# Experimental Study of Compression Behavior on Monolayer FFF Samples
## 2 Materials and Methods  
It is fundamental for a compression specimen to fail first due to compression before it fails due to buckling. Therefore, thin compression test specimens cannot adopt an approximately two-dimensional geometry, as such shapes would exhibit bending almost immediately under compressive loads. This fact implies the production of a three-dimensional monolayer structure. By curving a flat monolayer into a three-dimensional form, the resulting specimen satisfies the required failure mode criteria while still being manufacturable as a single extruded filament layer. The specimen geometry for compression tests is constrained by the following basic considerations:  
* Any type of closed geometry has to be hollow in order to conserve the character of a three-dimensional monolayer. * As the specimens are created with AM, the filaments cannot be manufactured parallel to the load direction without creating significant anisotropy. Hollow geometries cannot be manufactured lying on their side without using support structures, which would modify the mechanical behavior. Thus, the filaments have to be oriented perpendicular to the load direction, i.e., the load direction is parallel to the build direction. * The shape should have as little complexity as possible. Considering these criteria, alternative geometries for monolayer AM compression specimens can be identified based on the following guidelines:
* Circular: This geometry is the simplest closed geometry. It is easy to design, slice, and manufacture. While most slicer programs are not satisfactorily capable of preparing the manufacturing of three-dimensional monolayers, the G-code for the manufacturing of this geometry type can be created from an automatically calculated G-code for a solid circular cylinder. Therefore, a circular geometry was chosen as the main geometry type (type C) to be evaluated (Figure 2). * A variant of the circular design has a helicoidal print head trajectory. Avoiding a fixed layer transition point, the z-coordinate of the extruder constantly increases during the material deposition process. This design is more difficult to handle on the first layer, where the resulting layer thickness is not homogenous, but may avoid anisotropy at the layer transition points in the original circular trajectory design. Thus, this geometry type (type H) was also investigated (Figure 2). * S-shaped: This design is very similar to the circular geometry, and thus a very simple example of an open specimen design. Due to its radii, there should not be any especially high local accumulations of stress (as there would be at any sharp corners due to thermal stress) and the print head can easily create this geometry in a back-and-forth trajectory. The G-code for this geometry type can be created by mirroring one half of the G-code for the fully circular specimens. Thus, it was chosen as the third geometry type (type S) to be tested (Figure 2). * Sinoid: This design is open as well in the shape of a sine-wave. This shape might be less susceptible to local stress accumulations compared to the S-shaped geometry, which has a sudden change of curvature at the interface between the two half-circles. It is, however, much more difficult to model with little practical applications of the shape and was thus discarded from further investigation. * Angular designs, e.g., cubic: This type of geometry is not very suited to the use in monolayer mechanical testing as there would be local stress concentrations at sharp corners because of the thermal stress distribution from the manufacturing process. Due to this, angular geometry types are not investigated here. In order to reduce the volume of material used in this research, but still allow for the placement of the compression specimens between the platens without major readjustment of the universal testing machine position, a specimen height of 20 mm was chosen. The wall thickness of the specimens is determined by the layer thickness of the material deposition during manufacturing.