Inhomogeneity of Nacre Lamellae on the Nanometer Length Scale

Abstract

We present experimental evidence of inhomogeneous distribution of organic inclusions within individual aragonitic lamellae forming the nacre layer of mollusk shells. This nanoscale-inhomogeneity is visualized in the Perna canaliculus (green mussel) shells by the aid of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in the tomography mode. Electron tomography reconstructions of the three-dimensional distribution of intracrystalline organic inclusions, ranging in size between 2 to 25 nm, clearly reveal 50 nm wide depletion zones adjacent to the organic sheets between lamellae. The interrelation between the observed fine-scale inhomogeneity of nacre layer and its improved mechanical properties is discussed.

Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite

“…Gilow et al [12] also concluded that organic molecules attach preferentially to the highly charged f001g planes of the prismatic calcite of Pinna (which is a close relative of Atrina). A similar pattern has been recently observed in the nacre of the mussel Perna [33]. Organic molecules occluded within the crystals and distributed around the boundaries of 200-300 nm crystalline domains (similar to the ones which we report here; figure 6a-c,e-i) were imaged with TEM in the calcites of the pterioidean Pinctada and in the ostreoidean Crassostrea [13,34]; the boundaries are characterized by small misorientations, in coincidence with small distortions of the crystal lattice.…”

Probability distribution of fracture elongation, strength and toughness of notched rectangular blocks with lognormal Young's modulus

“…For each stage, we developed analytical or semi-analytical approximations to the distribution of elongation U , strength L, and the toughness modulus T . Qualitatively, our results are in agreement with previous work on fracture in materials with randomly disordered Young's modulus distributions (Dimas et al, 2014;Tai et al, 2007;Younis et al, 2012). The methods depend to some extent on the Euclidean dimension of the specimen (n 1 = or 2), the quantity considered (U, L, or T ), and the condition under which the probability distribution is assessed (fracture initiation I or ultimate failure U ).…”

“…Hence the force-displacement plots are consistent with what has been observed in layered materials when fracture propagates towards softer or stiffer layers (Delale and Erdogan, 1983;Fratzl et al, 2007;Romeo and Ballarini, 1995;Zak and Williams, 1963). To verify this interpretation, we considered a bi-material specimen patterned to idealize case (b).…”

“…Experimental investigations indicate that the elastic properties of many natural and man-made materials exhibit large random-looking local fluctuations (Gupta et al, 2006;Tai et al, 2007;Younis et al, 2012). A fundamental question is how these small-scale fluctuations affect the bulk strength of the material.…”

“…Some studies suggest that the fluctuations enhance fracture performance in natural materials such as bone and nacre (Dimas et al, 2014;Fantner et al, 2005;Hang and Barber, 2011;Hang et al, 2014;Hu et al, 2015;Jaasma et al, 2002;Tai et al, 2007;Younis et al, 2012). In layered materials it is well known that the crack driving stresses decrease when fracture propagates in the direction of increasing stiffness (Cook and Erdogan, 1972;He and Hutchinson, 1989;Romeo and Ballarini, 1995;Zak and Williams, 1963). Under certain conditions, the stress concentration at the crack tip completely vanishes and the uncracked part of the specimen behaves like pristine composite (Okumura and de Gennes, 2001).…”