Effects of nodal fillets and external boundaries on compressive response of an octet truss

Abstract

Additive manufacturing has enabled fabrication of weight-efficient periodic trusses over a wide range of length scales. The principal objective of the present study is to address two coupled aspects of truss design and performance: (i) the extent to which circular nodal fillets enhance node stiffness and alleviate stress concentrations, and (ii) the extent to which external boundaries affect local strut strains. To this end, octet trusses with and without filleted nodes were fabricated out of a hard thermoplastic and tested in compression, with digital image correlation used to monitor axial and bending strut strains and nodal rotations. Complementary finite element simulations were also performed. Macroscopic compressive failure occurs through bending and buckling of certain inclined struts followed by tensile rupture of struts oriented perpendicular to the loading direction. The struts experiencing the highest tensile strain and most prone to rupture are those located along the truss mid-plane and that intersect external edges. Additional boundary effects are manifested in exacerbated nodal rotations and bending of inclined struts intersecting the truss corners. Although buckling begins in the affected struts, large-scale buckling is preceded by tensile rupture. The implications for design and failure prediction of finite truss structures are discussed.