          

         Cellular structured materials have superior mechanical properties such as strength to weight ratio.  Lattices are a subset of the more general category of cellular solids, which includes naturally occurring structures such as honeycomb, cancellous bone, sponge, and synthetic materials such as polymer and metal foams. Lattices have the potential to reduce solid volumes, giving advantages such as weight reduction, decreased part production cost and ability to absorb energy under compressive and impact loading. Advances in field of lattice structure design has become possible mainly due to the emerging capabilities of additive manufacturing (AM) or 3D printing. Research in 3d-printed lattice structure material have produced functional materials that have improved properties, but are highly anisotropic. The anisotropy stems from stacking of 3d-printed layers forming the structure. The stacking scheme of layers causes an inherent disadvantage when the layers are parallel to direction of applied load, so called “along the grain”, and an advantage when the layers are normal to direction of applied load so called “against the grain”.   

          

            In an effort to reduce the directional dependency of these architectured materials, a study was undertaken to produce 3D printed structures that are isotropic and have relative density of 0.6 or lower. These lattice structures were formed using strut-based cell topologies that are adjoined as tetrahedrons to fill the volume of material. Tetrahedrons were selected because they form rigid units with minimal rotational capability at their joints. Other strut-based structures such as cube or atomic diamond tend to have multiple degrees of rotational freedom at the connection of one cell to another which reduces the rigidity of the material and render them useful in some directions and useless in others.  Tetrahedron based materials can also take advantage of non-uniformity in density by adjusting the scaling of tetrahedrons employed in the volume.  The non-uniformity enables architecting the structure in such a way to be denser where stress concentration is higher.

 

            Two different tetrahedron density v-notched specimens were produced. The samples were modelled following ASTM E23 standard for impact testing. They were printed with Acrylonitrile Butadiene Styrene (ABS) using a Stratasys uPrint SE 3D printer. The specimens were printed in two different configurations, ones where the struts were simply connected, and the other where the struts joints were strengthened by forming a spheres at their connections. The samples were tested using a Charpy impact testing instrument, where a 25-kg pendulum strikes the samples at speed of 5.5 m/s. Similarly v-notched solid sample with the same overall dimensions and highest infill ratio were also produced and tested.

 

            Four samples of each configurations were produced. For each configurations, two samples had impact along their grain and two had impact against the grain. The test results point that the impact energy absorption is highest for solid samples that are struck against their grain. The solid samples that were struck along their grain performs about 20% less. The high density tetra samples had a difference of about 16%, and the low tetra density specimen had a variation of about 8%. Some of these scatter is due to the ABS material that the vendor provides for printer. The average mass of each specimen were 6.3 g for low tetra density, 7.0 g for high tetra density sample and 10.5 g for the solid. When energy absorbed per unit mass of each specimen were compared, the high tetra density samples preform as well as that of solid samples. The data also point that inserting spherical joint at the connection of links is advantageous for low density mesh samples and does not affect high density samples. These results can lead to creating lattice structured materials that are lighter and yet stronger.


3D Printed Cellular Structure Materials Under Impact and Compressive Loading

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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