Multi-material 3D printing of bioinspired mechanical structures
Student thesis: Doctoral Thesis
Change has long assisted nature to build strong creatures. Species adapt every day obeying to the rule of withstanding loads. Inputs are gathered to grow hard and soft tissues where needed, saving life and preventing waste. Mimicking the same stunning mechanical performances is a hard challenge for man-made products obtained with top down processes. The aim of this project was to develop digital workflows for the generation of biologically inspired composites. The fabrication of hybrid composites inspired by tough biological materials such as nacre and compact bone were provided by the assembly of hard and soft phases through a multi-material 3D Printing (MM3DP) technique known as Multijet printing technology (MJP). To start, a systematic experimentation was conducted to quantify the mechanical properties of MJP base materials as well as the mechanical interplay between such multiple materials in a composites design. A workflow based on high contrast x-ray tomography (XCT) imaging data was then proposed to provide virtual models of less and more remodelled cortical bone. Mechanical properties of the cortical bone samples as well as the 3D printed replica were evaluated using acoustic measurements. Physical output show an elastic modulus almost ten times smaller (approximately 4 GPa) than the native bone (approximately 32 GPa), with the potential to tune the elastic modulus of the 3D printed replica depending on the employed manufacturing materials. The realization of controllable bioinspired 3D designs, represented the second goal of this project, in alternative to the extraction of key features from imaging data. Two workflows based on Generative Design (GD) were proposed to fluently draw and fabricate bioinspired compact bone and nacre. A parameterized version of the typical lamellar organization was presented for the first time to mimic compact bone and to supply a tool for the generation of a 3D crack blunting model. A further interactive workflow was proposed to produce a two-phase material inspired by nacre providing for a mechano-mimetic structure that dissipate energy via cracks delocalization, thus avoiding catastrophic failures. Results from tensile testing show increasing stress transfer from matrix to reinforcement by varying the diameter of the platelets from 2 to 9 mm. The resulting structures presented a volume fraction of the reinforcement that ranged from 54% to 69%, enough to absorb 20% energy than the stiffer base materials when subjected to dynamic loading. Finally, a semi-automated XCT based workflow was proposed to cover the absence of a volumetric metrology tool destined to multi-material AM when photo curable materials similar in density coexist.
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