FABRICATION AND CHARACTERIZATION OF 3D-PRINTED CARBON FIBER-REINFORCED EPOXY COMPOSITE HONEYCOMB STRUCTURES WITH TAILORED CORE CONFIGURATIONS FABRICATED USING ENGINEERED VISCOELASTIC INKS

Abstract

This study investigates the fabrication and mechanical characterization of lightweight, moderate-to-high-strength short carbon fiber-reinforced epoxy composite honeycomb cores and sandwich structures, produced using direct ink writing (DIW) 3D printing and customized viscoelastic inks. The engineered epoxy-based inks (I1–I4), containing 10–40 wt.% short carbon fibers, exhibit shear-thinning and yielding properties, enabling mold-free DIW fabrication of bioinspired cellular structures with hexagonal (H), triangular (T), and square (S) core geometries at infill densities of 30% – 50%. These inks were also used to fabricate sandwich structures with varied core configurations. This fabrication approach results in production of honeycomb cores and sandwich structures with superior surface finish and dimensional accuracy. The base materials (I1–I4) were characterized through tensile testing as per ASTM D638 Type V standards. Microstructural analysis of the printed specimens reveals print-induced fiber alignment, with fibers predominantly oriented along the a33 direction compared to a11 and a22. The mechanical behavior of the structures was evaluated under in-plane and out-of-plane compression, as well as three-point bending, to assess their load-bearing capacity, energy absorption and flexural performance. In-plane compression tests showed stiffness improvements of 59%, 43%, and 47%, and strength gains of 106%, 93%, and 162% in triangular, hexagonal, and square cores, respectively, as infill density increased from 30% to 50%, with specific strength following similar trends. Out-of-plane compression tests showed significant improvements in load-bearing capacity as the infill increased from 30% to 40%, enhancing energy absorption (EA) by 11.7% (hexagonal), 98% (square), and 80.8% (triangular). Square cores demonstrated the highest EA, outperforming hexagonal and triangular designs. Analysis of the specific energy absorption (SEA) and crushing force efficiency (CFE) metrics indicated that the DIW-printed composite honeycomb cores are very competitive when compared to conventional continuous fiber, metal, thermoplastic, and polymer-based cores. The DIW-printed sandwich structures with integrated skins also showed enhanced flexural stiffness and load-bearing capacity with increased core layers (1L to 2L), although flexural strength exhibited a decreasing trend. Varying core geometries yielded mixed results, yet the DIW-printed carbon fiber-reinforced epoxy composite sandwich structures exhibited competitive flexural properties compared to published studies. Overall, this study highlights the potential of DIW and engineered composite inks to produce high-performance cellular cores and sandwich structures with tailored mechanical properties, paving the way for advanced lightweight structural applications in crashworthiness, aerospace, and automotive engineering.