Influence of joint deformation on the auxetic behaviour of 3D printed polypropylene structures

Auxetic structures studied in the literature are often based on relatively stiff, metallic materials and theories regarding their response to mechanical loading cannot be translated directly to polymeric materials. As "soft" auxetics increase in popularity for applications in tissue engineering further investigation into the joint behaviour and effect on their Poisson's ratio is required. 3D printed polypropylene auxetic mesh structures were produced to compare to the requirements for biological cell-stretching devices while investigating the deformation mechanics. The behaviour of the meshes was characterised with tensile force-strain curves and high-definition imaging and the effect of joint behaviour on the Poisson's ratio was evaluated. Isolated unit cell samples of the re-entrant mesh were produced to characterise the in- and out-of-plane behaviour for geometries comprising re-entrant strut angles of 30, 45, and 60° to the tensile straining direction. Force-strain curves with three distinct phases were observed, with linear, plateau, and terminal regions characteristic of re-entrant honeycomb structures. A constant negative Poisson's ratio was measured up to a critical transition strain, at which point it is theorised that the onset of buckling triggers bending-dominated deformation to occur, out-of-plane. The production of full-scale mesh samples with the same 30, 45, and 60° geometry resulted in consistent values for critical transition strain and Poisson's ratios. An auxetic region of strain was defined, where the force is linear and a homogeneous negative Poisson's ratio can be maintained. This region represents the limit within which a biological cell-stretching device could operate successfully for the current mesh design.