## 4 Results and Discussion
### Material Properties
This may be why specimens with induced vibration had lower strain at fracture than specimens without induced vibration. While the air gap was defined as 0% in the software, it is clear that the air gap in the part is positive. While this gap can be reduced by carefully adjusting print parameters, it is difficult to eliminate completely. Induced vibration can compensate for this by pushing material into open gaps. The images of the broken cross-section are shown in Figure 16. In the figure, the gaps between some layers are visible, indicating a lack of bonding between the layers. The samples with vibration show a better distribution of material between the layers--this may likely lead to better bonding between them. Figure 16: Microscopic images of the cross-sections at the break. The results for the second print experiment are shown in Figure 17 and listed in Table 5. This experiment employed the same set of print parameters as experiment 1, but used a layer height of 0.35 mm. A larger layer height was used to maximize the presence of voids [24], which was expected to increase the effect of vibration. As vibration increased, so did tensile strength, elastic modulus, and fracture strength. However, as vibration level increased, strain at tensile strength and strain at fracture strength decreased. S16 and S19 had very similar mechanical properties, suggesting that using these motor speeds will yield similar results. S22 had the greatest effect on the mechanical properties of the part, increasing each metric except strain at fracture strength. As vibration increases, mechanical properties will improve, but the fracture characteristics become less desirable. Figure 18 shows the fracture surfaces for different levels of vibration. No vibration specimens tended to fail along the rasters, while specimens with vibration had relatively flat fracture surfaces. Specimens with vibration had saw tooth patterns along their fracture surface, oriented in line with the alternating 45\({}^{\circ}\) rasters. It should be noted that the samples in Figure 18 do not show their final two layers. For the final layers of printing, the vibration is typically turned off to obtain a smoother surface finish. In order to visualize the pores, the printing was discontinued before the final layers are printed. Figure 17: Stress–strain curve for 0.35 mm print layer thickness with varying levels of vibration. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Tensile Strength** \\ **(MPa)** \\ \end{tabular} } & \begin{tabular}{c} **Strain at Tensile** \\ **Strength** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Elastic Modulus** \\ **(MPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Fracture Strength** \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Strain at Fracture** \\ **(MPa)** \\ \end{tabular} \\ \hline No Vibration & 26.8 & 2.09 & 2427 & 19.3 & 3.5 \\ S16 & 29.2 & 2.00 & 2545 & 21.0 & 3.2 \\ S19 & 29.1 & 2.0 & 2548 & 21.3 & 3.2 \\ S22 & 29.6 & 1.97 & 2590 & 24.4 & 3.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results from the second experiment with a layer height of 0.35 mm. The vibration did not have a significant effect on dimensional accuracy. The length was also measured like in the first print experiment but not included as a metric for judging dimensional accuracy. Lengths were within \(\pm\)0.2 mm of 168.5 mm. As vibration increased, the dimensions increased slightly, but enough to be notable. The largest deviation occurs between the no vibration and the S22 vibration level. Vibration at this speed increased width and thickness by only 0.2%, as indicated in Table 6.  
Figure 19 shows porosity for different levels of vibration. As the vibration level increased, porosity decreased. Print experiment 2 featured a relatively large layer height (0.35 mm), which is why the no vibration specimens had large porosity (11.1% volume pores). As the vibration level increased, filament was pushed into the gaps between rasters, decreasing porosity.