Microstructure and mechanical properties of three dimensional-printed continuous fiber composites

Additive manufacturing processes have a demonstrated capacity for flexible production of metal and polymer components. More recently, capabilities for three dimensional printing of continuous fiber reinforced composites were developed. As with printed metal and polymer materials, printed composites will exhibit a unique microstructure with morphological features and process artifacts that manifest on multiple length scales. The aim of this research was to investigate the microstructures of various printed continuous fiber composites and determine linkages to consequent mechanical properties such as stiffness and strength. Samples investigated in this study comprised unidirectional carbon fibers in nylon matrix, unidirectional Kevlar fibers in nylon matrix, and Kevlar fibers in nylon matrix aligned at +/- 45 degrees directions. Tensile properties of the samples were evaluated along with comparison to expected properties. Fiber volume ratios were analyzed by thermogravimetric analysis. Scanning electron microscopy and optical microscopy were used to observe and characterize the hierarchical microstructure. Both strength and stiffness were approximately 30-40% weaker than traditionally produced composites, owing to features such as imperfect interfaces between printed layers, microvoids, incomplete fill density, and similar process artefacts. Future work will investigate mitigation of such effects through process modifications and post-processing to produce higher performance printed composites.