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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2.2.2. Mechanical Testing The samples were tested under monotonic loading under compression (ISO:13314), and tensile load. To determine the elastic modulus, low-cycle loading was carried out on four samples within the load limits of 20% to 70% of yield load obtained from monotonic test. Tests were carried out at room temperature, with a constant cross head speed of 1mm/min and a data sampling rate of 1kHz on Instron 8516 UTS. To calculate the displacement, a Linear Variable Displacement Transducer (LVDT) was used for compression test and extensometer for tensile testing. In each batch a minimum of 12 samples were subjected to compression-compression fatigue load. The load was applied in a staircase form within the range of 0.8-0.1F yield (Amin Yavari et al., 2015) to generate S-N curve, with a stress ratio, R=0.1. A minimum of two samples were tested at each load. The top and the bottom surfaces of the samples were polished to ensure that there were no surface irregularities. The test was stopped when the frequency of the sample decreased by 1 Hz or the number of cycles sustained by the samples exceeded 10 7 cycles.

3. Results and discussion 3.1. Porosity

Table 1. Measured porosity Structure

Measured Porosity (%)

Cubic

76.2 (0.15) 76.6 (0.32) 79.2 (0.32) 70.9 (0.18)

Star-shaped

X-shaped

Trabecular

Even though the design parameters were the same for all structures, the overall porosity displays differences among the four groups. The X-shaped samples had the highest porosity of 79%, while the trabecular samples had the lowest porosity of 71%. Cubic and star-shaped sample had similar porosity of ~76%. The differences are to be attributed to cell topology of the structure. The trabecular has the lowest number of struts per node and the highest variation in struts length, this makes the sample denser with thicker junctions thereby reducing the porosity. The properties of the structures will not be only influenced by the cell topology but also by the porosity. 3.2. Tensile and compression test The behavior of the four porous structures was studied under compression and tensile load. Stress-strain curves for representative samples are as shown in Figure 3 and Figure 4. Yield and Maximum strength values are obtained from the monotonic test, while stabilized stiffness is obtained from the low-cycle tests.