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Active materials, employed for actuation of functional structures, enable miniaturization and reduction of complexity. Such structures could find diverse applications in biomedical engineering, automotive, aerospace, civil engineering, and robotics. Materials, which react to an external stimulus by changing shape, are considered as active. Additive manufacturing of these materials holds great potential for fabrication of integrated, functional, multi-material systems with highly complex geometries. In recent years, the research on additive manufacturing of active materials has experienced a tremendous growth, with the emergence of 4D printing. Numerous structures with extraordinary shape changing behavior, or other novel properties have been presented, harnessing the actuation of active materials in conjunction with the design freedom of additive fabrication techniques. Many of these designs are based on shape memory polymers, which can be readily 3D-printed and show large shape memory strains. However, they often suffer from poor mechanical properties and low actuation force. Two major limitations, which hinder the advancement towards real-world applications. Metal-based active structures have the potential to overcome these drawbacks, however they are much less researched. The first part of this thesis investigates the fused filament fabrication process of passive structural steel and active shape memory alloys. The process relies on 3D printing of filaments, which consist of large fractions of metal powders, typically 50 to 60 vol.%, and a thermoplastic binder matrix. After printing, the binder is removed by a two-step solvent – thermal process, and the powder is sintered. A binder system is developed and optimized to improve the printability of 316L steel filaments. The capabilities of the filaments are demonstrated by fused filament fabrication and characterization of complex cellular solids, which show excellent ductility and low anisotropy. The sintering conditions of the stainless steel are optimized by microstructure studies. The mechanical properties of the optimally sintered steel are found to be comparable to those of conventionally manufactured steel. Two types of shape memory alloys are 3D-printed, and their microstructure is characterized, one showing superelastic properties, the other showing a shape memory effect at room temperature. This is the first time, functioning shape memory alloys are fabricated by filament-based additive manufacturing. Different actuator geometries are investigated to enlarge the deformation of the shape memory effect. The second part of the thesis revolves around the design and characterization of components for applications in 3D-printed active structure. 3D-printed polymeric flexural hinges, which enable controlled deformations by locally confining the strain in the material, are investigated. Three types of flexural hinges are fabricated from different additive manufacturing techniques and the influence of the design parameters on the bending stiffness and strength is examined. The effect of fatigue on the mechanical properties is studied and scanning electron microscopy is used to elucidate the fundamental mechanisms. In a second design study, a mechanical metamaterial is developed, which exhibits strain-stiffening characteristics, similar to the peculiar mechanical response of soft biological tissue. The non-linear elasticity is generated by formation of internal self-contacts under deformation. The metamaterial extends the capabilities of the contemporary designs, as is independent of the loading conditions. This was demonstrated for tension, compression, shear, and torsion. The mechanical behavior of the metamaterial is studied by finite element simulations and the results are corroborated by experimental testing of 3D-printed from silicone specimens. The concluding chapter of this thesis contextualizes the current state of filament-based additive manufacturing with the findings from this study and gives possible directions for future investigations. Furthermore, the general implications from the findings of this work for active structures and common limitations are discussed. |
