MEAM Seminar: “Engineering Mechanics of Architected Hard-Soft Composites: Experiment, Simulation, and Theory”

Enhancing the resistance of human-made brittle materials to fracture is challenging due to the limited microstructural toughening mechanisms. This seminar makes a case for engineering toughening mechanisms in brittle materials by developing purposeful architected arrangements of material inspired by natural systems. Experimental fracture mechanics in hard-soft (cementitious-elastomeric) ‘Nacre-like’ composites based on the tabulated brick-and-mortar arrangement of mollusk shells is presented. Tablet sliding and soft interlayer energy dissipation are among the nacre’s hierarchical toughening mechanisms, leading to its significantly higher fracture toughness than its major brittle constituent (~95%, aragonite). Here, it is hypothesized that tablet sliding and tortuous crack propagation (crack deflection and crack bridging) are the key mechanisms that promote inelastic deformation and increase the size of the fracture process zone in brittle material. These mechanisms significantly enhance fracture toughness and ductility by an order of magnitude compared to constituent hardened cement paste.

To better understand crack propagation in hard-soft composites a unified large-deformation constitutive framework was developed (implemented via user-element subroutine within the finite element software Abaqus). Interfacial properties play a crucial role in the fracture process. The proposed computational framework couples the phase-field approach for bulk fracture with a potential-based cohesive zone model (CZM) to study crack propagation in multi-material (e.g., hard-soft, hard-hard) containing an interface. The phase-field captures crack initiation and propagation in the bulk constituents, and CZM (PPR) captures the role of the interface failure (e.g., delamination, deflection). The framework’s validation against linear elastic fracture mechanics theory for hard-hard composites with an interface is discussed, including the competition between crack penetration into the bulk vs. crack deflection into the interface. The framework is a numerical tool for probing or designing architected hard-soft materials with enhanced performance characteristics and mechanisms. The seminar extends the design of architected materials beyond hard-soft composites using statistical mechanics. The degree of ‘order’ in material(s) arrangement can be quantified using proposed translational or orientational order parameters (T, Q) from perfectly ordered to disordered and ideal random. Proper quantification of disorder in contrast to other approaches (Voronoi tessellation or perturbation methods) allows for probing the disorder-property relationship. Combined with advanced manufacturing techniques (robotic additive, laser processing, etc.), disorder can be used as a new way to design engineering materials and structures.