PSI - Issue 28

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

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The tensile testing curves shown in Figure 3a indicate that the maximum stress is reached at strain values of approximately 0.02 for cubic, Star-shaped and trabecular structures. X-shaped samples had a long plateau before failure. Also, cubic samples underwent a sudden failure while in other samples the deterioration was slowed down by the presence of struts in other directions apart from vertical and horizontal direction. The compression testing curves in Figure 3b clearly indicate cubic samples had the shortest plateau compared to the other samples. On the other side, X-shaped samples had the most stabilized plateau before the initially failure at 0.175 strain values, indicating that the bending dominated structures were highly compliant under compression loading. The axial load on the vertical struts in the stretching dominated cubic samples led to higher strength followed by trabecular, star shaped, and X-shaped. As shown in Figure 4a and 4b, the curve is stabilized from the first cycle for tensile test while the it stabilizes after the first unloading cycle during the compression test. The Young’s modulus was obtained from the slope of the stabilized part of the curve. The Young’s modulus obtained from compression and tensile test is tabulated in Table 2. Comparing the values from monotonic and cyclic test a difference is observed. This difference is due to the stabilization of the structures after one cycle during the cyclic testing. This stabilization can be due to the irregularities in the struts and junctions. It can also be seen that difference in monotonic and cyclic test is large in compression loading compared to tensile. It is observed that the Young’s modulus recorded in tensile test is slightly higher than compression test for all the samples. This difference could be related to the increase in the strut thickness towards the solid grips in tensile test samples as discussed in section 2.1. The strength values tabulated in Table 3 consist of offset yield strength and maximum strength. Offset yield strength was calculated at 0.2% of the strain with a line parallel to the elastic region of the curve. Intersection of this line with the stress-strain curves was used to calculate this value. Offset yield strength in both tensile and compression are closer. Maximum strength values are higher in compression loading when compared to tensile loading. This difference is due to different deformation mechanisms, stretching in tensile and bending in compression.

3.3. Fatigue test

Figure 5 (a) S-N Curve (b) Normalized S-N Curve with respect to compression yield stress

Table 4 Fatigue strength from S-N Curve Sample

Fatigue Strength at 10 6 Cycles (MPa)

Cubic

49.50 13.50

Star-shaped X-shaped Trabecular

1.75 6.00