PSI - Issue 68

Shiyu Suzuki et al. / Procedia Structural Integrity 68 (2025) 596–602 S. Suzuku, N. Tsushima / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 4. Optical observations after fracture in fatigue tests at RT.

Figure 4 shows results of optical observations of the lattice specimens after the fracture in the three fatigue tests. From this figure, in all of the three fatigue tests, basically single main cracks propagated in the lattice structure resulting in the macroscopic fracture as similar to the static tensile test shown in Fig. 2(b). In the inserted figure in Fig. 4, a magnified image of the fractured lattice under σ a = 30.6 MPa condition is shown. It can be seen that each strut was fractured at points of intersections with other struts, which is probably because of the stress concentrations. In Fig. 3(a), the fatigue lives of the cylindrical specimens at RT are also displayed. In comparison with the cylindrical specimens, the lattice specimens exhibit significantly lower fatigue lives which is less than one-tenth of that of the cylindrical specimens under the same stress conditions. This is probably because of numerous sites of the stress concentration in the lattice structures due to their complex geometries. To properly evaluate the fatigue lives of the lattice structure, it is necessary to take the stress concentration related to the lattice geometry into account. 3.3. Fatigue tests at 200 °C Figure 5(a) shows a S-N diagram obtained by the four fatigue tests using the lattice specimens at 200 °C. Figure 5(b) shows load-displacement curves of one cycle at the half life, N f /2, of each lattice specimen along with the curve obtained by the static tensile test (see Fig. 2) for reference. From Fig. 5(b), it seems that in all of the four load conditions, the hysteresis is quite small, and the elastic deformation is dominant. Figure 6 shows results of optical observations of the lattice specimens after the fracture in the three fatigue tests. Note that for the test with the lowest stress condition ( σ a = 14.5 MPa), no image of the fractured specimen is shown since the specimen reached the run-out condition, 1×10 7 cycles. From this figure, under lower stress conditions, σ a = 30.6 MPa and 40.9 MPa, single main cracks propagated in the lattice structure resulting in the macroscopic fracture with no apparent inelastic deformation. On the other hand, under the highest stress conditions, σ a = 50.8 MPa, two main cracks propagated from two opposite sides of the lattice structure being accompanied by a certain amount of the plastic deformation as similar to the static tensile test shown in Fig. 2(b). From these results, although the load displacement curves seem linear in all of the four tests, the fatigue behavior under lower stress conditions of σ a = 14.5 MPa, 30.6 MPa and 40.9 MPa was the elasticity dominant, i.e. high cycle fatigue, whereas the fatigue behavior under the highest stress conditions of σ a = 50.8 MPa was strongly affected by the plasticity and can be considered the low cycle fatigue. Hence, Basquin’s law was applied to the two tests’ results under σ a = 30.6 MPa and 40.9 MPa and is plotted in Fig. 5(a). In Fig. 5(a), the fatigue lives of the lattice specimens at RT are also displayed. In comparison with RT, the lattice specimens at 200 °C exhibit equivalent or longer fatigue lives when the stress conditions are low, σ a ≤ 40.9 MPa, i.e. the high cycle fatigue regime. On the other hand, the lattice specimens at 200 °C exhibit a shorter fatigue life than at RT when the stress conditions are high, σ a ≥ 50.8 MPa, i.e. the low cycle fatigue regime. In the literature, the fatigue life of the additively manufactured AlSi10Mg is considered to decrease with increasing temperatures (Wang et al., 2019, Egidio et al., 2023). For example, Wang et al. (2019) conducted high cycle fatigue tests of the additively manufactured AlSi10Mg using flat specimens at temperatures from RT to 600 °C and showed that the fatigue life