Shear Rigidity of 3D Printed Infills

Abstract

This thesis studies the shear rigidity of 3D printed infills. The shear rigidity of 3D printed infills concerns the mechanical properties of sandwich elements. A sandwich element consists of a core and two face material sheets. This makes use of the fact that bending stress is only at the surface of a bent element. The closed standard to this topic is ASTM C393 concerning bending and shear rigidity of homogeneous cores materials. A 3D printed infill represents a sandwich element's discrete, inhomogeneous core material. There has been no standard for the shear rigidity of 3D printed elements. Finding the true shear rigidity can provide more precise deformation which affects heavily on the strength of objects. This is achieved through both FEA and three-point bending tests on 3D printed beams. Three-point-bending test records the sum of bending and shear de-formation defined as flexural deformation. The bending deformation is resisted by flexural rigidity, D=EI. The shear deformation is however resisted by shear rigidity, U=GA. In long homogenous beams, the shear deformation is very small. Two chosen infill pat-terns to study are Grid with 45-degree raster angle and Triangle with 90-degree raster angle. The core infill percentage range from 10% to 90%. The specimens are designed using SolidWorks software and 3D printed using Creality Ender 3 Pro. The 3D printing filament chosen is polylactic acid. The results show that increasing the infill percentage yields higher shear rigidity. Overall, the Triangle pattern provides 33% better shear resistance than the Grid pattern. Furthermore, at certain infill percentages, the 3D printed structures start to provide higher shear strength than the homogeneous ones. Therefore, instead of manufacturing 100% infill objects, choosing the right infill pattern and percentage can give better mechanical properties and save materials economically. This thesis can be viewed as guidelines for 3D printing production. Further studies on other infill patterns can provide more insights into the shear rigidity of 3D printed objects.