Macro-mechanical Response Research Articles

Continuum Model for Ion Gating in Cochlear Hair Cells

The details of ion flow in cells are important for the macro mechanical response of sensory organs, as in the inner ear of vertebrates. A continuum model for the flow of an ionized fluid through a pore in an elastic wall is considered. With this, the electro-mechanical behavior is readily computed. An interesting result is that two equilibria exist for the flow through a pore with a soft elastic entrance region. At one equilibrium point, the entrance opening is more restricted and the corresponding ion flow rate is much lower. Thus it appears that the tendency of ion channels to pop from an open to a closed state may be a consequence of basic fluid-elastic interaction. With the simple continuum model, various effects can be explored. One is the presence of an external spring force, corresponding to the force of the tip fiber between stereocilia. Another is the effect of an adjacent membrane and pore. The input physical parameters are reasonable and the results for flow rate are consistent with experimental measurements.

Micromechanics of granular media Part II: Overall tangential moduli and localization model for periodic assemblies of circular disks

In a previous paper, a mathematical model that captures the effects of microstructure on the macromechanical response of granular materials was presented. The overall response of the particle assembly derives the response of a macroscopic point using the conventional hypothesis in cmputational plasticity, i.e. a given overall displacement gradient determines the overall stress response. Numerical simulations with uniform as well as non-uniform assemblies of two-dimension circular disks capture, at least qualitatively, such mechanical responses as structural anisotropy, pressure dependency and strain-softening effects due to microstructural changes. The present paper attempts to re-analyze the problem from an opposite point of view, i.e. the overall imposed stresses determine the overall strain response. This formulation allows the model to capture naturally the volume-change characteristics of granular materials such as compaction as well as dilation, since the strains now result from the analysis rather than act as a prescribed force that drives the problem. The new formulation also requires the derivation of the so-called overall algorithmic tangential moduli from the consistent linearization of the elasto-plastic constitutive equation. The task becomes complicated because the linearization permeates down to the microstructural level, thus necessitating the extraction of the so-called algorithmic concentration tensor. To circumvent this difficulty, we approximate the algorithmic tensor of overall moduli with a secant approximation that retains the essential properties of the exact description. We then use this approximation to solve the inverse problem as well as perform some stability analyses that could be useful in predicting the onset of localized deformation in granular materials.

Microstructure‐property relationships in silicate‐matrix composites

SUMMARYElectron microscopy and associated analytical spectroscopic techniques have been used to characterize interfaces in SiC‐fibre/silicate‐matrix composites. Interface structure, formed via reaction during hot‐press fabrication, is a function of time, temperature, matrix composition and fibre type. Interfaces with Nicalon or Tyranno fibres vary from amorphous carbon in fine precipitated form to continuous graphitic layers.Interface behaviour in a stressed composite, and hence the matrix microcracking stress, is a sensitive function of microstructure. Interface debond and shear properties have been assessed using indent‐based ‘push‐down’ and ‘push‐through’ tests using a specially developed instrument within a scanning electron microscope. This uses piezoload measurement and translation, and is capable of dynamic image recording of the indentation sequence.Interface micromechanical (indent) measurements have been correlated with structure and macromechanical response in bend testing for a range of fibre/matrix types, processing and post‐processing thermal treatments. An example is also given of interfaces prepared by fibre precoating.

Effect of coiling in a cochlear model.

Transformation of the three-dimensional equations of fluid motion into cylindrical coordinates allowed analysis of a coiled cochlear model by the WKB technique. The model includes a single transverse mode of basilar membrane deflection and inviscid fluid. The results calculated using realistic parameters for the guinea pig show no significant difference in the basilar membrane amplitude and phase between the straight and coiled models. Some differences exist in the fluid pressure found in the scala. The conclusion is that the macromechanical response is not significantly affected by coiling.