Role of interphase layers in mechanical properties of nacreous structures

An exceptional combination of mechanical properties such as high stiffness, high strength, and high toughness makes nacre one of the most remarkable natural materials. It has been shown that these outstanding properties arise from the multilayered brick-and-mortar structure of the nacre and the vital nanoscale features in its structure. Moreover, problem-solving strategies of naturally growing composites such as nacre give us a unique vision to design, optimize and fabricate simultaneously tough, stiff, and strong composites. It is widely known that nature's biologically evolved, complex and effective functionally graded interfaces essentially enable the exceptional mechanical properties of biological composites. Particularly in nacre, the organic- inorganic interface referred hereto as interphase, in which the confined protein layer behaves stiffer and stronger in the proximity of the calcium carbonate minerals platelets, provides a key role in its toughness that is orders of magnitude higher than aragonite platelets as its main constituent. Hence, understanding the role of the interphase properties in the toughening mechanisms is paramount in the design and synthesis of high-performing bioinspired materials. In this study, a micromechanical model of the "Brick-Mortar"and "Brick-Bridge-Mortar"composites is presented focusing on the role of interphase properties on strengthening and toughening mechanisms. Shear-lag theory is employed on a simplified two-dimensional unit-cell of a multilayered composite. The computed closed-form solutions for the displacements as a function of constituent properties are used to calculate elastic modulus, strength, and work-of-fracture for a wide range of multilayered materials. Our results show that the properties of the soft phase in mineral bridges proximity can have a significant effect on the mechanical properties. The detailed relationships presented can help identify the future directions for advanced material design and development.