The spicules of sponges help prevent cracking via their long, thin shape and orientation transverse to load direction.

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"That comment about thin elastin fibers brings up still another way not to crack, one implied in the words about thin glass filaments…Use materials that are divided transversely to the direction of the load into thin fibers or filaments or, for bending loads…into layered sheets. Most sponges, for instance, incorporate tiny elongate spicules of calcium salts or glass, which provide stiffness with good material economy and little risk of fracture…For the values of the work of fracture and strain energy storage of the materials available to nature, fibers in the range of 1-10 micrometers in diameter correspond to the critical crack length range and so are especially appropriate." (Vogel 2003:337)

Comparative Biomechanics: Life's Physical World, Second EditionPrinceton University PressJune 17, 2013
Steven Vogel

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