Structure and moisture effect on the mechanical behavior of a natural biocomposite, buffalo horn sheath

Buffalo horn sheath is one of the typically natural composites with balanced strength and toughness. It demonstrates a structural hierarchy with stacked and corrugate-shaped lamellae, longitudinally sutured scaly cells and unidirectional keratin fibers. This work clarifies the influences of protein secondary structure, anisotropy microstructure, and moisture on flexural properties of sheath. From distal to proximal sections of sheath, flexural strength and modulus decrease, which may relate to the varied secondary structure. The mechanical properties are demonstrated to be enhanced along longitudinal direction correlated to the anisotropic microstructure. Besides, the strength and modulus can be weakened by the plasticization of moisture induced by rehydration. The fracture modes, which include matrix failure, interface dissociation and fiber breakage, are further correlated with the effects of anisotropy and moisture to illustrate damage patterns of horn sheath under different conditions. This study may provide reference to the structure-property relationship of natural composites and design strategy of bioinspired composites with tubular shape and anisotropy.