Hierarchical structure and mechanical properties of scales from grass carp

During the long-term natural evolution, the scales of teleost fish become ultra-thin and ultralight, and have excellent flexibility. In order to develop an innovative flexible protection system, the hierarchical structure and mechanical properties of scales from grass carp were investigated. Two types of samples for scales and collagen layers were extracted from the central regions of scales by three different orientations (0°, 45° and 90°) firstly. Then, tensile tests in uniaxial were conducted and mechanical properties of scales with different water contents as well as scales and collagen layers under different strain rates were investigated comparatively. The result shows that the scales of grass carp consist of outer hard bony layer and inner soft collagen layer, and the collagen layer consists of twisted overplayed collagenous fiber lamellas whose spiral angles are about 28°-31°. The mechanical properties of scale are in-plain anisotropic, while the mechanical properties of collagen layer are isotropic. The elastic modulus and tensile strength of dehydrated scale increase significantly, which are about 10 times and 3 times of those of hydrated scale, while the ultimate strain reduces by about 50%. The mechanical properties of hydrated scale and collagen layer are highly dependent on strain rate. The conclusions obtained provide ideas for the investigation of bio-inspired innovative composites and flexible protection systems.