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

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Fatigue crack growth measurements The following step after fatigue tests was fracture surface analysis with special attention devoted to the striation spacing and fatigue crack growth mechanism. Selected specimens strained with a total strain amplitude of 0.5% and 0.7% were thoroughly analyzed in SEM. The estimation of stress intensity factor range ∆ K needed in eq. (1) was carried out by the eq. (2) obtained from (Murakami and Aoki 1987) for a cylindrical specimen with an elliptical crack where crack length a was the distance between the crack initiation site and the start of each striation, F i is the correction factor and σ a stress amplitude. In eq. (1), C and m denoted the material constants of the Paris Law, while da/dN represented the fatigue crack growth rate. = ∆ (1) ∆ = 2 √ (2) 3. Results and discussion The initial microstructure of LPBF and HR alloy 699 XA is depicted in Fig.2. As can be seen in EBSD micrographs (Fig. 2b, d) the microstructure of both material batches consists of polyedrical grains with annealing twins. The average grain size excluding twin boundaries of HR and LPBF material is (63 ± 44) µm and (64 ± 55) µm, respectively. It is clear from EBSD maps, Fig. 2b, d show that the annealed microstructure exhibits no significant preferential orientation. A typical feature of LPBF material is the aluminium-rich oxide film that borders the grain boundaries (Fig. 2d). The presence of LPBF defects like lack of fusion or gas porosity were found scarcely. Which implies high quality of LPBF printing process. This finding is complementary to results from Archimedes method (relative density of 99.87%).

Fig. 2 The microstructure of 699 XA after solution annealing a) detail of HR material b) EBSD map with IPF coloring of HR sample horizontal section – L.D c) detail of LPBF materials (Electrolytic polishing) d) EBSD map with IPF coloring of LPBF horizontal section – L.D The fatigue life curves in the Basquin representation and Manson-Coffin representation of alloy 699 XA cyclically strained at room temperatures on specimens manufactured either by LPBF and HR are displayed in Fig.3. Experimental data in the case of Fig 3a specifically, the stress amplitude vs the number of cycles to fracture were