FIBER ASPECT RATIO CHARACTERIZATION AND STIFFNESS PREDICION IN LARGE-AREA, ADDITIVE MANUFACTURED, SHORT- FIBER COMPOSITES

Previously, the authors of this study developed a methodology for predicting the fiber orientation state and effective elastic modulus of a short-fiber, polymer composite bar fabricated using large-area additive manufacturing (LAAM). The predictions were made for a 13 wt% carbon fiber reinforced acrylonitrile butadiene styrene (ABS). Predicting the internal fiber orientation state of the sample and the elastic modulus (a function of the orientation state) requires knowledge of the fiber aspect ratio. Previously, the fiber aspect ratio was chosen arbitrarily, albeit within reason. In this study, we sought to experimentally characterize it and update our predictions. We found and present both the weighted average and the number average of the fiber aspect ratio of the 13 wt% carbon fiber ABS both before and after the material has been processed in the LAAM machine. We then use these aspect ratio values to make predictions of the fiber orientation state and the resulting elastic modulus of a tensile bar. Background and Requirements Composite materials and 3D printing technology both offer very attractive advantages to the manufacturing industry. Composite materials offer high strengthand stiffness-to-weight ratios and give engineers the ability to tailor parts to meet criteria for specific loading scenarios. 3D printing, also known as additive manufacturing, also has advantages. In most cases, 3D printing involves building a part from a CAD file in a layer-by-layer fashion based on many 2D print patterns laid up at increasing z-heights. This processing technique offers a designer the ability to design a part with little wasted material, add complexity to the part for free, and avoid the need for a mold, saving time and money. Large area additive manufacturing (LAAM) of composite materials is where large scale manufacturing, additive manufacturing, and composite materials meet. However, in order for LAAM of short-fiber composites to be a dependable means of manufacturing parts, it is important to devise a method of predicting the processing effects on the final properties of a LAAM-made, short-fiber composite part. To predict these properties requires a method of first predicting the fiber orientation state within the processed part, since a non-random fiber orientation state would give the part anisotropic properties. Predicting the fiber orientation state has been well researched when it comes to manufacturing short-fiber composites in the injection and compression molding industries but not in regards to 3D printing, specifically 3D printing of the polymer extrusion deposition variety like we investigate here. Predicting the fiber orientation state involves knowing the behavior of the polymer melt flow (the velocity profile), the shape of the fibers, and how the fibers interact with their surroundings. Traditionally, fiber orientation modeling is done with the flow and orientation state weakly coupled. That is, the flow affects the orientation state, but the orientation state is not allowed to affect the flow. (This is not actually true though since the flow viscosity is a function of the fiber orientation state.) In addition, inertial effects are neglected in fiber orientation modeling and the fibers are assumed to be rigid and of uniform distribution, shape, and aspect ratio. Generally, fiber interactions with walls are not taken into account.