Designing Tougher Synthetic Materials with Lessons from NatureAllison Juster, M.Sc., Northwestern University, 2010.
Major Professor: Dr. Horacio D. Espinosa.
Mother Nature creates many materials that employ hierarchical structures to achieve simultaneous high strength and toughness. Nacre, the mother-of-pearl material found in seashells, is one of nature’s hierarchical materials that has remarkable strength and toughness despite its composition of relatively weak constituents. Comprised of greater than 95% brittle ceramic, nacre’s toughness originates within its brick-and-mortar structure of ceramic bricks, or tablets, and biopolymer mortar. Under loading, the tablets slide relative to each other, activating a number of interlocking mechanisms. This propagates to spread inelastic deformation, which appears as a white region ahead of the crack tip. The spreading of inelastic deformation leads to the dramatic increase in performance. Through in-situ atomic force microscopy fracture experiments coupled with digital image correlation, the first direct quantitative proof that tablet sliding and interlocking are nacre’s primary toughening mechanism is reported. Preparation of fracture samples and a detailed account of the testing procedure will be discussed as well as the methodology behind this process.
With the quantitative results from natural nacre testing and the proof of nacre’s main toughening mechanisms, a scaled-up, model synthetic material is created that incorporates these main toughening mechanisms. The processing of this composite artificial nacre material will be discussed in detail, along with a discussion of the material selection used to choose the right tablet and filler material. Through tensile testing of the composite material, an optimal sample geometry is determined which leads to a 100% improvement in energy dissipation and a seven-fold increase in strain to failure.
The correlation between the structure and function of natural nacre versus the artificial composite will be discussed, with an emphasis on the similarities in modes of failure of the optimized artificial nacre sample and that of natural nacre. The impact of the overall work in relation to the biomimetics and nanocomposites fields will be briefly discussed, along with some future steps in this process.