Enhanced Toughening of the Lamellar Structure in a Black Coral by Interlocking Spines

The black corals possess a branched, tree-like skeleton that is composed of chitin fibrils embedded within a protein matrix. This skeleton exhibits growth rings interlocked by spines. The lamellae are tightly wrapped around the spines, creating a structure akin to an onion. The indentation hardness and Young's modulus of the spines are comparable to those of the chitin rings. The compressive stress and the fracture toughness are increased by approximately 14.6% and 32.2% at higher loading rate in the dry state, but remain comparable at different loading rates in the wet state. The lamellar interfaces have a tendency to resist sliding in the dry state. As a result, the lamellae that curve around the spines are prone to fracturing one by one, just like an onion being peeled. This allows the material to absorb more fracture energy, ensuring that the spines can effectively resist the lamellar delamination.