Abstract
Despite their low cost and design flexibility, additively manufactured, short fiber composites (SFCs) have low strength and stiffness compared to their continuous fiber counterparts. In this study, we overcame this limitation by developing a vibration integrated, auger extrusion system which allowed us the fabrication of SFCs at intricate geometries with high mechanical performance. This thesis treats three main research topics on the principle of using different short chopped fibers. First, short chopped carbon fibers were used as reinforcement which were considered to be too short to enhance mechanical strength of composites with unprecedentedly high strength (>400 MPa), stiffness (53GPa) and fiber volume (46%) up to now. We showed in this study for the first time that, at high fiber volumes of these fibers, a transformation takes place on load transport mechanism within the composites and higher levels of strength and stiffness enhancement were obtained. This fictitious transformation giving rise of short carbon fibers to act as if they are longer, helps effective transfer of tensile loads from matrix to fibers and this results in unprecedented mechanical performance of these material systems. Using these fibers also showed that the mechanical properties of the additively fabricated thermoset composites match those of commonly used structural metals. These properties show nearly isotropic behavior and therefore these composites have great potential to find immediate applications where weight reduction and component complexity are desired.
The second research goal in this work is to fabricate a thermoset-based Kevlar fiber using direct write additive manufacturing. This was also performed by utilizing the developed vibration integrated, auger extrusion system. This system enabled us to apply highly viscous materials based on the presence of Kevlar fibers. We found out that using 6.3% of Kevlar fibers into the epoxy matrix as volume percentage was possible and printed successfully. We also found out that additively manufactured thermoset-based Kevlar fiber with high mechanical performance such as low weight, high strength, and high ductility can be achieved which have a great potential to open doors for wide range of novel applications.
Our third aim was toward the additively manufactured syntactic foams composites. This is due to their advantages over traditionally fabricated foams in terms of design flexibility, in-field fabrication and the low investment cost. Unfortunately, current additive manufacturing methods developed for thermoplastic syntactic foams suffer from unavoidable porosity and low mechanical performance. likewise, in this topic, we overcame these limitations by fabricating thermoset based syntactic foams using direct write additive manufacturing which allowed us to fabricate buoyant syntactic foams with unprecedented strength (>100 MPa) and modulus (1.2 GPa). The achieved mechanical performance of these materials can be tailored by reinforcing the thermoset foams via short carbon fibers. Additively manufactured thermoset based syntactic foams with high scalability and tailored mechanical performance have great potential to find immediate applications where weight reduction, mechanical performance and component complexity are desired.