Evaluation of Fiber Orientation Measurement Techniques and Anisotropic Tensile Analysis of Additively Manufactured Carbon Fiber Poly-Lactic Acid

Additively manufactured parts are prone to anisotropic mechanical performance due to the time-based deposition of material, where decreased layer adhesion from the manufacturing process acts as a limiting condition for the part’s mechanical strength and applications. To increase the mechanical performance, chopped fibers can be added to the polymer matrix to increase feedstock strength while limiting process changes required. In additive parts containing short fibers, the direction of deposition is mechanically strongest due to the fibers orienting in line with material flow as a function of process parameters. The present work evaluated the relationships between fiber orientation and tensile strength for varying print parameters. Fiber orientation was measured using X-Ray Microscopy and image analysis, which was validated via a Monte Carlo simulation. A correlation between extrusion rate and fiber orientation was observed where the highest fiber orientation corresponded to the lowest extrusion rate with R² values of up to 0.90. Samples from the prints were tensile tested to determine correlation between fiber orientation and isotropy of tensile behavior. For all extrusion rates tested, as the extrusion multiplier increased, the ultimate tensile strength anisotropy decreased, however, the tensile strength of an estimated isotropic additive part was 236.78 ± 43.37 psi, approximately 25 % below that of the injection-molded samples. The use of image analysis for determination of fiber orientation shows micro scale analysis of additive parts is accessible in a research setting. The isotropy analysis of tensile strength in additive parts highlights the need to adjust print and process settings to specific boundary conditions and loading directions.