Effects of laminate architecture on fracture resistance of sponge biosilica: Lessons from nature

Hexactinellid sponges are known for their ability to synthesize unusually long and highly flexible fibrous spicules, which serve as the building blocks of their skeletal systems. The spicules consist of a central core of monolithic hydrated silica, surrounded by alternating layers of hydrated silica and proteinaceous material. The principal objective of the present study is to ascertain the role of the latter laminate architecture in the material's resistance to both crack initiation and subsequent crack growth. This has been accomplished through indentation testing on the giant anchor spicule of Monorhaphis chuni, both in the laminated region and in the monolithic core, along with a theoretical analysis of deformation and cracking at indents. The latter suggests that the threshold load for crack initiation is proportional to K(c)(4)/E(2)H where K(c) is fracture toughness, E is Young's modulus, and H is hardness. Two key experimental results emerge. First, the load required to form well-defined radial cracks from a sharp indent in the laminated region is two orders of magnitude greater than that for the monolithic material. Secondly, its fracture toughness is about 2.5 times that of the monolith, whereas the modulus and hardness are about 20% lower. Combining the latter property values with the theoretical analysis, the predicted increase in the threshold load is a factor of about 80, broadly consistent with the experimental measurements.