Modeling mechanical responses in a laminated biocomposite - Part II - Nonlinear responses and nuances of nanostructure

Accurate pseudo-hexagonal nanoarchitecture of nacre was used to design three dimensional finite element models of nacre, the inner layer of mollusk shells. Tensile tests were simulated introducing linear and nonlinear material properties in these models. Material parameters of components of nacre (aragonitic bricks and the complex organic phase) such as elastic modulus and hardness were obtained from bulk measurements reported in literature. In addition, nanoscale experiments conducted using atomic force microscopy and nanoindentation provided mechanical properties of aragonite platelets and organic phase. Linear simulations in the elastic regime at low stresses (2 MPa) was conducted on the new models. Our simulations indicate that a high modulus of organic phase (similar to 20 GPa) is necessary to obtain the experimentally obtained bulk phase elastic response of nacre. Our nanoindentation experiments also confirmed the simulations. Further nonlinear simulations were conducted under the assumption of a fully elastic behavior of aragonite and an elastoplastic model for the organic phase. Further the yield stress of the organic phase is varied over a wide range from 40 to 400 MPa. The resulting yield stress of nacre was compared to experimentally obtained value. Again our simulations indicate that an exceptionally high yield stress of the organic phase is necessary to obtain the yield behavior in nacre. Further, nanostructural nuances in the form of platelet-platelet mineral contacts were incorporated in the three dimensional models. The role of these mineral contacts on linear and nonlinear responses under high and low loads was quantitatively evaluated. Our simulations indicate for the first time that presence of these mineral contacts has minimal effect on both linear and nonlinear responses in nacre. As a matter of fact, the contacts are regions of high stress concentration and they break long before yield begins in nacre (similar to 50 MPa). These results have significant ramifications on a biomimetic design of scalable nanocomposites mimicking nacre. (C) 2005 Springer Science + Business Media, Inc.