Deformation mechanisms and post-yielding behavior of additively manufactured lattice structures

This study develops an anisotropic generalization of the volumetric hardening model (VHM) to investigate the governing deformation mechanisms at the onset of yielding of additively manufactured lattice structure (AMLS) made of a nickel-based superalloy, Inconel 718 (IN718), under quasi-static loading. The discussion of deformation mechanisms relies on defining a new yield surface using a combination of experimental measurements and finite element simulations that enable the representation of three distinct behavioral features of IN718 lattice structures under mechanical loading including (1) tension-compression asymmetry of strut-level response; (2) tension-compression asymmetry of the aggregate response; and (3) hydrostatic pressure sensitivity of the strut-level response. Typically, the VHM is used to describe the aggregate response of lattice or foam materials to global loading. The VHM model could be directly applied at the strut-level; however, this would assume a one-to-one correspondence between the local and global response. Such an assumption is not justified a priori and could alter the evolution of the local deformation mechanisms and the resulting analysis of failure modes and structural degradation. Therefore, we introduce a modified VHM (or MVHM), which represents a more appropriate yield criterion. The Johnson-Cook damage criterion and damage evolution law, which is based on Hillerborg's fracture energy method, are coupled with the MVHM to investigate the damage initiation and evolution, and their influence on the global stress-strain response using finite element simulations.