Small-scale Fracture and Size Effects in Bioinspired Nanoarchitected Materials

Abstract

How do we make any material tough?Fracture or premature failure is a pervasive issue across industries, materials, and length scales. While modern engineering approaches have developed effective methodologies to create damage-resistant materials, there remains a lack of fundamental understanding regarding the enhancement of a material’s fracture resistance. Natural materials, composed of diverse and readily available constituents, exhibit exceptionally high damage tolerance and serve as model systems for understanding mechanisms of toughness amplification. However, true utilization of bioinspired strategies requires evaluating fracture behavior at the appropriate (small-scale) architectural length scales, a task that has not yet been thoroughly addressed. This work aims to bridge this gap by developing fundamental design principles that govern both intrinsic (material) and extrinsic (architectural) fracture size-effects, that can then be applied to imbue any material with enhanced fracture resistance. This is accomplished by first developing a methodology to fabricate and test small-scale nanoarchitected materials via a combination of two-photon lithography, plasma etching, and in-situ nanomechanical testing. We first study how fracture behavior changes with sample size in polymers with varying degrees of crosslinking (DC) and find that ductility emerges when the sample width approaches a characteristic fracture length scale, independent of DC. Subsequently, various nanoarchitected materials were developed, utilizing small-scale constituents and exhibiting increasing architectural complexity. Layered materials with 1D stiffness heterogeneity showed increased fracture energy and stable crack propagation with increasing layer spacing, without significant loss in strength and stiffness. Bouligand-style materials comprising polymeric nanofibers demonstrated enhanced toughness due to size-enhanced ductility and nanoscale stiffness heterogeneity. Lastly, interpenetrating lattices, composed of ductile and brittle sub-lattices, exhibited improved toughness and resistance to crack propagation, underscoring the role of feature size in exploiting size effects to creatematerials with unprecedented properties. This work demonstrates the creation of fracture-resistant nanoarchitected materials through nanoscale toughening mechanisms and highlights the potential for re-investigating the origins of fracture toughness and cascading damage mechanisms in hierarchical architectures to develop highly tough, damage-tolerant materials with properties akin to natural structural materials.