1,370 research outputs found
Rate dependent shear bands in a shear transformation zone model of amorphous solids
We use Shear Transformation Zone (STZ) theory to develop a deformation map
for amorphous solids as a function of the imposed shear rate and initial
material preparation. The STZ formulation incorporates recent simulation
results [Haxton and Liu, PRL 99 195701 (2007)] showing that the steady state
effective temperature is rate dependent. The resulting model predicts a wide
range of deformation behavior as a function of the initial conditions,
including homogeneous deformation, broad shear bands, extremely thin shear
bands, and the onset of material failure. In particular, the STZ model predicts
homogeneous deformation for shorter quench times and lower strain rates, and
inhomogeneous deformation for longer quench times and higher strain rates. The
location of the transition between homogeneous and inhomogeneous flow on the
deformation map is determined in part by the steady state effective
temperature, which is likely material dependent. This model also suggests that
material failure occurs due to a runaway feedback between shear heating and the
local disorder, and provides an explanation for the thickness of shear bands
near the onset of material failure. We find that this model, which resolves
dynamics within a sheared material interface, predicts that the stress weakens
with strain much more rapidly than a similar model which uses a single state
variable to specify internal dynamics on the interface.Comment: 10 pages, 13 figures, corrected typos, added section on rate
strengthening vs. rate weakening material
Spontaneous thermal runaway as an ultimate failure mechanism of materials
The first theoretical estimate of the shear strength of a perfect crystal was
given by Frenkel [Z. Phys. 37, 572 (1926)]. He assumed that as slip occurred,
two rigid atomic rows in the crystal would move over each other along a slip
plane. Based on this simple model, Frenkel derived the ultimate shear strength
to be about one tenth of the shear modulus. Here we present a theoretical study
showing that catastrophic material failure may occur below Frenkel's ultimate
limit as a result of thermal runaway. We demonstrate that the condition for
thermal runaway to occur is controlled by only two dimensionless variables and,
based on the thermal runaway failure mechanism, we calculate the maximum shear
strength of viscoelastic materials. Moreover, during the thermal
runaway process, the magnitude of strain and temperature progressively localize
in space producing a narrow region of highly deformed material, i.e. a shear
band. We then demonstrate the relevance of this new concept for material
failure known to occur at scales ranging from nanometers to kilometers.Comment: 4 pages, 3 figures. Eq. (6) and Fig. 2a corrected; added references;
improved quality of figure
Strain localization in a shear transformation zone model for amorphous solids
We model a sheared disordered solid using the theory of Shear Transformation
Zones (STZs). In this mean-field continuum model the density of zones is
governed by an effective temperature that approaches a steady state value as
energy is dissipated. We compare the STZ model to simulations by Shi, et
al.(Phys. Rev. Lett. 98 185505 2007), finding that the model generates
solutions that fit the data,exhibit strain localization, and capture important
features of the localization process. We show that perturbations to the
effective temperature grow due to an instability in the transient dynamics, but
unstable systems do not always develop shear bands. Nonlinear energy
dissipation processes interact with perturbation growth to determine whether a
material exhibits strain localization. By estimating the effects of these
interactions, we derive a criterion that determines which materials exhibit
shear bands based on the initial conditions alone. We also show that the shear
band width is not set by an inherent diffusion length scale but instead by a
dynamical scale that depends on the imposed strain rate.Comment: 8 figures, references added, typos correcte
Anti-Pasch optimal packings with triples
It is shown that for v ≠ 6, 7, 10, 11, 12, 13, there exists an optimal packing with triples on v points that contains no Pasch configurations. Furthermore, for all v ≡ 5 (mod 6), there exists a pairwise balanced design of order v, whose blocks are all triples apart from a single quintuple, and that has no Pasch configurations amongst its triples
Spontaneous dissipation of elastic energy by self-localizing thermal runaway
Thermal runaway instability induced by material softening due to shear
heating represents a potential mechanism for mechanical failure of viscoelastic
solids. In this work we present a model based on a continuum formulation of a
viscoelastic material with Arrhenius dependence of viscosity on temperature,
and investigate the behavior of the thermal runaway phenomenon by analytical
and numerical methods. Approximate analytical descriptions of the problem
reveal that onset of thermal runaway instability is controlled by only two
dimensionless combinations of physical parameters. Numerical simulations of the
model independently verify these analytical results and allow a quantitative
examination of the complete time evolutions of the shear stress and the spatial
distributions of temperature and displacement during runaway instability. Thus
we find that thermal runaway processes may well develop under nonadiabatic
conditions. Moreover, nonadiabaticity of the unstable runaway mode leads to
continuous and extreme localization of the strain and temperature profiles in
space, demonstrating that the thermal runaway process can cause shear banding.
Examples of time evolutions of the spatial distribution of the shear
displacement between the interior of the shear band and the essentially
nondeforming material outside are presented. Finally, a simple relation between
evolution of shear stress, displacement, shear-band width and temperature rise
during runaway instability is given.Comment: 16 pages, 7 figures. Extended conclusion; added reference
Feasibility of Prehospital Emergency Anesthesia in the Cabin of an AW169 Helicopter Wearing Personal Protective Equipment During Coronavirus Disease 2019
OBJECTIVE: Pre-hospital emergency anaesthesia in the form of rapid sequence intubation (RSI) is a critical intervention delivered by advanced pre-hospital critical care teams. Our previous simulation study determined the feasibility of in-aircraft RSI. We now examine whether this feasibility is preserved in a simulated setting, when clinicians wear personal protective equipment (PPE) for aerosol-generating procedures (AGP) for in-aircraft, on-the-ground RSI. METHODS: Air Ambulance Kent Surrey Sussex is a Helicopter Emergency Medical Service (HEMS) which utilises an AW169 cabin simulator. Wearing full AGP PPE (eye protection, FFP3 mask, gown, gloves), 10 doctor-paramedic teams performed RSI in a standard “can intubate, can ventilate” scenario and a “can't intubate, can't oxygenate” (CICO) scenario. Pre-specified timings were reported, and participant feedback was sought by questionnaire. RESULTS: RSI was most commonly performed by direct laryngoscopy and was successfully achieved in all scenarios. Time to completed endotracheal intubation (ETI) was fastest (287s) in the standard scenario and slower (370s, p=.01) in the CICO scenario. Time to ETI was not significantly delayed by wearing PPE in the standard (p=.19) or CICO variant (p=.97). Communication challenges, equipment complications and PPE difficulties were reported, but ways to mitigate these also reported. CONCLUSION: In-aircraft RSI (aircraft on-the-ground) whilst wearing PPE for AGPs had no significant impact on time to successful completion of ETI in a simulated setting. Patient safety is paramount in civilian HEMS, but the adoption of in-aircraft RSI could confer significant patient benefit in terms of pre-hospital time saving and further research is warranted
Mapping interactions between the sustainable development goals: lessons learned and ways forward
Pursuing integrated research and decision-making to advance action on the sustainable development goals (SDGs) fundamentally depends on understanding interactions between the SDGs, both negative ones (“trade-offs”) and positive ones (“co-benefits”). This quest, triggered by the 2030 Agenda, has however pointed to a gap in current research and policy analysis regarding how to think systematically about interactions across the SDGs. This paper synthesizes experiences and insights from the application of a new conceptual framework for mapping and assessing SDG interactions using a defined typology and characterization approach. Drawing on results from a major international research study applied to the SDGs on health, energy and the ocean, it analyses how interactions depend on key factors such as geographical context, resource endowments, time horizon and governance. The paper discusses the future potential, barriers and opportunities for applying the approach in scientific research, in policy making and in bridging the two through a global SDG Interactions Knowledge Platform as a key mechanism for assembling, systematizing and aggregating knowledge on interactions
Infrastructures, processes of insertion and the everyday: towards a new dialogue in critical policy studies
This forum argues that the complex assemblages of infrastructures, and their reproduction in our everyday worlds, offer a privileged lens through which to explore the practices of much of what critical policy studies holds dear. It draws attention to processes of insertion that reproduce infrastructure in everyday lives, arguing that such processes cast new light on the work of the state, governance, and democratic struggles. It discerns three avenues as a means of exploring such infrastructural processes: first, an invitation to transcend the physical form and reflect on infrastructural temporalities; second on the transformation of spatial governance and policy through infrastructure; and third, a re-assessment in the relationship between infrastructures and the ‘modernist ideal’. Through these avenues, light can be shed on the often ‘hidden’ practices of policymaking. We conclude by calling for a dialogue across diverse disciplines, side-stepping embedded divides between academics-activists, cities-towns, and the global south-north
Vascular disrupting agent for neovascular age related macular degeneration: a pilot study of the safety and efficacy of intravenous combretastatin A-4 phosphate
BACKGROUND:
This study was designed to assess the safety, tolerability, and efficacy of intravenous infusion of CA4P in patients with neovascular age-related macular degeneration (AMD). METHODS:
Prospective, interventional, dose-escalation clinical trial. Eight patients with neovascular AMD refractory to at least 2 sessions of photodynamic therapy received CA4P at a dose of 27 or 36 mg/m2 as weekly intravenous infusion for 4 consecutive weeks. Safety was monitored by vital signs, ocular and physical examinations, electrocardiogram, routine laboratory tests, and collection of adverse events. Efficacy was assessed using retinal fluorescein angiography, optical coherence tomography, and best corrected visual acuity (BCVA). RESULTS:
The most common adverse events were elevated blood pressure (46.7%), QTc prolongation (23.3%), elevated temperature (13.3%), and headache (10%), followed by nausea and eye injection (6.7%). There were no adverse events that were considered severe in intensity and none resulted in discontinuation of treatment. There was reduction of the excess foveal thickness by 24.15% at end of treatment period and by 43.75% at end of the two-month follow-up (p = 0.674 and 0.161, respectively). BCVA remained stable throughout the treatment and follow-up periods. CONCLUSIONS:
The safety profile of intravenous CA4P was consistent with that reported in oncology trials of CA4P and with the class effects of vascular disruptive agents; however, the frequency of adverse events was different. There are evidences to suggest potential efficacy of CA4P in neovascular AMD. However, the level of systemic safety and efficacy indicates that systemic CA4P may not be suitable as an alternative monotherapy to current standard-of-care therapy
Tunable Superconducting Gravity Gradiometer for Mars Climate, Atmosphere, and Gravity Field Investigation
We are developing a compact tensor superconducting gravity gradiometer (SGG) for obtaining gravimetric measurements from planetary orbits. A new and innovative design gives a potential sensitivity of approximately 10(sup -4) E Hz(sup - 1/2)( 1 E = 10(sup -9 S(sup -2) in the measurement band up to 0.1 Hz (suitale for short wavelength static gravity) and of approximately 10(sup -4) E Hz(sup - 1/2) in the frequency band less than 1 mHz (for long wavelength time-variable gravity) from the same device with a baseline just over 10 cm. The measurement band and sensitiy can be optimally tuned in-flight during the mission by changing resonance frequencies, which allows meaurements of both static and time-variable gravity fields from the same mission. Significant advances in the technologies needed for space-based cryogenic instruments have been made in the last decade. In particular, the use of cryocoolers will alleviate the previously severe constraint on mission lifetime imposed by the use of liquid helium, enabling mission durations in the 5 - 10 year range
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