916 research outputs found
Low level of virological failure and drug resistance among patients receiving antiretroviral treatment under programme conditions in Maputo, Mozambique
Mexico AIDS Conference 200
Shakedown for slab track substructures with stiffness variation
In this paper, shakedown analyses are carried out to predict the long-term response of slab track substructures under repeated moving train loads. The train loads are converted into a distributed moving load on the substructure surface by using a simplified track analysis. Based on Melan’s static shakedown theorem, a well-established shakedown analysis method is extended to determine shakedown limits of the slab track substructures. The influence of a linearly increasing stiffness modulus on the shakedown limits is considered by conducting finite- element analysis with a user-defined material. It is found that a rise in stiffness modulus or stiffness variation ratio can either increase or decrease the shakedown limit, depending on the competitive effects of the two mechanisms. Furthermore, the subgrade thickness determines the dominant mechanism
Thermoelastic Damping in Micro- and Nano-Mechanical Systems
The importance of thermoelastic damping as a fundamental dissipation
mechanism for small-scale mechanical resonators is evaluated in light of recent
efforts to design high-Q micrometer- and nanometer-scale electro-mechanical
systems (MEMS and NEMS). The equations of linear thermoelasticity are used to
give a simple derivation for thermoelastic damping of small flexural vibrations
in thin beams. It is shown that Zener's well-known approximation by a
Lorentzian with a single thermal relaxation time slightly deviates from the
exact expression.Comment: 10 pages. Submitted to Phys. Rev.
Analysis of coupled heat and moisture transfer in masonry structures
Evaluation of effective or macroscopic coefficients of thermal conductivity
under coupled heat and moisture transfer is presented. The paper first gives a
detailed summary on the solution of a simple steady state heat conduction
problem with an emphasis on various types of boundary conditions applied to the
representative volume element -- a periodic unit cell. Since the results
essentially suggest no superiority of any type of boundary conditions, the
paper proceeds with the coupled nonlinear heat and moisture problem subjecting
the selected representative volume element to the prescribed macroscopically
uniform heat flux. This allows for a direct use of the academic or commercially
available codes. Here, the presented results are derived with the help of the
SIFEL (SIimple Finite Elements) system.Comment: 23 pages, 11 figure
High frequency poroelastic waves in hydrogels
In this work a continuum model for high frequency poroelastic longitudinal
waves in hydrogels is presented. A viscoelastic force describing the
interaction between the polymer network and the bounded water present in such
materials is introduced. The model is tested by means of ultrasound wave speed
and attenuation measurements in polyvinylalcohol hydrogel samples. The theory
and experiments show that ultrasound attenuation decreases linearly with the
increase of the water volume fraction "{\beta}" of the hydrogel. The
introduction of the viscoelastic force between the bounded water and the
polymer network leads to a bi-phasic theory showing an ultrasonic fast wave
attenuation that can vary as a function of the frequency with a non-integer
exponent in agreement with the experimental data in literature. When {\beta}
tends to 1 (100% of interstitial water) due to the presence of bounded water in
the hydrogel, the ultrasound phase velocity acquires higher value than that of
pure water. The ultrasound speed gap at {\beta} = 1 is confirmed by the
experimental results that show that it increases in less cross-linked gel
samples that own a higher concentration of bounded water
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