Fracture mechanisms of the Strombus gigas conch shell:: II -: Micromechanics analyses of multiple cracking and large-scale crack bridging

Micrornechanics analyses of the dominant energy-dissipating mechanisms responsible for the resistance to catastrophic fracture of the aeagonitic shell of the giant Queen conch, Strombus gigas, are presented. The crossed lamellar microstructure of the shell is associated with a work of fracture that is three orders of magnitude higher than that of non-biogenic aragonite [J. Mater. Sci. 6 (1996) 6583]. Previous energy-based models predict that multiple "tunnel" cracks in the weak layers of the shell account for a factor of 20 of this increase in fracture energy. We show that the additional factor of greater than or similar to300 results from the synergy between the tunnel cracking and crack bridging mechanisms, analogous to multiple energy dissipating mechanisms observed in brittle matrix composites. The theoretical models demonstrate that the microstructure of the shell of S. g f as is such that potential cracks evolve towards the desirable non-catastrophic ACK (Aveston-Cooper-Kelly) [Properties of fiber composites, Conference Proceedings 15, National Physical Laboratory. IPC Science and Technology Press, 1971] limit, a situation in which all bridging ligaments remain intact along the crack wakes. Load-deflection experiments at temperatures ranging from -120 to 200 degreesC suggest that a glass transition occurs within the organic (proteinaccous) phase at similar to175 degreesC, and demonstrate the critical role that this organic "matrix" plays in the resistance of the shell to catastrophic crack propagation. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.