Micromechanical Response Research Articles

Postnatal maturation of the organ of Corti in gerbils: Morphology and physiological responses

The organ of Corti, the sensory epithelium of hearing in mammals, matures postnatally in the gerbil. Quantitative analyses of the postnatal development of the organ of Corti, including supporting cells and the basilar membrane, were carried out. The morphological study confirmed that maturation of the sensory cells proceeds with a base-to-apex gradient, with the outer hair cells appearing to mature before the inner hair cells. Maturation of the supporting cells and the basilar membrane commenced first in the middle turn. Expansion of the second row of Deiters' cells began at 6 days after birth in the middle turn, before enlargement of the pillar cell heads at 8 days postnatally. Pillar cell head enlargement continued until 20 days postnatally in the middle turn. The tunnel of Corti and spaces of Nuel appeared first in the middle turn between 8 and 10 days postnatally. The maturation of the basilar membrane involved the thickening of the central hyaline layer and a reduction in the epithelial cells on the tympanic aspect. This process continued until about 20 days after birth. The cochlear microphonic potential, whole nerve action potential, and stimulus frequency otoacoustic emissions were recorded from 12 days after birth onward and related to changes in organ of Corti morphology. The results show that changes in the accessory structures continue throughout the period of onset and development of cochlear responses between 12 and 20 days after birth, and may therefore influence the micromechanical responses of the organ of Corti to acoustic stimuli during this period.

Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (i) hot-pressing with the addition of a Ca/B-containing glass and (ii) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2300°C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600°C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries.

Determination of the interface failure mechanism during transverse loading of single fibre SiC/Ti-6Al-4V composites from torsion tests

During transverse loading of continuous fibre-reinforced composites, large tangential shear stresses develop at the interface at about 45° to the loading axis. These shear stresses may initiate interface failure and frictional sliding, increasing the load in the matrix and lowering the applied stress to cause normal fibre-matrix separation. In the present study, an attempt has been made to measure the tangential shear strength of the interface for single fibre SiC/Ti-6Al-4V composites with varying interfaces using torsion testing and finite element stress analysis. An energy based fracture mechanics approach was developed to predict the micro-mechanical response of the composite to torsional loading following interface crack initiation. The fracture mechanics analysis suggests that a non-linearity would appear in the strain versus angle of twist curve once debonding initiates. Based on the onset of non-linearity and finite element stress analysis, the tangential shear strength for SCS-6/Ti-6Al-4V interface (carbon-rich coated) was estimated at 50 MPa; that for the SCS-0/Ti-6Al-4V interface (uncoated) was greater than 80 MPa, and that for the AC1/Ti-6Al-4V interface (carbon coated) was about 40 MPa. Comparison of these results with experimental observations and finite element analysis of transverse tension tests indicates that failure in SCS-6/Ti-6Al-4V and AC1/Ti-6Al-4V composites is initiated by tangential shear failure of the interface. In SCS-0/Ti-6Al-4V composites, the results are not conclusive.

Theoretical Model for Viscoelastic Sliding of Interfaces and Creep Behavior of Anion-Doped Si<sub>3</sub>N<sub>4</sub>/SiC Composites

A theoretical model for relating the interface viscosity to the creep behavior of highly refractory ceramics is proposed. Experimental confirmation of the model is given by monitoring the effect of systematic modifications in the interface viscosity on the creep behavior of Si3N4/SiC composites. A model material was selected for this basic study with a continuous film, consisting of glassy SiO2, at the interface, whose bulk anion composition was varied by incorporating increasing amounts of glass-network modifier ions (i.e., F or Cl anions). Internal friction measurements allowed to experimentally evaluate the micromechanical response of the internal interfaces, when subjected to an externally applied shear stress. Starting from this important experimental information, the short-term macroscopic creep behavior of the polycrystal could be predicted and consistently related to microscopic interface viscosity parameter.

FFT-BASED HOMOGENIZATION ALGORITHM USING DIGITAL IMAGES

The applicability of the FFT-based algorithm for evaluating the mechanical behaviors of composites is investigated from practical view points. After the iterative numerical algorithm is presented, the computational efficiency is compared with that for conventional FEM-based homogenization algorithm and then extended to nonlinear analyses such as elastoplastic problems. In addition to preferable features in microstructural analyses, there are several advantages over the conventional approach by the FEM-based homogenization method. While the method is for evaluating the micromechanical response to the macroscopic deformation, the solution method can also be utilized to estimate the homogenized material constants. In the numerical examples, the digital image processing technique is extensively utilized to define the geometry of microstructures so that the method would not suffer from modeling effort.

Interface effects on the micromechanical response of a transversely loaded single fiber SCS-6/Ti-6Al-4V composite

The ability of a fiber-matrix interface to support a transverse load is typically evaluated in straight-sided composite specimens where a stress singularity exists at the free surface of the interface. This stress singularity is often the cause of crack initiation and debonding during transverse loading. In order to develop a fundamental understanding of the transverse behavior of the fiber-matrix interface, it is necessary to alter the crack initiation site from the free surface to an internal location. To achieve this objective, a cross-shaped specimen has been recently developed. In this study, based on the experimentally observed onset of nonlinearity in the stress-strain curve of these specimens and finite element analysis, the bond strength of the SCS-6/Ti-6Al-4V interface was determined to be 115 MPa. The micromechanical behavior of these specimens under transverse loading was examined by finite element analysis using this interface bond strength value and compared with experimental observations. Results demonstrate that the proposed geometry was successful in suppressing de-bonding at the surface and altering it to an internal event. The results from numerical analysis correlated well with the experimental stress-strain curve and several simple analytical models. In an attempt to identify the true bond strength and the interface failure criterion, the present study suggests that if failure initiates under tensile radial stresses, then the normal bond strength of the SCS-6/Ti-6A1-4V composites is about 115 MPa; under shear failure, the tangential shear strength of the in-terface is about 180 MPa.

MICROSTRUCTURAL DESIGN OF COMPOSITES USING THE HOMOGENIZATION METHOD AND DIGITAL IMAGES

A microstructural design method is developed by utilizing the homogenization method and digital image-based (DIB) geometric modeling technique. The localization capability of the asymptotic homogenization method enables the evaluation of the microscopic deformation in the microstructure, i.e., representative volume element (RVE). This distinctive feature allows us to use the microscopic variables in the functional forms of constraints and objective function. In estimating the micromechanical responses, the systematic modeling method in terms of the pixel information of images and their processing make the construction of unit cell geometry easy and the homogenization results accurate. Therefore, while the specific microstructural geometry is taken into account in evaluating the micro and macroscopic variables, the optimal microstructural configurations can be determined depending on our objectives

Micromechanics of granular media Part I: Generation of overall constitutive equation for assemblies of circular disks

Many problems of special interest to engineers and earth scientists involve characterization of the overall engineering behavior of granular media. Granular materials respond quite differently from polycrystalline materials such as metals due to their particulate nature and random microstructure. Since the particle-to-particle interaction is frictional in nature, contacts in granular media can easily form or break as the particles move towards their new equilibrium positions. From a modeling standpoint, the prediction of the new microstructure of a granular material is a major source of numerical difficulty. Consequently, the effects of microstructure on the overall response of granular media remain hardly understood. This work attempts to capture the effects of microstructural changes on the overall response of granular media. A mathematical model is presented in which the response of a macroscopic point is derived from the overall response of the particle assembly. The conventional hypothesis in computational plasticity is employed, i.e. the overall stress response of the assembly is determined from the micromechanical responses of the constituent granules to a given overall displacement gradient. An important feature of the present formulation is the quasi-static nature in which the micromechanical responses are computed. A quasi-static approach is more appropriate for a wide class of engineering applications where inertia effects are negligible. To assess the performance of the model, numerical results obtained from two-dimensional plane-strain simulations with regular and random initial packings of circular disks are presented.

The rosette complex in gerbil Deiters cells contains γ actin

The relationship of selected cytoskeletal elements with the rosette complex of Deiters cells was examined immunocytochemically in the gerbil cochlea. By light microscopy, the staining pattern for actin in the apical portion of Deiters cells corresponded with the location of the rosette complex. At the ultrastructural level, the actin antibodies bound selectively at the periphery of the dense trabeculae in the center of the complex. Comparative staining with a battery of polyclonal and isoform-specific monoclonal antibodies revealed selective presence of the γ muscle actin isoform in this location. The loose meshwork at the periphery of the rosette complex stained selectively with a monoclonal antibody to vimentin. β-tubulin was not associated with the rosette complex but occurred in abundance in the microtubule-rich stalk. Actin and vimentin were not detected in the apical compartment of Deiters cells at the extreme base of the cochlea, thus confirming their association with the rosette complex which is not present in regions of the gerbil cochlea tuned to frequencies to 20 kHz or higher (Spicer and Schulte, 1993, 1994). The cytoarchitecture of the rosette complex and its preferential distribution along the place-frequency map promote speculation that Deiters cells may play a role in regulating ion homeostasis and/or micromechanical response properties of the organ of Corti.

Presynthesised interfaces for ceramic matrix composites via fibre coating

Chemical vapour deposited carbon and silicon carbide have been applied to Nicalon SiC fibres. The chemical compatibility and mechanical integrity have been evaluated via conventional techniques. The bi-layer coated fibres were hot pressed within a magnesium-aluminium-silicate (MAS) glass-ceramic matrix to determine thermal stability, structure and morphology of the embedded fibres. The micromechanical response of the fibre/matrix interface was determined using a unique scanning electron microscope (SEM) based microindentation system designed to measure interfacial shear strengths during fibre push down.

Response of metal matrix composites with imperfect bonding

The method of cells is employed to study the micromechanical response of metal matrix composites as a function of variable bonding parameters. Elastic constants and nonlinear stress-strain responses are predicted as functions of normal and tangential displacement discontinuities between the fiber and the matrix. Typical results are presented for axial, transverse and shear loading of unidirectional silicon-carbide/titanium. It is shown that imperfect bonding can degrade both the elastic constants and nonlinear response of composites with the degree of degradation dependent upon the type and degree of imperfect bond.

Microstructural characterization of interafaces in fibre-reiforced ceramics

A survey is presented of the principles underlying the design of model interface microstructures which have the required micromechanical response and thermal or environmental stability within fibre-reinforced ceramic matrix composites. Microstructural and compositional characterisation of interfaces, based on electron imaging and spectroscopic techniques, are reviewed with illustrative examples of interfaces formed by in-situ reaction during fabrication or by fibre precoating.

The Modified Mixture Theory for Fluid-Filled Porous Materials: Theory

The mixture theory by Green and Naghdi is modified and applied to the problems of flow-through porous materials. By introducing porosity, we make clear that two constituents occupying the same point in the original mixture theory are an equivalent homogeneous solid and an equivalent homogeneous fluid which, respectively, represent a porous solid and a porous fluid in the actual sample. The micro-mechanical response studied by Carroll and Katsube is introduced and detailed deformation and flow mechanisms are provided at each point of the mixture. The resulting theory is compared with Biot’s theory and in fact reduces to Biot’s theory when the fluid velocity gradient terms are ignored.

Ontogenetic Changes in Frequency Mapping of a Mammalian Ear

Cochlear microphonic iso-response functions reported here suggest an explanation of frequency-dependent changes in hearing sensitivity during early development. The work is a direct demonstration of developmental changes in the spatial frequency map of the mammalian hearing organ. Intracochlear recordings from the midbasal turn in a series of age-graded gerbils reveal a progressive increase in best frequency, spanning approximately two octaves, from the time of onset of function until adultlike responses are seen. It is, therefore, suggested that ontogenetic changes in the cellular structure of the organ of Corti contribute to an age-dependent shift in micromechanical response.