Fabrication and characterisation of novel functionally-graded lattice materials using additive manufacturing

Abstract

Porous and cellular materials are frequently found in nature as well as applications in biomedical, automotive, and aeronautical engineering. Their unusual properties are a direct consequence of their micro-structure. These materials have proven to be difficult to mimic synthetically. Additive manufacturing is a promising approach to fabricate this class of materials, as it provides great control over the lattice micro-architecture. A fabrication technique is developed using fused deposition modelling with affordable 3D printers, to produce extrusion with continuously varying cross-sectional size. The novel approach proposed here achieves this by manipulating process parameters. Significant intended variation in the axial and bending stiffness of extrusion is achieved. An excellent agreement between the target diameter of the extruded filament and the diameter of the fabricated filaments confirms a reasonably linear response of the machine following simple incompressible flow of molten material. The adaptability of a fixed bore nozzle to produce variable diameter extrusion was characterised via three quantities α, αS, αB that respectively represent adaptable range in geometry, axial stiffness and bending stiffness of the extruded filaments. The ovality of the extruded filaments thus produced was quantified and was seen to have a significant impact on their bending stiffness. Following successful fabrication of variable diameter extrusions, rectangular bi-layer lattice strips, with spatially varying bending stiffness, were fabricated. Their bending response is asymmetric about their length-wise centre. This asymmetric response was found to be consistent with a simple one-dimensional theory of post-buckled mode shape arising from a functionally graded beam. The response shows high curvature in parts of the structure with relatively softer struts compared to the stiffer regions, which is consistent with expectations. Finally, bi-layer square lattice films with spatially varying stiffness were fabricated. The bent surface of the planar structure shows strong spatial variations in bending response. This asymmetric response is well captured by the linear buckling mode shapes obtained from finite element analysis. Encouraged by the success in fabrication and analysis, a host of mathematical problems including response of woodpile lattices when properties vary spatially were solved.

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