Additive Manufacturing of Gradient Stiffness Honeycombs Using Thermoplastic Polyurethane Composite Material Variations

Gradient stiffness structures are gaining attention for their energy absorption characteristics. This article explores an approach to designing honeycomb structures with gradient stiffness achieved through material variations in porous, solid, and carbon fiber (CF)‐reinforced thermoplastic polyurethane (TPU). Unlike conventional methods using thickness variation, the gradient stiffness is engineered by varying TPU composition within the honeycomb. Using a dual‐head 3D printer, one head dispensed TPU with a chemical blowing agent for porous and solid TPU, while the other extruded CF‐reinforced TPU. Tensile testing and scanning electron microscopy are employed to characterize stiffness variations and microstructure of the materials. Gradient honeycombs with up to three stiffness levels are fabricated by alternating print heads and adjusting temperatures. Compression tests are conducted at a strain rate of 0.025 s ⁻¹ , incorporating digital image correlation, to generate stress–strain curves, which highlighted unique densification and plateau regions. Honeycombs made entirely of CF‐reinforced TPU excelled at absorbing energy at stresses above 0.2 MPa, whereas porous TPU structures performed best at lower stresses. Gradient stiffness honeycombs exhibited superior densification strains, reaching up to 59%, compared to nongradient counterparts (48%–52%). CF TPU demonstrated the highest specific energy absorption value of 0.09 J cm ⁻³ , while the honeycomb structure made from porous TPU showed the lowest at 0.02 J cm ⁻³ up to the densification point. Gradient stiffness honeycombs exhibited values ranging from 0.028 to 0.077 J cm ⁻³ . However, when subjected to varying application stresses, these gradient structures display favorable energy absorption performance across a broader range of stress conditions.