Abstract
The brittlestar Ophiocoma wendtii is theorised to employ a technique already used in metallurgy in order to optimise the mechanical properties of calcitic microlenses within their skeletons. These microlenses contain arrays of Mg-rich nanoprecipitates, which are proposed to inhibit crack propagation through the compression of the local host lattice. Here, we employ classical molecular dynamics in order to study the effects of Mg-rich nanoprecipitates on lattice strain, stress distributions and crack propagation in calcite. Our quantitative results on lattice strain and stress induced on the host matrix are compatible with empirical estimates. Simulations of crack propagation demonstrate that the inclusion of a Mg-rich region results in an increase in stress required to fracture the crystal, as well as higher residual stress in the fractured crystal. This is the result of an inhomogeneous stress distribution causing a more disordered fracture, as well as deflections of the crack away from the lowest energy (10.4) surface. The results agree with the proposal that the compression of the host lattice inhibits propagation, and offer insight into other mechanisms through which the nanoprecipitates affect crack propagation.
Highlights
The ability to produce minerals with bespoke properties is crucial for the survival of living organisms
Biological systems are limited to the chemical composition and ambient conditions of their environment when forming biominerals; evolution has provided living organisms with the means to produce a range of highly sophisticated materials with properties tailored to their respective purposes
The results are compared with the empirically derived relations for the lattice parameters in the a and c-directions as a function of Mg concentration as found by Polishchuck et al It is clear from the results that the force fields predict a reasonably accurate relationship between strain and concentration, especially in the more elastic c-direction
Summary
The ability to produce minerals with bespoke properties is crucial for the survival of living organisms. Polishchuk et al propose that the nanoprecipitates have a different effect on the mechanical properties of calcite: rather than increasing the tensile strength by inhibiting dislocation motion, as in metal alloys, the nanoprecipitates in calcite increase the fracture toughness, by inducing a compressive stress in the host matrix. Such a prestressing mechanism is employed in other brittle materials, such as tempered glass and prestressed concrete. We use crack propagation simulations to examine the effect of magnesium nanoprecipitate incorporation on calcite fracture toughness
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