Convergent Evolution in Biological Lamellar Systems: Multiscale Structures and Mechanics

A. Zaheri

2 (2019) 89-89

The focus of this thesis is as follows. The first system of study is the lamellar keratinous structure with incorporated tubules. For this case, horns and baleens are selected as model materials and their compressive deformation behavior are investigated from quasi-static to high strain rate impact testing. The tests are performed under different loading directions and hydration states, revealing a strong strain rate dependency for both dried and hydrated conditions. A strong anisotropy behavior is observed under impact for the dried state. Detailed failure mechanisms along with impact resistivity are discussed. As the second architecture of the lamellar systems, mechanics of helicoidal (Bouligand) fibrous structural motif is studied. To this end, first, a nanoscale characterization of Bouligand structure through utilization of atomic force microscopy (AFM) is presented. Using a figeater beetle cuticle as a model material, quantification of pitch size, diameter, and twisting angle of the helicoidal structures are performed by exploiting an experimental–theoretical framework that combines AFM nanoindentation and anisotropic contact 4 mechanics analysis. The developed methodology is general and can be applied to the study of other types of biocomposites that have features with characteristic dimensions under 100 nm. Next, the role of pitch rotation angle between fibers in helicoidal motif is examined for beetle cuticle during beetle’s ontogeny. This is conducted through continuum mechanics analysis by consideration of flexural behavior and the requirement of toughness property. As complement of the study, testing of 3D-printed samples and a systematic analysis of the effect of pitch angle in the inherent mechanics of helicoidal architectures are performed. Experimentation and analysis reveal improved isotropy and enhanced toughness at lower pitch angles, highlighting the flexibility of the helicoidal architecture. The final system of the lamellar cases is the arrangement of lamellar plates combined with dispersed fiber-like system, which is common within high-mineralized teeth. The wearing mechanism and sharpness preservation of teeth is crucial for survivability of species in nature. In this thesis work, hypothetical self-sharpening of sea urchins teeth is examined through direct in-situ SEM scratching experiment as well as finite element simulation. Using contact mechanics analysis, developed experimental methodology could reveal how chipping of plates and wearing of the region called stone (i.e. polycrystalline matrix with dispersed fiber like crystals) contribute to the sharpening mechanism of the teeth. Finite element simulation highlights how the material properties such as, namely interfacial properties together with arrangement of structural elements such as lamellar plates could lead to self-sharpening teeth. Finally, the methods and findings of this thesis can serve as a source of inspiration of design principles for manufacturing multifunctional manmade composite systems