4D Printed Auxetic Structures with Tunable Mechanical Properties

Abstract

Additive manufacturing (i.e., 3D printing) has revolutionized the entire design cycle from prototyping, machining, and assembly. The majority of the materials suitable for 3D printing result in rather rigid structures with a fixed set of properties. Four-dimensional printing (4D printing) addresses this limitation through the use of active and smart materials during 3D printing. The careful selection of structural design and suitable stimulus-activated smart material enables a unique set of shapes and properties to be programmed and achieved. This deviation from rigid structures with a fixed set of properties enables novel and unique applications in robotics, deployable structures, biomedical, and aerospace industries. The current work is focused on an interesting class of structures, 2D auxetic cellular solids, which exhibit distinctive mechanical properties (e.g., negative Poisson’s ratio) due to their carefully designed porous structure. 4D printing of such structures using smart materials allows for changes in the unit cell shape and dimensions to be made and thus resulting in tunable structural stiffness, Poisson’s ratio, and rigidity. This work aims to experimentally investigate the tunability of stiffness and Poisson’s ratio in 4D printed auxetic structure through utilization of programming and shape recovery properties found in Shape-Memory Polymers (SMP). Auxetic honeycomb structures with tunable mechanical properties were successfully manufactured and evaluated in this work. The attained structural stiffness was tuned in the range of 0.179-0.242 kN/mm while the Poisson’s ration was controlled from -0.33 up to even positive magnitudes of +0.69. This wide range of elastic properties was obtained from a single structure programmed to different deformation levels. Experimental evaluation of 4D printed SMP structures under constant cyclic programming/recovery conditions induced residual strains which consequently affected the mechanical properties by degrading the Poisson’s and structural stiffness. The magnitude of induced residual strains depend on the level of applied deformation during programing and the number of applied programing/recovery cycles. At the local level, full-field measurements revealed the localization of strains within the complex heterogeneous structure of the axially loaded auxetic honeycomb samples.