  Triply periodic minimal surface (TPMS) shell lattices are attracting
increasingly attention due to their unique combination of geometric and
mechanical properties, and their open-cell topology. However, uniform thickness
TPMS shell lattices are usually anisotropic in stiffness, namely having
different Young's moduli along different lattice directions. To reduce the
elastic anisotropy, we propose a family of variable thickness TPMS shell
lattices with isotropic stiffness designed by a strain energy-based
optimization algorithm. The optimization results show that all the six selected
types of TPMS lattices can be made to achieve isotropic stiffness by varying
the shell thickness, among which N14 can maintain over 90% of the
Hashin-Shtrikman upper bound of bulk modulus. All the optimized shell lattices
exhibit superior stiffness properties and significantly outperform
elastically-isotropic truss lattices of equal relative densities. Both uniform
and optimized types of N14 shell lattices along [100], [110] and [111]
directions are fabricated by the micro laser powder bed fusion techniques with
stainless steel 316L and tested under quasi-static compression loads.
Experimental results show that the elastic anisotropy of the optimized N14
lattices is reduced compared to that of the uniform ones. Large deformation
compression results reveal different failure deformation behaviors along
different directions. The [100] direction shows a layer-by-layer plastic
buckling failure mode, while the failures along [110] and [111] directions are
related to the shear deformation. The optimized N14 lattices possess a reduced
anisotropy of plateau stresses and can even attain nearly isotropic energy
absorption capacity.
