Understanding Mixed-Mode Fracture Mechanics in Additively Manufacturable Functionally Graded Microcellular Solids

The use of materials having cellular structures is rapidly growing in various engineering applications ranging from biomedical to aerospace, civil, and automotive industries. If designed accurately, cellular structures can be both high strength and light weight. In contrast to a traditional uniform pattern of cells across a structure, optimal designs usually require a non-uniform grading of cells, called a functionally graded structure. Recent advances in additive manufacturing and computational techniques have enabled researchers to precisely build functionally graded cellular structures with complex patterns. However, most available designs for optimized cell pattern configurations ignore the notches or cracks that can be formed in the printed components either during the fabrication process or due to excessive loads. The main goal of this research is to understand fracture mechanics near such critical areas in additively manufactured functionally graded cellular structures exposed to complex loading conditions. This fundamental understanding can then be integrated into next-generation design of engineered cellular structures with enhanced fracture resistance. The mathematical models and mechanics developed in this research will advance the fields of fracture mechanics, theoretical and computational mechanics, composite structures, and additive manufacturing. Additionally, through this project, graduate, undergraduate, and K-12 students will engage in several professional, educational, and outreach activities. This project aims to provide a greater understanding of fracture behavior in additively manufacturable functionally graded microcellular structures. The scientific objectives of this work are to i) demonstrate that different microcellular structures with different pattern functions can be successfully produced by additive manufacturing, ii) experimentally and computationally characterize their constitutive response, iii) develop a novel computationally efficient multiscale approach for prediction of their mixed mode fracture behavior, and iv) provide detailed information regarding how patterns and distributions of cells (i.e., topology and morphology) should be configured near the stress concentrations. To achieve these goals, different spatially pattered microcellular structures will be built by additive manufacturing to experimentally investigate their constitutive response, mixed-mode fracture toughness, and the crack propagation mechanism. A synergistic experimental/computational framework will be developed to predict and optimize fracture behavior by considering the material anisotropy induced from both the microcellular patterns and the printing orientation. 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.