MEAM Ph.D. Thesis Defense: “Controlling Fracture Behavior Through Architecture”

Many natural materials achieve excellent combinations of mechanical properties through their micro- and nano-scale structures, which leverage a level of complexity currently unmatched in engineering design. Recent advances in digital manufacturing have enabled the introduction of these fine-scale architectures to improve the mechanical properties of materials, but their intricacy still lags far behind that of natural materials. In particular, the potential of these structures to create materials with enhanced fracture resistance has remained limited, primarily due to a narrow design focus on simple, repetitive structures optimized for idealized materials. Improving the damage-tolerance of materials is critical to the mechanical performance of structures and interfaces, as cracks and defects often lead to failure at far-field loads that are significantly lower than the theoretical strength of the system. This thesis will demonstrate how leveraging disordered structures and considering material behavior beyond the idealized elastic-brittle regime can significantly enhance the fracture resistance of architected interfaces. Specifically, three key aspects influencing the failure of architected interfaces are examined: the effects of plasticity, the advantages of disordered structures, and the impacts of stochastic material failure. Through a synthesis of mechanics frameworks, computational modeling, and experimental mechanics including full-field analyses using digital image correlation and photoelasticity, it is shown that properly designed architectures lead to tunable and enhanced fracture resistance. These architectures enlarge the region of damage around the crack tip, delocalizing stresses and increasing the resistance to crack propagation, while also revealing novel properties such as the decoupling of toughness and strength.