# Towards Self-Reinforced PLA Composites for Fused Filament Fabrication
## 3 Methods
### Geometry of Tensile Test Specimens  
For determining the mechanical properties of thermoplastic polymers, there exist standards for polymers but they are not specifically designed for 3D printing. They even date back before 3D printing such that their applicability is possible but not guaranteed. Therefore, it is important to select and interpret the recommendations adequately for the intended research and use case. In this paper, the main priority was to 3D print the tensile specimens to determine the tensile properties of the material(s), similar to in [59]. The different test standards for conventional specimen production and their application to additive manufacturing are compared in [60]. From the various norms recommended in [60], the ASTM D3039 was chosen as the testing standard with the following justifications:  
1. The ASTM D3039 is specified for polymer matrix composite materials, which applies to self-reinforced PLA as well;
2. Although not specified in the ASTM D3039 itself, the standard has been used in various studies for unreinforced FFF 3D printed tensile specimens [61; 62];
3. Since the specimen geometry of the ASTM is a simple rectangular geometry, the slicer fine-tuning compared to dog bone geometries is less extensive. For dog bone geometries, the area of the curvature is critical. Abrupt endings of the layer paths in the area  
Figure 10: Top: Microscopic image of unmelted polymer for base matrix materials A and B. Bottom: Photos of unmelted granules while filament processing.  
of the curvature can cause premature failure due to shear stress concentrations, we refer to [61]. This problem is eliminated with a rectangular specimen geometry;
4. The rectangular specimen geometry is also well-suited for printing continuous fibre reinforced specimens.  
The ASTM D3039 only provides concrete information on the form tolerance of the specimen geometry, but not on the dimensions. The ASTM describes that different studies have found different width to thickness ratios to work, but ultimately this ratio needs to be adapted according to the individual experiments (we refer to Section 8.2.2 in the ASTM D3039).  
The first step was to determine the cross-sectional area of the specimen regarding the tensile machine's maximum force. Here, the machine used for tensile testing was the MTS Tytron 250 (MTS Systems Corporation, Eden Prairie, USA), which is capable of applying a maximum force of 250 N. To be able to test until failure with an adequate accuracy, the maximum force used for sizing the specimens \(F\) was set to 200 N. In the end, two geometries were established; one for testing the matrix material and one for testing the composite material where the load capacity was expected to be higher. For the unreinforced specimens (printed only with matrix material), the cross-section \(A\) was set to 4 mm\({}^{2}\). This cross-section allows (in combination with the previously mentioned testing machine) for the following maximum stress:  
\[\sigma=\frac{F}{A}=\frac{200\,\mathrm{N}}{4\,\mathrm{mm}^{2}}=50\,\mathrm{MPa}. \tag{2}\]  
As a preliminary estimation, the tensile cross-section for the reinforced, composite specimens was set to 2 mm\({}^{2}\) to compensate for the probable increase in load capacity with fibres.  
With the cross-section being set, the width \(w\) and thickness \(t\) of the specimens needed to be sized. Due to the rectangular geometry of the specimen and the clamping devices at the chair, friction clamps were employed. Therefore, it is crucial to have a sufficient gripping surface, for which the width of the specimen had to be quite high. From previously conducted experiments on commercially available PLA filament, a thickness to width ratio of 1:16 was chosen. The final geometry is shown in Figure 11.  
Figure 11: Geometry of the tensile specimen in accordance with the ASTM D3039 used for testing unreinforced specimens. Commonly used in technical drawings, A and B are the reference surfaces for the parallel tolerances as seen in the ASTM D3039.