NSF-DFG: Hierarchical Design and Additive Manufacturing of Metallic Programmable Metamaterials

Programmable mechanical metamaterials (PMMs) have unique mechanical properties and functionalities through specific geometric designs without changing the material composition. Current PMMs are primarily made of polymer, which cannot sustain high stress and temperature. Metallic PMMs can be used under these extreme conditions, and have potential applications for aerospace, automobile and biomedical industries. However, an existing scientific challenge is that a simple translation of the proven polymer PMM design into metals is unsuccessful because most metals have magnitudes lower elastic strain limit than polymers leading to a low lifetime under cyclic loading. Without a complete structural redesign, the required mechanical properties would be extremely challenging to achieve in metallic PMMs. Although additive manufacturing (AM) offers the potential to fabricate complex geometry, the small dimension and complex geometry required to achieve functionality in metallic PMMs have reached its resolution limit. Hence, there is an urgent need to explore the fundamental AM processing mechanism that can achieve desirable small features with acceptable defect density and residual stresses. This project leverages the unique expertise of the Purdue team on manufacturing and Freiburg team on design to tackle this challenge. The international collaboration will enable students from both institutes to develop a solid foundation in both experiments and simulations through videoconferences and annual student exchanges. The objective of this project is to apply laser powder bed fusion to fabricate metallic PMMs and understand fundamentally the influence of hierarchical design and AM processing on microstructures, defect density, elastic strain limit and fatigue resistance. The project team plans to develop an integrated experimental and modeling platform that can significantly improve fundamental understandings on the manufacturing of metallic PMMs with superior mechanical performance (large global strain and fatigue resistance). This research will integrate AI-assisted computer design, AM modeling and processing, characterization and mechanical testing to identify architectures that can sustain large global strain with minimal local elastic strain. Purdue’s capability on in-situ small scale mechanical testing in SEM and Freiburg’s capability on fatigue testing of small structures will be integrated to understand the underlying deformation and fatigue mechanisms. If successful, this project will generate new knowledge about the influence of AM processing conditions on generation of internal defects and residual stress, as well as the consequent impact on fatigue properties of metallic PMMs. This project is also expected to lead to new structural redesign of PMM coupled with AM for metallic materials that offers the promise to sustain large elastic deformation, high stress and fatigue resistance, not attainable in polymeric PMMs. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.