Toughening in a nacre-like soft-hard layered structure due to weak nonlinearity in the soft layer

Abstract

Recently, it has been found experimentally that hydrated nacre exhibits a nonlinear mechanical response. While mechanical nonlinearity has been shown to be important in other biological structures, such as spider webs, the implications of mechanical nonlinearity in nacre have not been explored. Here, we show that the nonlinear mechanical response of nacre can be reproduced by an analytical model, which reflects a nacre-like layered structure, consisting of linear-elastic hard sheets glued together by weakly nonlinear-elastic soft sheets. We develop scaling analysis on this analytical model, and perform numerical simulations using a lattice model, which is a discrete counterpart of the analytical model. Unexpectedly, we find the weak nonlinearity in the soft component significantly contributes to enhancing toughness by redistributing the stress at a crack tip over a wider area. Beyond demonstrating a mechanism that explains the unusual properties of biological nacre, this study points to a general design principle for constructing tough composites using weak nonlinearity, and is useful as a guiding principle to develop artificial layered structures mimicking nacre. I. INTRODUCTIONNatural materials often exhibit remarkable hierarchical structures leading to outstanding mechanical characteristics [1-5], as observed in bone, spider silk [6], and the exoskeletons of crustaceans [7,8]. Nacre, which is found in many seashells and protects shellfish from their environment, is composed of soft and hard layers, and has been studied as a prototype material for several decades [9][10][11]. Researchers have been inspired by nacre's remarkable structure to develop new materials that demonstrate excellent mechanical performance. [12][13][14][15][16][17][18].There have been many studies discussing the mechanisms responsible for the toughness of nacre. Toughening mechanisms have been proposed based on a variety of experimental observations, such as (1) step-wise elongation [19], (2) micro-cracking and crack bridging [20], (3) thin compressive layers [21], (4) rough layer interfaces [22], (5) mineral bridges [23], and (6) wavy surface of the plates [24]. On the theoretical side, various approaches have been explored, including (1) elastic models [21] based on analytical solutions [25, 26] and on scaling arguments [27, 28], (2) viscoelastic

Investigation of the multiple-fragmentation process and post-fragmentation behaviour of dense and nacre-like alumina ceramics by means of tandem impact experiments and tomographic analysis

“…Increasing the interface strength compared to the matrix strength (ratio tending to 1) should lead to a brittle and catastrophic failure and should be avoided. More details upon the role of the polymer (or even metallic [31]) phase can be found in other works ( [32][33][34][35]). Several processing strategies were attempted to replicate the layering of nacre.…”