14 research outputs found
Fronts Propagation at the Onset of Plastic Yielding
The existence of a finite threshold, the yield stress, for the onset of
plastic yielding is a universal feature of plasticity. This jamming-unjamming
transition is naturally accounted for by the dynamics of a bistable internal
state field. We show, within the athermal Shear Transformation Zones (STZ)
theory of amorphous plasticity, that the transition is accompanied by the
propagation of plastic fronts. We further show that the mean-field theory
cannot select the velocity of these fronts and go beyond the mean-field
description to include fluctuations and correlations effects, resulting in new
nonlocal terms in the equations. Finally, we demonstrate that the new terms,
with an associated intrinsic lengthscale, provide a velocity selection
mechanism for the plastic fronts.Comment: 6 pages, 1 figure; reorganized paper, friction part omitte
Elastic nonlinearities in a one-dimensional model of fracture
The dynamics of rapid brittle cracks is commonly studied in the framework of
linear elastic fracture mechanics where nonlinearities are neglected. However,
recent experimental and theoretical work demonstrated explicitly the importance
of elastic nonlinearities in fracture dynamics. We study two simple
one-dimensional models of fracture in order to gain insights about the role of
elastic nonlinearities and the implications of their exclusion in the common
linear elastic approximation. In one model we consider the decohesion of a
nonlinear elastic membrane from a substrate. In a second model we follow the
philosophy of linear elastic fracture mechanics and study a linearized version
of the nonlinear model. By analyzing the steady state solutions, the
velocity-load relations and the response to perturbations of the two models we
show that the linear approximation fails at finite crack tip velocities. We
highlight certain features of the breakdown of the linear theory and discuss
possible implications of our results to higher dimensional systems.Comment: 9 pages, 6 figures, related papers: arXiv:0807.4866 and
arXiv:0807.486
Finite-time Singularities in Surface-Diffusion Instabilities are Cured by Plasticity
A free material surface which supports surface diffusion becomes unstable
when put under external non-hydrostatic stress. Since the chemical potential on
a stressed surface is larger inside an indentation, small shape fluctuations
develop because material preferentially diffuses out of indentations. When the
bulk of the material is purely elastic one expects this instability to run into
a finite-time cusp singularity. It is shown here that this singularity is cured
by plastic effects in the material, turning the singular solution to a regular
crack.Comment: 4 pages, 3 figure
Dynamic Failure in Amorphous Solids via a Cavitation Instability
The understanding of dynamic failure in amorphous materials via the
propagation of free boundaries like cracks and voids must go beyond elasticity
theory, since plasticity intervenes in a crucial and poorly understood manner
near the moving free boundary. In this Letter we focus on failure via a
cavitation instability in a radially-symmetric stressed material, set up the
free boundary dynamics taking both elasticity and visco-plasticity into
account, using the recently proposed athermal Shear Transformation Zone theory.
We demonstrate the existence (in amorphous systems) of fast cavitation modes
accompanied by extensive plastic deformations and discuss the revealed physics.Comment: 4 pages, 4 figure
Free-Boundary Dynamics in Elasto-plastic Amorphous Solids: The Circular Hole Problem
We develop an athermal shear-transformation-zone (STZ) theory of plastic
deformation in spatially inhomogeneous, amorphous solids. Our ultimate goal is
to describe the dynamics of the boundaries of voids or cracks in such systems
when they are subjected to remote, time-dependent tractions. The theory is
illustrated here for the case of a circular hole in an infinite two-dimensional
plate, a highly symmetric situation that allows us to solve much of the problem
analytically. In spite of its special symmetry, this example contains many
general features of systems in which stress is concentrated near free
boundaries and deforms them irreversibly. We depart from conventional
treatments of such problems in two ways. First, the STZ analysis allows us to
keep track of spatially heterogeneous, internal state variables such as the
effective disorder temperature, which determines plastic response to subsequent
loading. Second, we subject the system to stress pulses of finite duration, and
therefore are able to observe elasto-plastic response during both loading and
unloading. We compute the final deformations and residual stresses produced by
these stress pulses. Looking toward more general applications of these results,
we examine the possibility of constructing a boundary-layer theory that might
be useful in less symmetric situations.Comment: 30 pages (preprint format), 9 figure
Emission Factors for a Taxi Fleet Operating on Liquefied Petroleum Gas (LPG) as a Function of Speed and Road Slope
Real-driving emissions of NOx, CO, and THC, as well as fuel consumption (FC) were studied from 18 liquefied petroleum gas (LPG) fueled taxis operating in a metropolitan road network. Euro 2 to Euro 5 technology vehicles were measured with the use of portable emission measurement systems (PEMS). Statistical processing was implemented to derive mean emission levels for the different technologies. The taxis were measured from 6 months to 2.5 years after their catalysts and lambda sensors were replaced. The emission levels of Euro 4 taxis after catalyst replacement appear higher compared to pre-replacement levels, while pre-Euro 4 taxis emission levels were moderately reduced by the catalyst replacement. Overall, Euro 5 LPG taxis exhibit the lowest emissions, even below the respective regulated limits. The NH3 and N2O pollutant levels of a Euro 5 LPG taxi measured in the lab were found at about half its NOx emissions. Different integration methods of PEMS data were investigated toward the development of emission factors, including both time-based and distance-based approaches at different resolutions. Distance-based integration in sections of 500 m was considered suitable, as this provides a large dataset for statistical confidence and sufficient resolution for link-based modeling. Based on this, FC and emission factors of NOx, CO, and THC as a function of speed and road slope are presented, separately for vehicles considered as normal and high emitters. Volatile organic compounds speciation of Euro 5 taxis showed that methane and butane are the most abundant hydrocarbon species in the exhaust
Reynolds number dependence of drag reduction by rodlike polymers
We present experimental and theoretical results addressing the Reynolds number (Re) dependence of drag reduction by sufficiently large concentrations of rod-like polymers in turbulent wall-bounded flows. It is shown that when Re is small the drag is {\em enhanced}. On the other hand when Re increases the drag is reduced and eventually the Maximal Drag Reduction (MDR) asymptote is attained. The theory is shown to be in excellent agreement with experiments, rationalizing and explaining all the universal and the non-universal aspects of drag reduction by rod-like polymers
Real-World Measurement of Hybrid Buses’ Fuel Consumption and Pollutant Emissions in a Metropolitan Urban Road Network
This study investigates pollutant emissions and fuel consumption of six Euro VI hybrid-diesel public transport buses operating on different scheduled routes in a metropolitan urban road network. Portable emission measurement systems (PEMS) are used in measurements and results are compared to those obtained from a paired number of Euro V conventional buses of the same body type used as control over the same routes. The selected routes vary from urban to highway driving and the experimentation was conducted over the first half of 2015. The available emissions data correspond to a wide range of driving, operating, and ambient conditions. Fuel consumption, distance- and energy-based emission levels are derived and presented in a comparative manner. The effect of different factors, including speed, ambient temperature, and road grade on fuel consumption and emissions performance is investigated. Mean fuel consumption of hybrid buses was found 6.1% lower than conventional ones, from 20% lower up to 16% higher, over six routes tested in total. The mean route difference between the two technologies was not statistically significant. Air conditioning decreased consumption benefits of the hybrid buses. Decrease of the mean route speed from 15 km h−1 tο 8 km h−1 increased the hybrid buses consumption by 63%. Nitrogen oxides (NOx) emissions of the Euro VI hybrid buses were 93 ± 5% lower than conventional Euro V ones. Nitrous oxide (N2O) emissions from hybrid Euro VI buses made up 5.9% of total greenhouse gas emissions and largely offset carbon dioxide (CO2) benefits. The results suggest that hybrid urban buses need to be assessed under realistic operation and environmental conditions to assess their true environmental and fuel consumption benefits
Evaluation of real-world gaseous emissions performance of selective catalytic reduction and diesel particulate filter bus retrofits
This study reports on the results of gaseous pollutants emission measurements of double-decker buses in an urban road network, using portable emission measurement systems (PEMS). Measured vehicles were tested by following in-service buses on regular routes. Six Euro II and Euro III buses were retrofitted with diesel particulate filters (DPF) and selective catalytic reduction (SCR) or a combined SCR+DPF (SCRT) device. Substantial and statistically significant technology impacts were observed for several pollutants. Optimized SCR and SCRT retrofit technology reduced real-world NO emissions by approximately 70%, on average. Retrofit DPF slightly reduced NO emissions but increased direct NO emissions by more than a factor of 8, on average. SCRT led to about 70% lower NO levels than DPF alone, but for some vehicles higher NO levels were observed as compared with the "no retrofit" situation, warranting further investigation. None of the SCR systems were found to lead to a substantial increase in NH emissions after operation optimization. High NH and N O emissions were occasionally observed while experience with the system calibration was being accumulated. Observed average N O emission levels for "DPF+SCR" technology were relatively high at 182 mg/kg fuel, corresponding to 1.5% of total greenhouse gas emissions. The study shows that SCR retrofit programs can be effective for NO reduction of transit buses but that proper calibration and regular emission monitoring are required