Constitutive equations for ligament and other soft tissue: evaluation by experiment

Ligaments, tendons and other soft tissues are nonlinearly viscoelastic. To discriminate among various constitutive equations which may be used to describe the tissue, appropriate experimental modalities are requisite. Ideally, testing should span physiologic ranges for load (or strain), load history (recovery and reloading), and load onset and duration, and a robust model will fit all data. Methods to expand the experimental window of time for relaxation and creep are presented and evaluated. The role of ramp, relaxation and recovery protocols is studied in the context of viscoelasticity describable by linear, quasi-linear, nonlinear superposition, Schapery, and multiple integral formulations. The advantages associated with testing protocols that expand the time windows for creep or relaxation are presented.     

Observation of the retina using the tandem scanning confocal microscope

A tandem scanning confocal microscope (TSCM) is currently being used to obtain high-resolution images of the human cornea in vivo. Advantages of confocal microscopy for in vivo imaging include optical sectioning and increased contrast through removal of scattered light. We have adapted the TSCM to view the retina in vivo by constructing an applanating lens and fitting the microscope with an imaging-intensifying camera of increased sensitivity. The microscope uses a spinning disc with 40,000 holes, each of 30 microns diameter, and a 100 W mercury arc lamp light source with a 455 nm long pass filter. The applanating lens is composed of three elements, two of which are movable for focusing. Images of a rabbit retina were obtained in vivo revealing the nerve fiber layer and blood vessels around the optic disc. The power density at the retina was calculated to be 3 mW/cm², which is well below the power levels of a direct or indirect ophthalmoscope. Magnification of the retinal image was approximately 60x and a 1 mm wide area of retina was in view. This prototype TSCM system demonstrates that images of a retina in vivo are obtainable with confocal microscopy and that the sharpness is comparable to standard fundus camera photography. Further modifications to improve the light level and alterations in the design of the objective should improve the quality of the images obtained and achieve the enhanced resolution of which, in theory, the confocal microscope is capable.     

Characterization of high-loss viscoelastic elastomers

The behaviour of a variety of high-loss viscoelastic elastomers is described. Measurements were conducted using a novel micromechanics apparatus which is capable of creep, constant load rate, subresonant dynamic and resonant dynamic experiments in bending and torsion upon a single specimen. The range of equivalent frequency is from 10^–6 Hz to several kilohertz under isothermal conditions.     

The Role of Gradient Effects in the Piezoelectricity of Bone

Stress-gadient effects in piezoelectricity are obtained from general nonlocality considerations. A nonlocal continuum representation of bone is appropriate, in view of bone's structure.     

Resonant ultrasound spectroscopy of cylinders over the full range of Poisson's ratio

Mode structure maps for freely vibrating cylinders over a range of Poisson's ratio, ν, are desirable for the design and interpretation of experiments using resonant ultrasound spectroscopy (RUS). The full range of isotropic ν (-1 to +0.5) is analyzed here using a finite element method to accommodate materials with a negative Poisson's ratio. The fundamental torsional mode has the lowest frequency provided ν is between about -0.24 and +0.5. For any ν, the torsional mode can be identified utilizing the polarization sensitivity of the shear transducers. RUS experimental results for materials with Poisson's ratio +0.3, +0.16, and -0.3 and a previous numerical study for ν = 0.33 are compared with the present analysis. Interpretation of results is easiest if the length∕diameter ratio of the cylinder is close to 1. Slight material anisotropy leads to splitting of the higher modes but not of the fundamental torsion mode.     

Properties of a chiral honeycomb with a poisson's ratio of — 1

A theoretical and experimental investigation is conducted of a two-dimensionally chiral honeycomb. The honeycomb exhibits a Poisson's ratio of —1 for deformations in-plane. This Poisson's ratio is maintained over a significant range of strain, in contrast to the variation with strain seen in known negative Poisson's ratio materials.     

Anisotropic polyurethane foam with Poisson'sratio greater than 1

Anisotropic polymer foams have been prepared, which exhibit a Poisson's ratio exceeding 1, and ratios of longitudinal to transverse stiffness exceeding 50. The foams are as much as 20 times stiffer in the longitudinal direction than the foams from which they were derived. The transformation process involved applying to open-cell polyurethane foam an axial strain of 25 to 45%, at a temperature above the softening point, followed by cooling under axial strain. This revised version was published online in November 2006 with corrections to the Cover Date.     

Multiwavelength Methods in Holographic Interferometry

Abstract

Techniques are presented which take advantage of the wavelength dependence of various phenomena in holographic interferometry. Image-plane interferograms illuminated with light containing multiple wavelengths exhibit colour dispersion of the fringes. From this dispersion, we extract full-field information concerning displacement components which are not disclosed by monochromatic illumination.     

The viscoelastic behavior of β-In3Sn and the nature of the high-temperature background

The damping behavior in torsion of single phase -In₃Sn has been evaluated at room temperature over a broad range of frequencies (10^–4 to 10³ Hz) and as a function of various mechanical and thermal treatments. The results are consistent with a model for the power-law (tan f ^–n), high-temperature-background absorption being effected by diffusional processes on grain and, particularly, on subgrain boundaries. The results are compared/contrasted with those for damping in single-phase -InSn₄ and in the two-phase - eutectic. Failure of the eutectic material to follow a composite model for damping, combined with a thermal aging effect that lowers damping only for certain frequencies, shows that the boundary-based absorption model for the high-temperature background applies, too, to phase boundaries.