## 3 Results and Discussion
### Effects of Infill Patterns on Mechanical Properties  
This research is conducted to analyse the behaviour of infill patterns, various densities, and layer bonding in dog-bone samples via CAD and FDM processes. Failures in each printed specimen were recorded in both simulation and experiment procedures. Internal structure and defects were examined properly to investigate material properties. The main goal of FEA was to determine the patterns' reactions under the tensile test. Sometimes, slicing softwares are not able to generate specific patterns in printed products. Hence, modifying personalized infill patterns with high strength and stiffness can be achieved by CAD design. The von Mises and stresses were examined with the same condition under mentioned loads. Analysing stress-strain of CAD parts was done by choosing one element in the middle of dog-bone samples. The maximum von Mises of 4 CAD samples is recorded in Table 7 to compare each infill pattern and find out the weakest and strongest under the tensile test. Figure 8: 2D graph of SR measurements in CAD parts. Figure 7: 2D graph of SR measurements in FDM parts. As shown in Table 7, cracks and failures happened in interlayer bonding between each layer inside the samples. Initially, internal patterns inside the samples started to crack, followed by a failure in the area of the gauge length. Concentric and grid specimens were the strongest samples, among others, under 2500 N force. This means weight can be minimized by choosing an appropriate infill pattern without sacrificing strength. However, increasing density in printed samples led to heavier and stronger products. It can be seen that grid and concentric samples are the strongest while the zigzag and triangle with 10% density samples are the weakest. It is also shown how infill density can affect products' strength and stiffness. In spite of infill density effects, concentric samples have almost corresponding strength. In brief, the grid sample with 60% infill density was the strongest, followed by the concentric sample with 30% density. Figure 9 shows failure (load-extension) for each pattern with different densities in the printing process. A wide range of loads was applied to crack grid, concentric, triangle, and \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **von Mises (MPa)** \\ \hline \hline \end{tabular}
\end{table} 
Table 7: von Mises in CAD samples. zigzag patterns in dog-bone samples. Figure 9b shows that concentric FDM specimens are strongest, among other parts. Each sample cracked at a specific load which meant pattern and density were effective in their mechanical properties. A concentric pattern with 30% infill density generated by the FDM printer was the strongest sample with 2731.65 N maximum tensile load, which was close to that of the FEA process. The maximum stress at this load was 30.02 MPa. In contrast, the zigzag pattern with 30% density generated by CAD was the weakest part with 1264.45 N, among other 3D printed samples. Almost every sample cracked around 2 mm extension, while a few parts were displaced more because the infill patterns inside the parts stuck together. That was the reason to examine the internal structure to achieve a reliable result and determine the differences with FEA. Figure 9: Failure graphs of (**a**) grid, (**b**) concentric, (**c**) triangle, and (**d**) zigzag patterns for CAD and FDM parts in the experiment. A comparison between all samples was conducted to show at what load specimens started to crack. Figure 10 indicates concentric samples in both FDM and CAD had better strength while the density was lower. The following strong pattern was a grid with different densities, the same as the FEA procedure. Further, in triangle patterns, the FDM part with 10% density was almost as good as the CAD sample with 40% density. In contrast, as shown in Table 8, the weakest CAD samples were for the zigzag pattern with 1264.45 N and 1893.47 N for 30% and 50% density, respectively.