Heterogeneity in millimeter-scale Ti-6Al-4V lattice primitives: Challenges in defining effective properties for metamaterial design

The effective design of metallic metamaterials, characterized by interconnected struts or 'lattices,' hinges on the ability to predict strut and strut intersection ('node') responses. This is critical for predicting the macroscopic properties of structures comprised of thousands of struts. Computationally efficient beam descriptions, defined by strut properties like cross-sectional area, modulus, and yield stress, can significantly expedite the prediction of lattice structures and ultimately enable topology optimization. This paper provides a comprehensive examination of the properties of electron-beam melted three-dimensional printed struts and their 'nodes'—four intersecting struts. The findings elucidate the efficacy of various strategies for defining effective properties that accurately capture mechanical response. The study reveals that using a single set of effective properties can introduce inconsistencies between strut stiffness, peak load, and critical displacement. Stiffness correlates with averaged cross-sectional areas, while the peak load capacity correlates more closely with minimum inscribed cross-sectional areas. Analysis of nodes indicates that the interaction of surface defects and heterogeneity within the node strongly influences multi-strut response. Data from CT scans, EBSD scans, and nanoindentation maps highlight spatial variations comparable to the strut diameter, posing a significant challenge in defining effective homogenized properties. This study emphasizes the need for future efforts to integrate statistical property distributions with high throughput simulations to overcome the difficulty in defining a representative volume element (RVE) at the strut scale.