A volume ratio method for determining the onset of yielding in lattice structures

This paper introduces a novel method for determining the yield stress of lattice structural materials fabricated through metal additive manufacturing (MAM). The new method, named the ‘volume ratio method’, defines yielding based on a threshold volume fraction of the lattice material that has undergone plastic deformation. The volume ratio method is applied to multiple finite element lattice models with five distinct unit cell geometries and across a broad range of relative densities. The finite element simulations employ experimentally calibrated material properties for pure iron, and the results are compared to the traditional 0.2% offset method. Results show that the 0.2% offset method often fails to accurately capture the early onset of plastic deformation, leading to an overestimation of yield stress values by as much as 78.6%, primarily due to geometric nonlinearity and localized stress fields inherent in lattice structures. In contrast, the volume ratio method provides earlier detection of plasticity, accurately reflecting the heterogeneous stress distribution typical of these structures. Overall, the volume ratio method offers a practical, intuitive, and computationally feasible alternative for accurately identifying the yield point in lattice structural material produced by MAM, thereby enhancing design reliability in applications where premature plasticity is a critical concern.