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.
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.
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.
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.
IMPORTANCE: Patients with frailty have higher risk for postoperative mortality and complications; however, most research has focused on small groups of high-risk procedures. The associations among ...frailty, operative stress, and mortality are poorly understood. OBJECTIVE: To assess the association between frailty and mortality at varying levels of operative stress as measured by the Operative Stress Score, a novel measure created for this study. DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study included veterans in the Veterans Administration Surgical Quality Improvement Program from April 1, 2010, through March 31, 2014, who underwent a noncardiac surgical procedure at Veterans Health Administration Hospitals and had information available on vital status (whether the patient was alive or deceased) at 1 year postoperatively. A Delphi consensus method was used to stratify surgical procedures into 5 categories of physiologic stress. EXPOSURES: Frailty as measured by the Risk Analysis Index and operative stress as measured by the Operative Stress Score. MAIN OUTCOMES AND MEASURES: Postoperative mortality at 30, 90, and 180 days. RESULTS: Of 432 828 unique patients (401 453 males 92.8%; mean (SD) age, 61.0 12.9 years), 36 579 (8.5%) were frail and 9113 (2.1%) were very frail. The 30-day mortality rate among patients who were frail and underwent the lowest-stress surgical procedures (eg, cystoscopy) was 1.55% (95% CI, 1.20%-1.97%) and among patients with frailty who underwent the moderate-stress surgical procedures (eg, laparoscopic cholecystectomy) was 5.13% (95% CI, 4.79%-5.48%); these rates exceeded the 1% mortality rate often used to define high-risk surgery. Among patients who were very frail, 30-day mortality rates were higher after the lowest-stress surgical procedures (10.34%; 95% CI, 7.73%-13.48%) and after the moderate-stress surgical procedures (18.74%; 95% CI, 17.72%-19.80%). For patients who were frail and very frail, mortality continued to increase at 90 and 180 days, reaching 43.00% (95% CI, 41.69%-44.32%) for very frail patients at 180 days after moderate-stress surgical procedures. CONCLUSIONS AND RELEVANCE: We developed a novel operative stress score to quantify physiologic stress for surgical procedures. Patients who were frail and very frail had high rates of postoperative mortality across all levels of the Operative Stress Score. These findings suggest that frailty screening should be applied universally because low- and moderate-stress procedures may be high risk among patients who are frail.
Use of pneumococcal conjugate vaccines has caused emergence of non-vaccine serotypes. No Brazilian data specifically about serotype 19A are available. We aimed to evaluate the frequency of ...occurrence, susceptibility profile and molecular epidemiology of serotype 19A before and after vaccine introduction in Brazil. Pneumococcal identification was performed by the conventional method. Strain serotype was determined by multiplex polymerase chain reaction (PCR) and/or Quellung reaction. Resistance was determined by Etest® and PCR was performed to determine the presence of macrolide resistance genes, ermB and/or mefA. Pneumococci were typed by Multilocus Sequence Typing. Thirty-eight serotype 19A Streptococcus pneumoniae were recovered, mostly from invasive diseases. Prevalence of serotype 19A increased following vaccination (from 3.5% before vaccination to 8.1% after, p = 0.04196). Non-susceptibility increased to most antimicrobials after vaccine introduction and was associated with clonal complex (CC)320. MLST showed nine different STs, which were grouped in one main CC: CC320 (63.9%). During the post-vaccination era, the frequency of this serotype increased significantly from 1.2% in 2011 to 18.5% in 2014 (p = 0.00001), with a concomitant decrease in the genetic variability: ST320 consistently predominated after vaccine-introduction (61.1%). Overall, our results showed a post-PCV10 increase in the frequency of serotype 19A. This was accompanied by a selection of CC320 and antimicrobial resistance.
We report a demonstration of vapor-phase Rubidium (Rb) density stabilization in a vapor cell using a solid-state electrochemical Rb source device. Clear Rb density stabilization is observed. Further ...demonstrations show that the temperature coefficient for Rb density can be reduced more than 100 times when locked and the device's power consumption is less than 10 mW. Preliminary investigation of the locking dynamic range shows that the Rb density is well stabilized when the initial density is five times higher (33 × 10
/cm
) than the set point density (6 × 10
/cm
). Active stabilization with this device is of high interest for portable cold-atom microsystems where large ambient temperature working ranges and low power consumption are required.
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