# Experimental Study of Compression Behavior on Monolayer FFF Samples
## 3 Results
### Mechanical Properties for PLA and Post-Test Geometrical Analysis  
The compression properties of the three types of specimen were determined experimentally. The mean resulting compression strength and modulus of elasticity values can be found in Table 1. The data obtained are compared with the literature values obtained on standardized non-monolayer specimens. Figure 9 shows the comparison of the mechanical compression behavior of the three geometry types. All data obtained for each sample have been represented, the compression strength in Figure 10, and the compression modulus of elasticity in Figure 11.  
The compression strength values were very similar for all three types of specimens, with the helicoidal trajectory type performing slightly better. This suggests that the defects associated with the manufacture of each specimen do not significantly influence the compression behavior of the specimens. As the specimen Type H was both a closed geometry as well as fairly uniform due to the missing fixed layer transition point (as it was present compared to circular trajectories), this was expected. This can be seen both in the mean values and in the behavior of the stress-strain curve. In all three cases, a very similar elastic zone can be observed, with very similar behavior. The specimens with closed geometry \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen Characteristics** & \begin{tabular}{c} **Compression** \\ **Test Speed** \\ **[mm/min]** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile/Compression** \\ **Strength [MPa]** \\ \end{tabular} &
\begin{tabular}{c} **Modulus of** \\ **Elasticity** \\ **[MPa]** \\ \end{tabular} \\ \hline Circular monolayer specimen for compression testing & 2 & 133.3 \(\pm\) 1.7 & 4005 \(\pm\) 92.0 \\ Helicoidal monolayer specimen for compression testing & 2 & 135.9 \(\pm\) 3.0 & 2783 \(\pm\) 74.0 \\ S-shaped monolayer specimen for compression testing & 2 & 132.6 \(\pm\) 2.0 & 3355 \(\pm\) 126.0 \\ Literature value [14] & 0.34 & 108 & 2300 \\ Literature value converted to SI units from [7] & 1.33 & 93.8 & n.a. \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the literature and experimental compression properties of PLA. Figure 9: Comparison of mechanical behavior of three sample types under compression conditions. have a similar maximum stress, with Type S being lower because it is open. In the plastic deformation zone, Type S has the steepest drop, suggesting a lower ductility or capacity to maintain strength after the elastic limit, which may be caused by its open geometry. Thus, it appears that the helical path is the one that best withstands the compressive conditions although with little difference with respect to Type C, while the open geometry offers a worse result, with a more premature collapse. However, the compression modulus of elasticity varied significantly for the different types of specimens. While it was highest for Type C specimens (which had added stiffness due to the layer transition point), the helicoidal routes had a severely reduced modulus. The end point of the compression specimen extruder paths protruded from the geometry in z-direction. This end point was clipped manually for one of the specimens with no strong effect on mechanical properties. This validated the assumption that the end  
Figure 11: Compression modulus of elasticity of different samples obtained. Figure 10: Compression strength of different samples obtained. point did not invalidate the compression specimens' CAD and CAM design for systematic mechanical testing. During the compression tests, no fracture of the specimens occurred. The specimens ultimately fail due to local or global buckling. The plastic deformation of the specimens after reaching their compression strength was characteristic of the geometry and trajectory type under test. At the start of the compression tests, when the upper platen already touched the specimen with the necessary pre-stress applied, only the extrusion end point is in contact with the platen, see Figure 12a.