Small-Scale Deformation and Fracture of Hard Biomaterials

Abstract

Structural materials engineering often aims to realize materials that are simultaneously strong, tough, and lightweight — a combination classically considered mutually exclusive. Natural composite materials such as bone exhibit a combination of these properties far exceeding that of their constituents, a feat generally credited to their hierarchical structure — all the way down the nanoscale. To date, a quantitative description of how this property combination arises in such microstructurally complex materials has remained elusive due to challenges in experimentally isolating and probing the salient deformation and toughening mechanisms at the micro and nanometer scales — length scales on the order the constituents of many natural composites. In this thesis, we first investigate the site-specific nanoscale structure of human bone using transmission electron microscopy. We show the presence of previously undiscovered disordered arrangement of collagen and mineral — alongside a well known ordered structure — within the trabecular architecture of bone. We perform micro- and nano-mechanical compression experiments to probe strength and deformation of each of these microstructures, revealing a size-dependent strength of bone attributed to the limited number of failure-initiating critical defects (e.g pores) in the small-scale samples relative to macro-scale tissue. Unlike experiments for investigating strength at small-scales, fracture experiments are standardized for the macroscale. To address this, we developed an in situ SEM/nanoindenter methodology that enables 3-point bending fracture experiments with observation and measurement of crack growth and toughening behavior at nano and micrometer scales. Using this technique, we discuss the crack initiation and growth toughness arising primarily from the underlying fibril microstructure in bone. In the context of a crack growth resistance, we describe a transition in the toughening behavior of bone originating from different levels of hierarchy. Given its versatility, this experimental technique establishes a platform for understanding the coupling between structure and fracture behavior of micron-sized materials.