Abstract
Alligator is a well-adapted living fossil covered with a dorsal armor. This dermal shield consists of bony plates, called osteoderms, interconnected by sutures and non-mineralized collagen fibers, providing a dual function of protection and flexibility. Osteoderm features a sandwich structure, combining an inner porous core and an outer dense cortex, to offer enhancements for bending stiffness and energy absorbance. In this study, hierarchical structure and mechanical behaviors of the American alligator (Alligator mississippiensis) osteoderm were investigated. Micro-computed tomography was applied to reveal the complex 3-dimesional neurovascular network. Through the observation under optical and scanning electron microscopes, the osteoderm was found to consist of woven bone in the dorsal region and lamellar-zonal bone in ventral region. Nanoindentation and compressive tests were performed to evaluate the mechanical properties of osteoderms. The varying mineral contents and porosity resulted in a graded mechanical property: from a hard and stiff dorsal cortex gradually transform to a more compliant ventral base. Three protective mechanisms were proposed and observed for alligator osteoderms: (1) flexibility provided by sutures and non-mineralized collagen fibers; (2) energy absorption under compression contributed from the interior cellular foams; (3) non-uniform microstructure and graded mechanical properties offer load re-distribution and impact resistance. The inspirations from alligator osteoderms may lead to the optimized design of novel synthetic armors and advanced composites.