Vertical stiffness of functionally graded hexagonal honeycomb structures for non-pneumatic tyres produced by fused filament fabrication

Non-pneumatic tyres (NPTs) are gaining popularity in applications that value durability, puncture resistance, and low maintenance. Current designs range from foam-based constructions to intricate cellular configurations. Cellular structures offer unique mechanical properties, and recent advancements in material science have introduced functionally graded materials (FGMs) as a novel class with spatially optimized properties. The design freedom offered by additive manufacturing techniques makes them ideal for producing complex NPT structures. Specifically, Fused Filament Fabrication (FFF) has been explored to produce NPTs, but examples remain limited, and the use of FGMs in NPT design is largely unexplored. This study investigates the use of FFF and elastomers, specifically thermoplastic polyurethane (TPU), to produce bicycle-sized NPTs. TPU is a promising material for NPTs mostly due to its recyclability and reusability compared to traditional rubber. Four honeycomb-based NPT designs varying in cell orientation and the presence of radial grading were developed. Vertical stiffness and energy absorption capabilities were experimentally analysed under compression. Results indicate that graded structures exhibited greater specific mechanical properties compared to uniform structures, with the highest performance observed in graded designs with horizontally oriented cells and the lowest in uniform designs with vertical cells. Additionally, an elastoplastic material model, defined by a bi-linear stress-strain relationship was employed to simulate the initial vertical behaviour of the NPTs, achieving a reasonable agreement with experimental results, with deviations ranging from 7% to 29%. These findings demonstrate the potential of using FFF to produce NPTs, and that the insights provided can help advance NPT technology.