Geometry and Constraint Dependent Energy Absorption and Mechanical Responses of Finite Elastomeric 3D-Printed Auxetic Structures

Auxetic structures, characterised by negative Poisson’s ratios, exhibit exceptional energy absorption and controlled deformation making them ideal for applications in soft robotics, wearable sensors, biomedical devices, and structural monitoring. This study investigates the mechanical behaviour of novel hybrid re-entrant chiral geometry structures utilising re-entrant arms that facilitate chiral rotation during deformation. The hybrid re-entrant chiral auxetic structures fabricated from thermoplastic polyurethane, focusing on influence of geometric aspect ratio and boundary constraints on the compression deformation responses. Small-scale finite 2x2 cell array auxetic structures were investigated to provide insights into edge-dominated deformation responses critical to structures with constrained dimensions such as wearable sensors and soft robotics. Five structural variants with constant material volume and aspect ratios ranging from 0.83 to 1.33 were fabricated by fused deposition modelling. Compression testing and structural modelling revealed substantial dependence of the mechanical properties, including stiffness, buckling, and energy absorption, on the aspect ratio and boundary conditions. The lowest aspect ratio (0.83) structure maintained linear elasticity to 1.25% strain without constraint, reducing to 0.50% with constraint. Conversely, the highest aspect ratio (1.33) structure had reduced elastic strain capacity (0.30%) due to more prominent axial stiffening. Constrained specimens had higher initial stiffness, increasing from 6 kPa to 14.1 kPa as aspect ratio was increased from 0.83 to 1.33, while equivalent counterparts without boundary constraints ranged from 2.8 kPa to 8.7 kPa. For lower aspect ratio designs, the specific energy absorption was up to 180% higher at 30% strain with the constraints. Energy absorption was diminished for higher aspect ratio designs due to greater influence of geometric stiffening. The findings provide new insights into coupled effects from geometry and constraints on mechanical responses to guide design of auxetic structures for small-scale flexible load-bearing devices. • Two-by-two TPU auxetic arrays show tuneable stiffness and deformation modes. • Lower aspect ratios improve flexibility; higher ratios increase stiffness but reduce stability. • Lateral constraints increased stiffness by 221% but reduced strain capacity. • Aspect ratio and boundary conditions govern buckling and auxetic responses in small arrays. • Constrained cells absorbed up to 180% more energy than equivalent unconstrained cells at 30% strain.