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A fundamental challenge for lightweight architected materials is their propensity for localized failure due to layered buckling, plastic shear-banding or fracture. Recent research efforts have used disorder to interrupt localization and enhance deformation, but most design strategies simply distribute the accumulation of damage, they do not prevent it from developing and propagating. This work explores how gradient architecture can be designed to hinder crack propagation and promote recoverability in nanostructured ceramic metamaterials. We experimentally and numerically investigated five different shell-based spinodal ceramic nanoarchitectures with 10-80 nm thick alumina films. These were fabricated using atomic layer deposition on sacrificial polymeric scaffolds written using two-photon lithography. All thin-walled (<40 nm) architectures underwent shell buckling-dominated deformation and showed nearly full recovery after compression to 45% strain, an expected result for this class of nanoarchitected materials. Thick-walled (>40 nm) isotropic and anisotropic architectures experienced considerable local damage during compression and predictably showed permanent failure even at low strains. Unexpectedly, thick-walled conch-shell inspired gradient architectures showed localized damage but experienced a full recovery after compression to 45% strain. This degree of recoverability has never been observed in this high density of a nanostructured ceramic, particularly one with visible local cracking during compression. This result stems from the length scale of the structural heterogeneity - the gradient layers were sufficiently small so as to inhibit the local damage development needed for crack propagation, thereby preventing catastrophic failure. Our findings have significant implications for how length scales and heterogeneity can be used to design failure-resistant materials from brittle constituents. 

Mechanical deformation of polymer networks causes molecular-level motion and bond scission that ultimately lead to material failure. Mitigating this strain-induced loss in mechanical integrity is a significant challenge, especially in the development of active and shape-memory materials. We report the additive manufacturing of mechanical metamaterials made with a protein-based polymer that undergo a unique stiffening and strengthening behavior after shape recovery cycles. We utilize a bovine serum albumin-based polymer and show that cyclic tension and recovery experiments on the neat resin lead to a ~60% increase in the strength and stiffness of the material. This is attributed to the release of stored length in the protein mechanophores during plastic deformation that is preserved after the recovery cycle, thereby leading to a “strain learning” behavior. We perform compression experiments on three-dimensionally printed lattice metamaterials made from this protein-based polymer and find that, in certain lattices, the strain learning effect is not only preserved but amplified, causing up to a 2.5× increase in the stiffness of the recovered metamaterial. These protein–polymer strain learning metamaterials offer a unique platform for materials that can autonomously remodel after being deformed, mimicking the remodeling processes that occur in natural materials. 

Abstract The enhanced properties of nanomaterials make them attractive for advanced high‐performance materials, but their role in promoting toughness has been unclear. Fabrication challenges often prevent the proper organization of nanomaterial constituents, and inadequate testing methods have led to a poor knowledge of toughness at small scales. In this work, the individual roles of nanomaterials and nanoarchitecture on toughness are quantified by creating lightweight materials made from helicoidal polymeric nanofibers (nano‐Bouligand). Unidirectional ( = 0°) and nano‐Bouligand beams ( = 2°–90°) are fabricated using two‐photon lithography and are designed in a micro‐single edge notch bend (µ‐SENB) configuration with relative densities between 48% and 81%. Experiments demonstrate two unique toughening mechanisms. First, size‐enhanced ductility of nanoconfined polymer fibers increases specific fracture energy by 70% in the 0° unidirectional beams. Second, nanoscale stiffness heterogeneity created via inter‐layer fiber twisting impedes crack growth and improves absolute fracture energy dissipation by 48% in high‐density nano‐Bouligand materials. This demonstration of size‐enhanced ductility and nanoscale heterogeneity as coexisting toughening mechanisms reveals the capacity for nanoengineered materials to greatly improve mechanical resilience in a new generation of advanced materials.