Design, fabrication and investigation of mechanical behavior of carbon fiber reinforced composite honeycomb structures

Sandwich structures are widely used in sectors such as aviation, aerospace, automotive and defense industries due to their superior mechanical properties, high energy absorption capability and high specific strength. These structures are formed by combining different surface materials and core structures such as foam, lattice and honeycomb. Among these, honeycomb core structures are frequently preferred due to their advantages such as superior mechanical behavior and ease of fabrication. On the other hand, the design and material properties of honeycomb core structures have a significant effect on their mechanical behavior and energy absorption capabilities. In this context, honeycomb core structures produced from materials such as Nomex and aluminum in various geometries are widely used in applications. However, the tendency of thin-walled metallic honeycombs to buckling is the most important weakness of these structures, and these traditional configurations often fail to meet the desired performance requirements in applications requiring high strength. At this point, fibre reinforced composite materials offer an invaluable opportunity for the development of high strength and low weight honeycomb structures. Motivated by these facts, this thesis focuses on the design, fabrication and testing of honeycomb structures made of carbon fiber-reinforced polymer (CFRP) composite material, aiming for high specific strength and energy absorption performance. The mechanical and energy absorption behaviors of these structures have been examined experimentally in detail, considering various design parameters. In the fabrication process of CFRP honeycombs, a molding and bonding technique was applied, which allows low-cost and mass production of corrugated fiber composite sheets. CFRP composite honeycombs were produced using 3K Twill [0/90] fibre-oriented fabric and epoxy resin. In the context of examining the effects of cell topology on the mechanical behavior of honeycomb structures, three different core topologies namely square, circle and hexagon were taken into consideration in the designs and these structures were fabricated by combining corrugated sheets produced in aluminum moulds using epoxy resin. In addition, the effects of wall thickness and height on the mechanical behavior of the structures were investigated for three different values of each parameter. Following the fabrication process, the mechanical behavior and energy absorption properties of the designed honeycomb structures were examined under both in-plane and out-of-plane quasi-static loading conditions. In this context, experimental tests were performed for 81 scenarios for the design parameters considered. For each scenario, at least three repetitions of the tests were performed and the results were analyzed to reveal the effects of the design parameters on the mechanical behavior and energy absorption performance of the honeycomb structures. In this context, criteria such as strength, total absorbed energy, specific energy absorption, mean crushing force, peak crushing force and crush force efficiency were considered to evaluate the structures' performance. The performances of CFRP honeycomb structures under both in-plane and out-of-plane loading conditions were also compared with the performances of various porous structures in the literature and their superior aspects were revealed. The results showed that CFRP honeycomb structures have high mechanical strength and energy absorption performance under both in-plane and out-of-plane loading conditions, and the crushing performance of these structures can be significantly improved by selecting appropriate design parameters. The experimental results show that CFRP honeycomb structures with a density range of 155-283 kg/m3 have a strength of up to approximately 61 MPa and a specific energy absorption capacity of up to 192 J/gr. These findings reveal the potential of these structures to be used in applications requiring high specific strength and energy absorption.