Poisson's ratio, ν, is a fundamental parameter characterizing the mechanical behavior of a material. Because the ratio of the bulk to the shear modulus, B/G, becomes infinite when ν=1/2, it is often ...assumed that the bulk modulus becomes very large as a material approaches “incompressibility.” This is incorrect; experimental results for viscoelastic materials show that changes in the bulk modulus are actually negligible as ν approaches 1/2. An analysis is performed to clarify the apparent conflict between the classic elastic equations and the experiments. At ν=1/2, the bulk modulus is shown to exhibit a singularity, but this is irrelevant to real materials.
Stress–strain measurements are reported for an elastomeric polyurea in uniaxial tension over a range of strain rates from 0.06 to 573
s
−1. The experiments were carried out on a new drop weight test ...instrument, which provides mechanical data at strain rates up to 1000
s
−1, filling the gap between conventional low speed instruments and split Hopkinson bar tests. The tensile data obtained herein are compared with recent high strain rate compression data on the same material Yi et al. Polymer 2006;47:319–29. Advantages of the present measurements include a more uniform strain rate and the ability to ensure homogeneous strain.
Infrared thermography was carried out on a polyurea, stretched to failure, over four decades of strain rates (0.026–400 s−1). A correction for convective heat transfer was developed that enabled the ...thermal response of slower experiments to be compared to adiabatic measurements. Overall the deformation was exothermic, but in contrast to simple, homogeneous elastomers, the temperature change was a complicated function of strain and rate. The largest temperature rise was 20 °C, which in comparison to other rubbers (e.g., natural and styrene butadiene rubber, both neat and reinforced with filler), is about twice that at failure and 5–10 times that at comparable strains. These temperature changes in the polyurea correspond to a half decade shift toward higher frequency of the soft segment dynamics. At low rates (<1 s−1), the temperature increased up to a strain of ca. 3, with the subsequent decline corresponding to an upturn in the stress. At high strain rates (>1 s−1), the temperature increased monotonically. For samples stretched to failure, there was a maximum in the temperature increase versus strain rate at an intermediate rate = 1.2 s−1, due to the competing effects of greater heat generation and lower failure strain. Thermoelastic inversion was observed at low rates at ca. 4% strain, consistent with the thermal expansion coefficient, indicating that entropic elasticity is a dominant mechanism at low strains. However, at higher strains the deformation departs from this behavior, with endothermic processes commencing as the material begins to yield. These processes are identified with plastic deformation and breakup of the hard domains within the phase-separated polyurea structure. An energy balance indicates that, notwithstanding the large temperature increases, structural changes account for the largest part of the strain energy.
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•Polyurea heats 5–20 °C during strain (5–10 × normal rubbers).•Heat increases with rate.•75% of energy goes to structural changes.•Low strain, low rate thermoelastic inversion: elasticity is entropic.•Highest rates: full recovery, constant thermal energy fraction; entropic elasticity.
The lower bound customarily cited for Poisson's ratio ν, −1, is derived from the relationship between ν and the bulk and shear moduli in the classical theory of linear elasticity. However, ...experimental verification of the theory has been limited to materials having ν 0.2. From consideration of the longitudinal and biaxial moduli, we recently determined that the lower bound on ν for isotropic materials from this theory is actually . Herein we generalize this result, first by analyzing expressions for ν in terms of six common elastic constants, and then by considering arbitrary strains. The results corroborate that for classical linear elasticity to be applicable. Of course, a few materials exist for which ν < 0.2, thus deviating from this bound; accurate analysis of their mechanical behavior requires more sophisticated elasticity models.
Extensional stress−strain measurements on a polyurea (PU) were carried out at strain rates up to 830 s−1, in combination with ex post facto small-angle X-ray scattering (SAXS) measurements and ...temperature-dependent SAXS. The elastomer is of interest because of its application as an impact-resistant coating. The highest strain rates used herein fall within the softening, or transition, zone of the viscoelastic spectrum and are thus relevant to the working hypothesis that the performance of a polyurea impact coating is related to its transition to the glassy state when strained very rapidly. While quasi-static and slow deformation of the PU gives rise to irrecoverable strain and anisotropic SAXS patterns, when stretched at high rates the PU recovers completely and the scattering is isotropic. Thus, the deformation of the hard domains observed at low rates is absent at high strain rates. Linear dynamic mechanical measurements were also carried out, with the obtained segmental relaxation times in good agreement with dielectric relaxation measurements on this material. The PU exhibits the usual breakdown of time−temperature superposition in the transition zone. This thermorheological complexity underlies the fact that published time−temperature shift factors for this material are unrelated to the segmental dynamics, and therefore use of these shift factors to predict the onset of glassy dynamics during impact loading of the PU will be in error.
The most common form of non-insulin-dependent diabetes mellitus (NIDDM) is characterized by obesity, insulin resistance, insulin secretory dysfunction, and overproduction of glucose in the liver. The ...relative roles of these metabolic abnormalities in the causation of NIDDM remain controversial,
1
,
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because once the disease has developed it is impossible to determine the initial events. Cross-sectional studies of subjects at high risk for NIDDM provide some information about the characteristics that may lead to the development of the disease, but these studies are limited by the lack of knowledge of which subjects will indeed go on to have the disease. Only . . .
Assessment of language organization is crucial in patients considered for epilepsy surgery. In children, the current techniques, intra-carotid amobarbital test (IAT) for language dominance, and ...cortical electrostimulation mapping (ESM), are invasive and risky. Functional magnetic resonance imaging (fMRI) is an alternative method for noninvasive functional mapping, through the detection of the hemodynamic changes associated with neuronal activation. We used fMRI, to assess language dominance in children with partial epilepsy.
Eleven right handed children and adolescents performed a word generation task during fMRI acquisition focused on the frontal lobes. Areas where the signal time course correlated with the test paradigm (r = 0.7) were considered activated. Extent and magnitude of signal changes were used to calculate asymmetry indices. Seven patients had IAT, ESM, or surgery outcome available for comparison.
fMRI language dominance always agreed with IAT (6 cases) and ESM (1 case), showing left dominance in six and bilateral language in one. fMRI demonstrated left dominance in three additional children, and right dominance in one with early onset of left temporal epilepsy. Four children whose initial studies were equivocal due to noncompliance or motion artifacts were restudied successfully.
fMRI can be used to assess language lateralization noninvasively in children. It has the potential to replace current functional mapping techniques in patients, and to provide important data on brain development.
Poisson's ratio, ν, was measured for four materials, a rubbery polymer, a conventional soft foam, and two auxetic foams. We find that for the first two materials, having ν ≥ 0.2, the experimental ...determinations of Poisson's ratio are in good agreement with values calculated from the shear and tensile moduli using the equations of classical elasticity. However, for the two auxetic materials (ν < 0), the equations of classical elasticity give values significantly different from the measured ν. We offer an interpretation of these results based on a recently published analysis of the bounds on Poisson's ratio for classical elasticity to be applicable.
Mice expressing an error-prone mitochondrial DNA polymerase rapidly accumulate random mutations in mitochondrial DNA. Expression of the transgene in the heart leads to dilated cardiomyopathy ...accompanied by a wave of apoptosis in cardiomyocytes, and a vigorous and persistent protective response, including upregulation of the anti-apoptotic protein, Bcl-2. To investigate the role of the mitochondrial permeability transition pore in the development of disease, we treated mice with cyclosporin A (CsA), an inhibitor of pore opening. Drug treatment prevented cardiac dilatation, transgene-specific apoptosis, and upregulation of Bcl-2. It also rescued hearts from the profound decrease in connexin 43, which characterizes the dilatated heart. Treatment with FK506, which like CsA inhibits cytoplasmic calcineurin but not the mitochondrial pore, did not affect disease development, suggesting that the relevant target of CsA was the mitochondrial pore. These data implicate breakdowns in the mitochondrial permeability barrier in pathogenesis of elevated frequencies of mtDNA mutations.
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