Oliver K. Johnson

Inferring grain boundary structure–property relations from effective property measurements

AbstractGrain boundaries strongly affect many materials properties in polycrystalline materials. However, very few structure–property models exist for grain boundaries, due in large part to the complicated and poorly understood way in which the properties of grain boundaries vary with their crystallographic structure. In the present work, we infer grain boundary structure–property correlations from measurements of the effective properties of a polycrystal. We refer to this approach as grain boundary properties localization. We apply this technique to a simple model system of grain boundary diffusivity in a two-dimensional microstructure, and infer the properties of low- and high-angle grain boundaries from the effective diffusivity of the grain boundary network. The generalization and use of these methods could greatly reduce the computational and experimental effort required to establish structure–property correlations for grain boundaries. More broadly, the technique of properties localization could be used to infer the properties of many microstructural constituents in complex microstructures.

Nano-composite Sensors for Wide Range Measurement of Ligament Strain

Biological tissues routinely experience large strains and undergo large deformations during normal physiologic activity. In order to quantify this strain, researchers often use optical marker tracking methods which are tedious and difficult. This paper investigates a new technique for quantifying large strain (up to 40%) by use of piezoresistive composite strain gages. The High Displacement Strain Gages (HDSGs) being investigated are manufactured by suspending nickel nanostrands within a biocompatible silicone matrix. The conductive nickel filaments come into progressively stronger electrical contact with each other as the HDSG is strained, thus reducing the electrical resistivity which is measured using conventional techniques. In the present work, HDSGs measured the strain of bovine ligament under prescribed loading conditions. Results demonstrated that HDSGs are an accurate means for measuring ligament strains across a broad spectrum of applied deformations. The technique has application to most biological tissue characterization applications.

Convergence of the hyperspherical harmonic expansion for crystallographic texture

Advances in instrumentation allow a material texture to be measured as a collection of spatially resolved crystallite orientations rather than as a collection of pole figures. However, the hyperspherical harmonic expansion of a collection of spatially resolved crystallite orientations is subject to significant truncation error, resulting in ringing artifacts (spurious oscillations around sharp transitions) and false peaks in the orientation distribution function. This article finds that the ringing artifacts and the accompanying regions of negative probability density may be mitigated or removed entirely by modifying the coefficients of the hyperspherical harmonic expansion by a simple multiplicative factor. An addition theorem for the hyperspherical harmonics is derived as an intermediate result.