PSI - Issue 18

S. Raghavendra et al. / Procedia Structural Integrity 18 (2019) 93–100 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Table 1. Pore size and strut thickness of different batches (nominal values, from CAD files) Sample Pore Size (μm) Strut thickness(μm) 1550 1500 500 0720 700 200 1520 1500 200

2.2 Porosity and Morphology The porosity of the samples was calculated by the relative density method. The cellular specimens were cleaned and subjected to ultrasound cleaning. Later the specimens were weighed in a gravimetric weighing scale with a precision of 0.0001g. The volume of the specimen was calculated by measuring the height and diameter values at various locations. Porosity values were calculated using equation 1, where ρ is the density of the specimen and ρ o is the density of the titanium alloy Ti-6Al-4V alloy (4.42g/cm3). ܲ݋ ݎ ݋ ݏ ݅ ݕݐ ൌ ఘ ೚ ఘ ିఘ ೚ (1) The difference between the as-designed and the as-built structures are characterized by the morphological analysis. Two important parameters are considered for the analysis, strut thickness and the pore size, pore size is compared with the minimum feret diameter of the pores for comparison. JEOL JSM-IT300LV scanning electron microscope (SEM) is used for the strut thickness measurements, and Nikon stereomicroscope is used to measure the minimum feret diameter (minimum distance between the parallel tangents of the circumference of the pore). 2.3 Tensile and compression test The mechanical characterization was carried out using monotonic tensile and compression test at room temperature for three specimens in each configuration. Both the tensile and compression test (ISO 13314:2011, ISO Standards,2011) were conducted on an Instron universal testing machine at a constant cross head speed of 1mm/min and a sampling rate of 1kHz. The displacement in compression and tensile test is measured using a LVDT and 12.5mm Instron extensometer. Data acquisition was carried out using a series IX and SAX V9.3 softwares. The elastic modulus of the specimens is calculated on 0.2% strain line parallel to the elastic region of the stress strain curve. The strength of the material in tensile and compression loading is calculated by ultimate tensile strength and offset compressive strength. Ultimate tensile strength is obtained directly from the stress-strain curve while offset compressive strength is obtained along a 4% offset line from 0.2% strain line. 2.4 Finite element analysis Due to the variation in the as-designed and as-built structures, finite element analysis of the as-designed structures was carried out in both compressive and tensile loading conditions. A mesh convergence study was carried out for all the batches of specimen. The STL files are converted into volume using Autodesk Inventor, the CAD files are meshed using 10 noded tetrahedron elements (SOLID187) in HYPERMESH V12. Analysis and the post processing of the results are carried out using ANSYS V16. A multilinear material model is considered from the tensile test data of the specimen produced from SLM, with a Young’s modulus of 109 GPa. Initially, a mesh convergence study is carried out for all the batches of specimens due to the variation in strut thickness values. For regular structures as FE model of 5x5x5 unit cells were considered (Yang, 2016), for the irregular and random structures 5x5x5mm cube is considered for the analysis. To simulate the testing conditions, the bottom face of the sample is completely fixed while the top face of the sample is subjected to tensile and compressive loads. Displacement loads are applied to reduce the computation time (Helou et al., 2016) Since it is not easy to replicate the as-designed structure into a FE model. Only regular cell topology was replicated based on the average thickness values and the measured porosity, regular cubic structures are developed using by