Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xiaodong Li

Abstract

Conch shells are natural nanocomposites with an exquisite multiscale hierarchical architecture which exhibit coupled ultrahigh mechanical strength and toughness. What materials design strategy renders conch shells such mechanical prowess? In this study, micro/nanoscale structural and mechanical characterization of conch shells (Busycon carica) has been carried out. We demonstrate, for the first time, direct evidence that the previously claimed single-crystal third-order lamellae - the basic building blocks in conch shells are essentially assembled with aragonite nanoparticles of the size ranging from 20 to 45 nm. The third-order lamellae exhibit not only elasticity but also plasticity with the strain up to 0.7% upon mechanical loading, due to the unique nanoparticle-biopolymer architecture in which the biopolymer mediates the rotation of aragonite nanoparticles in response to external loading. Our finding - metal like deformation behavior overturns the previous assumption that aragonite lamellae are brittle in nature. The three-order crossed-lamellar architecture interlocks cracks via crack deflection along the biopolymer interfaces in a three-dimensional manner. The interlocking mechanism and the plasticity of third-order lamellae jointly contribute to the remarkable mechanical prowess.

We report that conch shells display an unusual resilience against high strain rate predatory-attack vis-à-vis under quasi-static loading. Upon dynamic loading, conch shells trigger a new defense mechanism - intra-lamella fracture, involving nanoparticle rotation and formation of trapped dislocations, which differs from the inter-lamella fracture damage under quasi-static violation.

Another fascinating design principle with the curve-shaped third-order lamellae is uncovered in conch spines. Such architecture enhances the fracture strength up to 30 % compared with that of conch shell bodies with straight reinforcements, unveiling the roles of spines in protection from predators.

Moreover, the effects of electron beam irradiation and heat treatment on the structural and mechanical stability of conch shells were investigated. Both conditions can induce phase transformation from aragonite to calcite, to lime, altering the aforementioned properties.

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