| Mineralized biological materials such as bone and seashells are made of relatively weak building blocks, yet they exhibit remarkable combinations of stiffness, strength and toughness. In particular, fracture toughness of these structures exceeds that of their components by several orders of magnitude. This performance can be largely explained by the "staggered microstructure" of these materials: stiff inclusions of high aspect ratio are laid parallel to each other with some overlap, and bonded by a softer, more ductile and tougher matrix. Under tensile stress the inclusions "slide" on one another, which generates large deformations and energy dissipation. This mechanism is predominant in mother of pearl, also known as nacre, a remarkable material which is now serving as model for biomimetic materials. As such, nacre is used as the representative of staggered structures throughout the present study. First, in order to better identify which type of nacre should serve as biomimetic model, the structure and mechanics of four different types of nacre are characterized. In particular, the respective deformation and fracture behaviour of these nacres is examined using four point bending experiments with and without initial notches. The transition between different failure modes is captured with in-situ testing techniques. In particular, nacre from pearl oyster is found to be the toughest, and for the first time remarkable deformation and fracture patterns are observed using in-situ optical and atomic force microscopy. Under stress, stair-like deformation bands form at an angle from the loading direction, forming a dense, tree-like network. This marks a clear difference from the now well documented "columnar" failure mode where deformation bands are perpendicular to the loading direction. By contrast, brittle tablet fracture is shown to be the predominant failure mode in pen shell due to its excessively high tablet aspect ratio. Analytical models supported by existing numerical simulations reveal the conditions for the transition between columnar to stair failure modes, namely large or random overlap between inclusions and local shear stress generated by inhomogeneities in the material. "Stair" failure promotes spreading of nonlinear deformation and energy dissipation, which translates into a higher overall toughness. Finally, in order to relate the material properties of staggered structures directly to those of their ingredients, a closed-form fracture model is developed based on the pre-existing models for stiffness and strength. This model shows that a combination of inclusions pullout and large process zone leads to tremendous toughness amplification. The model also suggests that a material like nacre cannot reach steady state cracking, with the implication that the toughness increases indefinitely with crack advance. Transition from steady state to non-steady state toughening is shown to be controlled by the values of a newly-found non-dimensional material property called "process zone parameter". These findings agree well with existing fracture data, and for the first time relate microstructural parameters with overall toughness. The insights gained throughout this study are applicable to other mineralized biological materials such as bone. In addition, the findings related to the mechanics of natural staggered structure will contribute to the development of upcoming biomimetic materials. |
