Biological armors under impact--effect of keratin coating, and synthetic bio-inspired analogues.

A number of biological armors, such as turtle shells, consist of a strong exoskeleton covered with a thin keratin coating. The mechanical role upon impact of this keratin coating has surprisingly not been investigated thus far. Low-velocity impact tests on the turtle shell reveal a unique toughening phenomenon attributed to the thin covering keratin layer, the presence of which noticeably improves the fracture energy and shell integrity. Synthetic substrate/coating analogues were subsequently prepared and exhibit an impact behavior similar to the biological ones. The results of the present study may improve our understanding, and even future designs, of impact-tolerant structures.