## 5 Observations and Discussion  
Figure 5 and Table 4 show the data collected during the experiments, where the consistency between replications was good with the exception of Case 6. Note also that the pattern of response values is very similar for both axial and radial cases, suggesting that the expected behavior of each factor combination is similar regardless of loading orientation. For the axial cases, only the wall thickness was found to be significant, while all the factors except the use of the soft printed infill were significant on the radial loadings. However, it should be noted that the \(p\)-values for Factor A and Factor C were below \(p=0.10\) for axial loading, so a larger sample size may show significance in a more extensive experiment. The dramatic difference between replications for Case 6 may have been caused by a hidden defect in the shell for Replication 2 or may be a feature of this combination of factors; this requires further analysis in future work. Figure 6 shows examples of some of the buckled ABS cylinders, both with and without foam reinforcement. Note that the axial loading (both cases) and radial loading (with foam) resulted in fracture on shell collapse, while the other case shows more traditional buckling behavior expected (such as seen in [12]). The foam-filled shell in Case 4 (Figure 6a) also shows this, even though fracture did happen upon shell collapse.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multirow{2}{*}{**A**} & \multirow{2}{*}{**B**} & \multirow{2}{*}{**C**} & \multirow{2}{*}{**D = ABC**} & \multirow{2}{*}{**Run**} & \multirow{2}{*}{**Loading**} & \multicolumn{3}{c}{**Critical Load (\(N\))**} & \multirow{2}{*}{**Mean (\(N\))**} & \multirow{2}{*}{**Stdev (\(N\))**} \\ \cline{6-9}  & & & & & & & & **Rep 1** & **Rep 2** & & \\ \hline
1 & - & - & - & - & (1) & Axial &  
Before the experiments were completed, it was reasonable to expect that the choice of PLA (far more stiff and dense than ABS) for the build material and the use of a thicker wall would provide much higher strength in general. However, the dramatic effect of using the foam and soft infill reinforcements was not anticipated, since these media are too soft to support much load on their own. It could be concluded from this that the infill, especially in the axial loading cases, provided a \"pressure\" on the cylinder wall that prevented buckling far more effectively than the strength of the reinforcing material itself.  
The fact that these factors can be observed to have a large impact on the results but not be significant factors (especially in the axial case) demonstrate that a more rigorous experiment is needed to explore the factor interactions. An initial exploration of this was done (Figure 7) along with this study, but a two-way ANOVA with a full factorial experiment would be able to provide more information. Some interactions that can be observed are the use of an infill structure with material choice for the axial case and material choice with the use of foam reinforcement for both loading cases.  
Figure 6: Examples of post-buckling specimens under: (**a**) axial and (**b**) radial loadings.