Additive manufacturing of short fiber reinforced thermoset composites with unprecedented mechanical performance

Additively manufactured, short fiber reinforced polymer composites have advantages over traditional continuous fiber composites, which include low cost and design flexibility. However, these composites suffer from low strength and stiffness as compared to their continuous fiber counterparts due to the limitation of low fiber volume. In this study, we overcame this limitation by developing a vibration integrated auger extrusion system. This direct write additive manufacturing technique allowed us to fabricate short fiber reinforced thermoset composites in intricate geometries, with unprecedented high compression strength (673 MPa), flexural strength (401 MPa), flexural stiffness (53 GPa), and fiber volume ratio (46 %). Milled carbon fibers were used as the reinforcing fibers, which were considered too short to have the ability to enhance the mechanical strength of composites. However, in this study we show for the first time that milled carbon fibers have the ability to significantly reinforce the thermoset matrix and the composites reinforced with these fibers achieve mechanical performances similar to those of composites reinforced with longer fibers. We believe that a transformation takes place at high fiber volumes on the load transport mechanism within the composites, leading to higher levels of strength and a stiffness enhancement. This pseudo transformation can give rise to short fibers that act as if they are longer, which aids in the effective transfer of tensile loads from the matrix phase to the fibers. This study also showed that the mechanical properties of the additively fabricated thermoset composites match those of ubiquitous, denser structural metals, and these properties show nearly isotropic behavior. Therefore, these systems have great potential to find immediate applications where weight reduction and component complexity are both desired.