Natural materials such as bone, tooth, and nacre are nanocomposites of proteins and minerals with superior strength. Why is the nanometer scale so important to such materials? Can we learn from this to produce superior nanomaterials in the laboratory? These questions motivate the present study where we show that the nanocomposites in nature exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (robustness). We further show that the widely used engineering concept of stress concentration at flaws is no longer valid for nanomaterial design.

Both elastic modulus and fracture stress are known to increase with the amount of mineral deposited within collagen fibrils. Current mechanical models of mineralized fibrils, where mineral platelets are arranged in parallel arrays, reproduce the first effect but fail to predict an increase in fracture stress. Here, we propose a model with a staggered array of platelets that is in better agreement with results on molecular packing in collagen fibrils and that accounts for an increase of both elastic modulus and fracture stress with the amount of mineral in the fibril. Finally, we explore the dependence of the mechanical properties within the model, when the degree of mineralization and the thickness of the platelets as well as their distance varies.

It is seen by the wide ternary homogeneity ranges of the high-temperature heazlewoodite phases b 1 and b 2 that iron is solved to a large extent. ConclusionThe sublattice models chosen for the Gibbs energies of the high-temperature heazlewoodite phases b 1 and b 2 provide a satisfactory and thermodynamically consistent description of a large amount of experimental data. The introduction of the second ternary high-temperature heazlewoodite phase b 2 improves the agreement between calculations and experiments. The present assessment can be useful in connection with the development of refined thermodynamic databases for multicomponent metal-sulfur systems.

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The deformation behavior of certain biologic macromolecules is modeled by the "sticky chain," a freely jointed chain with weak bonds between subsequent joints. Straining the chain leads to thermally assisted breaking of the weak bonds, yielding a characteristic shape of the force-elongation curve, usually with a pronounced plateau, but sometimes displaying a pseudo-Hookean behavior over a wide range of deformations. The number of individual links is assumed to be large, so the stochastic time evolution of the individual events can be approximated by a differential equation. The cases of individual and collective bond breaking are treated and formulae given for various measurable quantities. A threshold strain rate is found, below which the deformation force no longer depends on the deformation velocity. The method is applied to experimental results for the deformation of single molecules like titin or DNA and the results agree with the parameters deduced from the same experiments by the original authors using Monte Carlo (MC) calculations. Despite its intrinsic continuous character, the model, therefore, is applicable even for the deformation of macromolecules with only a few discrete unfolding elements, yielding physical quantities from experimental results using simple formulae instead of a host of MC computations.

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The aim of the investigation was to study the influence of indenter tip geometry on the conventionally obtained indentation modulus of enamel by nanoindentation. Indentation tests on bovine enamel using three different diamond pyramidal indenters with half face angles 65.27°, 45°, and 35.26° were conducted to evaluate the indentation modulus using the Oliver–Pharr method [W.C. Oliver and G.M. Pharr, J. Mater. Res. 7, 1564 (1992)]. In addition, three different dehydration conditions were studied: wet under Hank’s balanced salt solution, laboratory dried, and vacuum dehydrated. For the Berkovich indenter (65.27°) and 45° pyramidal indenters, there was only a small difference between indentation modulus values, whereas for the cube-corner indenter (35.26°) a ratio of 2.4 between laboratory dry and wet samples was found. A detailed evaluation, including indentation creep and recovery as well as pileup, resulted in a reduction of this latter ratio to 1.7. This still large difference was rationalized on the basis of the different deformation mechanisms generated by indenters of different face angles

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