PSI - Issue 28

S. Raghavendra et al. / Procedia Structural Integrity 28 (2020) 517–524 Author name / Structural Integrity Procedia 00 (2019) 000–000

523

7

Figure 6 Fracture surface images for star-shaped structure (a) fracture at junction (b) crack initiation at surface defect

The fatigue test results are indicated in the form of S-N curve in Figure 5. The curves clearly indicate the effect of unit cell type on the fatigue properties of the cellular structures. Cubic samples sustained 10 7 cycles for all loads below the maximum limit of 0.8F yield . Similar results for compression-compression fatigue of cubic samples was obtained by Yavari.et.al. (Amin Yavari et al., 2015). An approximate fatigue strength was tabulated from the S-N curve of cubic samples. Instead, a clear S-N curve was obtained for Star-shaped, trabecular and X-shaped samples as shown in Figure 4. Among these three structures, Star-shaped samples had the highest fatigue strength value. Despite the lower number of struts per node, trabecular samples showed better fatigue resistance than X-shaped samples: this is due not only to the slightly lower overall porosity, but most probably to the fact that the struts are oriented in all the directions. The better outcome of Star-shaped samples is related to the presence of one extra vertical strut when compared to X-shaped sample which increased the fatigue strength by almost 8 times. In trabecular specimens the failure was not clearly visible due to their dense configuration. In cubic, star-shaped and X shaped samples the fracture is closer to the junctions as shown in Figure.6. The crack initiation in the structures was at the surface irregularities at the junctions. The propagation of the cracks was influenced by the presence of internal defects such as voids and unmelted powder. 4. Conclusion Ti-6Al-4V cellular structures with four different topologies were manufactured using LPBF process with an actual porosity of 70-75%. The topologies considered were cubic, star-shaped, X-shaped and trabecular. The samples were subjected to tensile and compression loading under monotonic and cyclic loading to obtain their strength and Young’s modulus values. Further they were subjected to compression-compression fatigue load. The following conclusion were obtained from the study:  Even if the porosity values were slightly different, the strength and stiffness values obtained from the static testing clearly indicate the effect of cell topology. Cubic samples had the highest Young’s modulus and strength while X-shaped samples had the least values. However, cubic structures displayed a catastrophic fracture behavior, while the other structures were more resilient.  X-shaped and trabecular structures which were bending dominated are highly compliant for applications with compression loading while stretching dominated structures are suitable for structural applications with axial loading.  Comparison of monotonic and cyclic test results indicated that stabilization of the structure during cyclic load was predominant in compression compared to tensile test.  S-N curves from the fatigue test indicate that cubic specimens sustained fatigue load up to 10 7 cycles for the load range considered in this study. For other structures, the fatigue strength decreased with increase in the bending dominated behavior of the unit cell topology.  Despite being bending dominated and with the lowest number of struts per node, the bone mimicking trabecular structures showed good fatigue resistance thanks to the presence of struts in all the directions.