Toward Fatigue-Tolerant Design of Additively Manufactured Strut-Based Lattice Metamaterials

The advent of additive manufacturing (AM) has enabled the prototyping of periodic and non-periodic metamaterials (a.k.a. lattice or cellular structures) that could be deployed in a variety of engineering applications where certain combinations of performance features are desirable. For example, these structures could be used in a variety of naval engineering applications where lightweight, large surface area, energy absorption, heat dissipation, and acoustic bandgap control are desirable. Furthermore, combining the multifunctional design optimization of these structures with progressive degradation due to cyclic loading could lead to fatigue-activated attritable systems with potentially tailorable performances not yet in reach by current conventional systems. Nevertheless, in order to deploy these complex geometry structures their multiphysics response has to be well understood and characterized. The objective of the current effort is to describe an initial approach for designing a uniaxial metamaterial specimen for fatigue testing as the first step toward the design of multi-axial fatigue test coupons. In order to compare bending- and stretching-dominated structures, two strut-based lattices made of Ti-6Al-4V alloy consisting of the octet and tetrakaidecahedron (or Kelvin) cells are examined. The specimens are designed to fail in the central area of the specimen where edge effects are minimized. Finite element results of the relevant structural mechanics are implemented and exercised to compare the performance of the eight relevant geometries and to evaluate the effect of relative density on fatigue life.