Towards a Micromechanics Model for Continuous Carbon Fiber Composite 3D Printed Parts

Material extrusion is transitioning from a technology mainly for rapid prototyping to one that is increasingly finding use in manufacturing functional parts. Of particular interest in this regard is the reinforcement of extruded parts with Continuous Carbon Fiber (CCF). However, predicting the effective properties of 3D printed composite parts presents a unique challenge because of the strong effects of meso-structure on the mechanical behavior of printed parts. This work aims to develop a mathematical model that would enable such a prediction of behavior by incorporating the rule of mixtures commonly used in micromechanics modeling. Results from tensile tests on composite specimens with varying volume fractions produced from a blend of onyx (nylon and chopped carbon fiber) and CCF are reported. Volume fractions are varied through a range of factors including the layers with fiber, the distribution of fiber within layers and the angle of the fibers relative to the loading direction, though findings suggest that this has no significant influence on the model itself, and that volume fraction is a sufficient parameter. The predictive ability of the micromechanics model is put to the test for composite honeycombs under compression, and a wide discrepancy between model and experimental result is demonstrated, demonstrating the limitations of such a model and suggesting pathways for improvement.