražina et al. / Procedia Structural Integrity 52 (2024) 43 – 51 Tomáš Vražina et. al./ Structural Integrity Procedia 00 ( 2022) 000 – 000 5 plotted in the bilogarithmic representation and fitted by eq. (3) where ´ is the fatigue strength coefficient and b the fatigue strength exponent. In the high cycle fatigue region of the Basquin representation, the curve representing HR material has sharper steepness in comparison with LPBF material. This does not usually happen in the case of heat-treated state or in the as-build state as proposed in (Hosseini and Popovich 2019). log = b log(2 )+log ´ (3) The plastic strain amplitude vs the number of cycles to fracture was represented by bilogarithmic scale in Fig 3b with experimental data fitted by eq. (4) where ´ is the fatigue ductility coefficient and c is the fatigue ductility exponent. Estimated coefficients obtained using the least square fit procedure from eq. (3) and eq. (4) are incorporated in Table 2. The curve representing LPBF material is slightly shifted above HR samples this deviation is low and almost hidden in the scatter of the experimental points. log = log(2 )+log ´ (4) Table 2 Calculated LCF parameters of VDM 699 XA. LPBF HR b (-) -0.08 ± 0.01 -0.06 ± 0.01 ´ (MPa) 970 ± 50 790 ± 80 c (-) -0.67 ± 0.15 -0.68 ± 0.59 ´ (-) 0.17 ± 0.03 0.18 ± 0.13 47

Fig. 3 Fatigue life curves a) Basquin representation b) Coffin-Manson representation.

The softening/hardening curves of 699 XA tested in the total strain amplitude range of 0.2%-0.7% are presented in Fig. 4 a for HR material and Fig. 4 b for LPBF material. Both materials show the identical character of cyclic hardening/softening curves. Significant cyclic hardening can be seen for both material batches in the whole range of total strain amplitudes. Cyclic straining with total strain amplitude ε a ≤ 0.25 % resulted in fatigue hardening until the fracture of specimens. In the case of cyclic straining with ε a ≥ 0.3 5 %, the initial fatigue hardening is replaced by a cyclic softening phase until the final fracture. This particular behavior is typical for materials that have undergone solution annealing. As noted by (Pham et al. 2011) and later proven by (Heczko, Polák, and Kruml 2017), the inceptive hardening observed in this case is related