*3.3. Bccz*

The fatigue Wohler curves for the Bccz specimens, presented in Figure 10, remained partially completed due to the different resistance behaviors found. The specimens already reached the preset limit of 1.5 × 107 cycles at 60% of the load for the 5 mm cells and 40% for the 7 mm cells; for this reason, it was impossible to obtain complete Wohler curves. Further improvements will enhance the span of loads tested by introducing a setup to produce between 80% and 100% of the yield load. The greatest fatigue resistance of the Bccz specimens was given by the vertical struts. During the static compression tests, the vertical struts deformed and then broke with buckling-like deformation. The photos of the Bccz specimens before the fatigue test are shown Figure 11 while pictures of specimens after the test are shown in Figure 12.

In the fatigue tests, the imposed load was not sufficient to reach the buckling limit for the vertical struts. As visible in Figure 12, the failure of the specimen, when it occurs, was due to the breaking of the struts at the central nodes, as seen for Rhombic and Octet cells. The fracture then followed the plane at 45◦ only in some points, but in most cases, there was a sparse and irregular separation of the cells. Further analysis will be dedicated to a higher *σM*.

**Figure 10.** Wohler curves for Bccz specimens.

**Figure 11.** Lateral view of the Bccz AlSi10Mg specimens for fatigue test: (**a**) Bccz 5-25, (**b**) Bccz 5-30 and (**c**) Bccz 7-30.

**Figure 12.** Bccz cells subjected to fatigue: (**a**) cell size 5 mm, relative density 25%, max load 80% *σ*02; (**b**) cell size 5 mm, relative density 30%, max load 80% *σ*02; (**c**) cell size 7 mm, relative density 30%, max load 80% *σ*02; (**d**) cell size 7 mm, relative density 30%, max load 60% *σ*02.

#### **4. Discussion**

Several reports have shown that different cell topologies provide different fatigue life behaviors [40,41]. This section will provide a comparison of the three architected cells subjected to the same load path.

What is striking about the graphs reported in Figure 13 is the homogeneous trend. Bccz cells, as commented above, always performed better, followed by Octet cells and Rhombic cells. The last two cells' topologies presented similar curves, which showed different trends than that of the Bccz cells. A possible explanation for this might be that Rhombic and Octet cells are bending-dominated cells [42] while Bccz cells are stretch-dominated cells that benefit enormously when the compression load on the vertical cells does not reach the buckling limit.

(**b**)

**Figure 13.** *Cont*.

**Figure 13.** Wohler curves: (**a**) cell size 5 mm and 25% relative density; (**b**) cell size 5 mm and 30% relative density; (**c**) cell size 7 mm and 30% relative density.

The results provided are significant in at least two major respects. Firstly, they provide reliable encouraging data on the fatigue life of different cells with various cell sizes and different relative densities. Secondly, they establish some useful correlations between design parameters and fatigue life.

Despite these promising results, further work is still required to establish the real fatigue limit of Bccz cells and deeply evaluate the effect of relative density on the fatigue performance for bending-dominated cells type.
