Deformation-induced accelerated dynamics in polymer glasses

Molecular dynamics simulations are used to investigate the effects of deformation on the segmental dynamics in an aging polymer glass. Individual particle trajectories are decomposed into a series of discontinuous hops, from which we obtain the full distribution of relaxation times and displacements under three deformation protocols: step stress (creep), step strain, and constant strain rate deformation. As in experiments, the dynamics can be accelerated by several orders of magnitude during deformation, and the history dependence is entirely erased during yield (mechanical rejuvenation). Aging can be explained as a result of the long tails in the relaxation time distribution of the glass, and similarly, mechanical rejuvenation is understood through the observed narrowing of this distribution during yield. Although the relaxation time distributions under deformation are highly protocol specific, in each case they may be described by a universal acceleration factor that depends only on the strain.Comment: 15 pages, 15 figure

Effect of Heat-Treatment Time on Bending Properties of Cobolt-Chromium Orthodontic Wires

Different protocols appear in the literature with respect to heat-treating cobalt-chromium orthodontic wires. The objective was to determine the effect of variable heat-treatment time and method on the bending properties of CoCr wires. Two tempers of CoCr ‘Elgiloy’ wires (Rocky Mountain Orthodontics), blue (B) and yellow (Y), were heat-treated for different durations. The groups (n=20/group/temper) included: 1) as-received (control); 2) brush-flame; 3) 480oC for 5sec; 4) 480oC for 10min; 5) 480oC for 2hr; and 6) 480oC for 5hr. Wire segments were tested by a three-point bend test. Stiffness/flexural modulus, percent recovery, and force values at select deflections were statistically compared using ANOVA/Bonferroni post-hoc test (p\u3c0.05). A T-test compared the different tempers. Longer heat-treatment (2hr/5hr) increased % recovery, flexural modulus, and force values when compared to the as-received counterparts. Heat treatment for 10 minutes resulted in intermediate increases. Using a brush-flame technique reduced elastic recovery and resulted in greater bending variability. Similar mechanical properties can be achieved in just 2 hours compared to the manufacturer recommended 5 hours of heat-treatment of Elgiloy wires. Ten minutes of heat-treatment, which may be more realistic in a busy orthodontic practice, can increase bending properties 50-75% compared to the 5-hour group. The brush-flame technique is not recommended due to inconsistent heating conditions resulting in varying bending properties

Mechanistic and pathological study of the genesis, growth, and rupture of abdominal aortic aneurysms

Postprint (published version

Structure-stiffness relation of live mouse brain tissue determined by depth-controlled indentation mapping

The mechanical properties of brain tissue play a pivotal role in neurodevelopment and neurological disorders. Yet, at present, there is no consensus on how the different structural parts of the tissue contribute to its stiffness variations. Here, we have gathered depth-controlled indentation viscoelasticity maps of the hippocampus of isolated horizontal live mouse brain sections. Our results confirm the highly viscoelestic nature of the material and clearly show that the mechanical properties correlate with the different morphological layers of the samples investigated. Interestingly, the relative cell nuclei area seems to negatively correlate with the stiffness observed

The Compression-Mode Giant Resonances and Nuclear Incompressibility

The compression-mode giant resonances, namely the isoscalar giant monopole and isoscalar giant dipole modes, are examples of collective nuclear motion. Their main interest stems from the fact that one hopes to extrapolate from their properties the incompressibility of uniform nuclear matter, which is a key parameter of the nuclear Equation of State (EoS). Our understanding of these issues has undergone two major jumps, one in the late 1970s when the Isoscalar Giant Monopole Resonance (ISGMR) was experimentally identified, and another around the turn of the millennium since when theory has been able to start giving reliable error bars to the incompressibility. However, mainly magic nuclei have been involved in the deduction of the incompressibility from the vibrations of finite nuclei. The present review deals with the developments beyond all this. Experimental techniques have been improved, and new open-shell, and deformed, nuclei have been investigated. The associated changes in our understanding of the problem of the nuclear incompressibility are discussed. New theoretical models, decay measurements, and the search for the evolution of compressional modes in exotic nuclei are also discussed.Comment: Review paper to appear in "Progress in Particle and Nuclear Physics