Multiaxial mechanical characterization of additively manufactured open-cell Kelvin foams

Additively manufactured lattice structures (AMLS) exhibit substantially more freedom in design compared with traditional cellular materials such as honeycombs and stochastic foams. Prior to extensive applications, multiaxial mechanical behaviors of AMLS need to be thoroughly understood. In this study, open-cell Kelvin lattice structures (named as Kelvin foams) are fabricated through the selective laser melting (SLM) process using 316L stainless steel powder. Multi-cell numerical models are developed and validated against the uniaxial compressive results and then adopted to simulate virtual triaxial experiments. An energy dissipation point is adopted to define onset of yielding. Multiaxial yield surface of Kelvin foams is characterized in the von Mises and mean stress plane, which can be well fitted in terms of an elliptical or a parabolic yield function. The normalized yield surface using uniaxial yield strength is still relied on the strut diameter. It is found that the yield surface is gradually shrunk with increasing strut-diameter dimensional tolerance induced by the SLM process, which becomes more pronounced when its stress state is close to a hydrostatic compression. The yield surface is found to expand when the expanded polystyrene (EPS) foam is used as a filler material, but the influence of foam filler becomes weaker when gradually experiencing hydrostatic compression.