# Experimental Study of Compression Behavior on Monolayer FFF Samples
## 3 Results
### Mechanical Properties for PLA and Post-Test Geometrical Analysis
The rest of the specimen is separated from the platen by a gap. By design, the force is introduced only at that point at first until the platen has lowered far enough to make contact with the whole specimen's cross-section (Figure 12b). At this point, the compression strength has not yet been reached, so the raised starting point does not affect the measured mechanical properties too much. In fact, some comparison tests of specimens with a clipped end point resulted in almost the same experimental values for compression strength and modulus of elasticity as for those with it still attached. A while after reaching compression strength, the first outward signs of out-of-axis deformation was visible. The geometry surrounding the starting point had not yet deformed much. As the starting point of each layer was located on top of each other, the geometry had slightly different mechanical properties in this zone compared to the rest of the cross-sectional area. The progression of the specimen buckling is shown in Figure 13. The behavior of the S and H type specimens is similar to that of the C type specimens. Figure 12: (**a**) For Type C specimens, only the extrusion end point has contact with the top platen at the start of the test. (**b**) Type C specimen is flush with the top platen after initial compression. Figure 13: Asymmetric buckling of a Type C specimen (time-lapse). Normally, the compression test would be stopped before reaching the point where buckling became this pronounced. The compression strength had already been reached a while before the onset of buckling. For this study, the test was continued in order to be able to document the deformation more easily with photographs. Type C specimens were prone to asymmetric buckling due to the starting points of the individual layers influencing the specimen deformation. Most Type H specimens showed symmetric buckling. The buckling was often composed of two bulges with one buckling inwards and the other outwards. After the pure buckling phase was over, the specimen filaments started to move relative to each other. Two interesting phenomena were detected: the out of load axis displacement of individual filaments and stress-whitening (crazing) of the mauve PLA. The mauve PLA does not typically show crazing in tensile tests, but they were very noticeable in the compression tests. The stress lines were perpendicular to the filament deposition direction (radial lines compared to the load axis). Figure 14 shows the last stage of plastic deformation if the compression test is continued far past the buckling stage. The filaments started to completely separate at the junction between neighboring filaments, similar to a partial delamination process seen for composite materials. In general, the open geometry of the S-shaped specimens behaved differently from the tube-shaped geometries under compression loads. Figure 15 shows the deformed Type C specimen after the mechanical test, Figure 16 shows the Type H, and Figure 17 shows the Type S. In all cases, the plastic deformation is clearly visible. Type C and type H specimens show similar geometries, irrespective of the load supported. This proves two things, the final geometry can be independent of the compressive load and that the interlayer seam of the Type C specimen does not have a negative influence, contrary to what might be thought. However, it is true that the Type H specimens show a higher strength, the highest, despite having a similar geometry. Thus, the collapse of the layers occurs in a similar way, although the trajectory is continuous. On the other hand, without exception, the ends of the S-shape buckled inwards with only one bulge forming along the height of the specimen. With both half-circles of the S-shape bulging outwards (away from the imagined circles' centers), the center part of the specimen remained relatively free of buckling. The deformation out of the load axis in the center portion of the S-shaped specimen was negligible for most specimens.