Structural basis for the fracture toughness of the shell of the conch Strombus gigas

Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component con stituting not more than a few per cent of the composite material, the fract ure toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude(1). The judicious placement of the organic matri x, relative to the mineral phase, and the hierarchical structural architect ure extending over several distinct length scales both play crucial roles i n the mechanical response of natural composites to external loads(.)(2-4) H ere we use transmission electron microscopy studies and beam bending experi ments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two e nergy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shell's tougher middle lay ers at higher loads. Both mechanisms are intimately associated with the so- called crossed lamellar microarchitecture of the shell, which provides for 'channel' cracking in the outer layers and uncracked structural features th at bridge crack surfaces, thereby significantly increasing the work of frac ture, and hence the toughness, of the material. Despite a high mineral cont ent of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a 'ceramic plywood', and can guide the biomimetic design of tough, lightweight structures.