# Enhancing Structural Performance of Short Fiber Reinforced Objects through Customized Tool-Path
## 3 Material Characterization  
### Mechanical Property Measurement  
As various materials for FDM have been developed, associated test methods have been reported. Nevertheless, American Society for Testing and Materials (ASTM) test protocols for additive manufactured objects officially has not been developed [32; 33]. In this study, CFRP is treated as a laminated plastic polymer. CFRP-Nylon (20% fiber volume fraction) coupon specimens with printing orientation of 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\) were fabricated, and relevant mechanical properties were measured. As shown, direction 1 (red color) is the FDM line of deposition, and direction 2 is perpendicular to this line of deposition. From the printed specimens oriented at 0\({}^{\circ}\), tensile modulus in the deposition direction E1, Poisson ratio \(\
u_{12}\) and tensile strength S1 were determined. From the specimens oriented at 90\({}^{\circ}\), tensile modulus perpendicular to the deposition direction E2, Poisson ratio \(\
u_{21}\) and tensile strength S2 were determined. From the specimens oriented at \(\pm\)45\({}^{\circ}\), shear modulus at the 1-2 plane \(G_{12}\) and shear strength S12 were determined. Three specimens per sample were tested for each one of the three orientation cases, and each printing material, totalizing 9 test runs.  
For the determination of stiffness and strength properties at material directions 1 and 2, the ASTM D638 test standard was applied. As shown in Figure 4, I-type (dog-bone) specimen was used, with length and width of 165 mm and 19 mm, respectively. Letter-size sheets (11\({}^{\prime}\)\(\times\) 8.5\({}^{\prime\prime}\)) were printed by the designated printing orientations and cut by a CNC machine to obtain specimens. The nominal specimen thickness was 3.3 mm, with 11 printed layers. ASTM D3518 test method was conducted to obtain the 1-2 plane shear stiffness and strength properties. Rectangular specimens (25 mm \(\times\) 200 mm) were printed. The nominal thickness is 4.8 mm with 16 printed layers, and the stacking sequence was [\(\pm\)45\({}^{\circ}\)]4s (symmetric).  
Ultimaker 2+ was used to produce test specimens. It was one of the popular 3D printers at the time of study, allowing users to upgrade it to apply CFRP filaments. The printing envelope of 215 mm \(\times\) 210 mm \(\times\) 180 mm was used. For the size of the nozzle, it has been reported that a 0.4 mm nozzle is frequently clogged with short fibers. On the other hand, 0.8 mm is too big to obtain precise fabrication. Manufacturer of Ruby nozzle, which can endure weariness from short fibers, recommended 0.6 mm. Therefore 0.6 mm was chosen in this study. The printing parameters employed were; nozzle extrusion temperature: 260 \({}^{\circ}\)C, heat bed temperature: 100 \({}^{\circ}\)C, deposition line (layer) height; 0.15 mm, and printing speed: 20 mm/sec. Carbon-fiber nylon filament (NylonXTM from Matterhackers, filament diameter: 2.85 mm) made of 4043D resin reinforced with chopped short carbon fibers (20% weight fraction) was used. The experiments were performed using SHIMAZUTM AGS-X HC universal testing machine (Figure 5b). A fully calibrated extensometer was utilized to precisely capture displacement change. All the specimens were loaded up to material failure at a displacement rate of 1 mm/min. The data acquisition rate was 10 Hz.  
Figure 6 shows the stress-strain curves for the specimens printed in the three different directions. Captured data from the machine was 10 points-average smoothed. As shown, the directional dependency of material properties is noted. Table 1 shows the mechanical properties of CFRP-Nylon (NylonXTM) which were extracted from the stress-strain curves and used in the FE modeling analysis of FDM parts. [PERSON]'s moduli and tensile yield strengths, as well as Poisson's ratios, were obtained for the printing directions of 0\({}^{\circ}\) and 90\({}^{\circ}\). Moreover, shear strength was experimentally determined per ASTM D3518.