2,914 research outputs found
Bursts in a fiber bundle model with continuous damage
We study the constitutive behaviour, the damage process, and the properties
of bursts in the continuous damage fiber bundle model introduced recently.
Depending on its two parameters, the model provides various types of
constitutive behaviours including also macroscopic plasticity. Analytic results
are obtained to characterize the damage process along the plastic plateau under
strain controlled loading, furthermore, for stress controlled experiments we
develop a simulation technique and explore numerically the distribution of
bursts of fiber breaks assuming infinite range of interaction. Simulations
revealed that under certain conditions power law distribution of bursts arises
with an exponent significantly different from the mean field exponent 5/2. A
phase diagram of the model characterizing the possible burst distributions is
constructed.Comment: 9 pages, 11 figures, APS style, submitted for publicatio
Natural Aging and Vacancy Trapping in Al-6xxx
Undesirable natural aging (NA) in Al-6xxx delays subsequent artificial aging
(AA) but the size, composition, and evolution of clustering are challenging to
measure. Here, atomistic details of early-stage clustering in Al-1\%Mg-0.6\%Si
during NA are studied computationally using a chemically-accurate
neural-network potential. Feasible growth paths for the preferred
precipitates identify: dominant clusters differing from motifs;
spontaneous vacancy-interstitial formation creating 14-18 solute atom
-like motifs; and lower-energy clusters requiring chemical
re-arrangement to form nuclei. Quasi-on-lattice kinetic Monte Carlo
simulations reveal that 8-14 solute atom clusters form within 1000 s but that
growth slows considerably due to vacancy trapping inside clusters, with
trapping energies of 0.3-0.5 eV. These findings rationalize why cluster growth
and alloy hardness saturate during NA, confirm the concept of ''vacancy
prisons", and suggest why clusters must be dissolved during AA before formation
of . This atomistic understanding of NA may enable design of
strategies to mitigate negative effects of NA
Failure Probabilities and Tough-Brittle Crossover of Heterogeneous Materials with Continuous Disorder
The failure probabilities or the strength distributions of heterogeneous 1D
systems with continuous local strength distribution and local load sharing have
been studied using a simple, exact, recursive method. The fracture behavior
depends on the local bond-strength distribution, the system size, and the
applied stress, and crossovers occur as system size or stress changes. In the
brittle region, systems with continuous disorders have a failure probability of
the modified-Gumbel form, similar to that for systems with percolation
disorder. The modified-Gumbel form is of special significance in weak-stress
situations. This new recursive method has also been generalized to calculate
exactly the failure probabilities under various boundary conditions, thereby
illustrating the important effect of surfaces in the fracture process.Comment: 9 pages, revtex, 7 figure
Lattice density-functional theory of surface melting: the effect of a square-gradient correction
I use the method of classical density-functional theory in the
weighted-density approximation of Tarazona to investigate the phase diagram and
the interface structure of a two-dimensional lattice-gas model with three
phases -- vapour, liquid, and triangular solid. While a straightforward
mean-field treatment of the interparticle attraction is unable to give a stable
liquid phase, the correct phase diagram is obtained when including a suitably
chosen square-gradient term in the system grand potential. Taken this theory
for granted, I further examine the structure of the solid-vapour interface as
the triple point is approached from low temperature. Surprisingly, a novel
phase (rather than the liquid) is found to grow at the interface, exhibiting an
unusually long modulation along the interface normal. The conventional
surface-melting behaviour is recovered only by artificially restricting the
symmetries being available to the density field.Comment: 16 pages, 6 figure
Crack tip blunting and cleavage under dynamic conditions
In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures (Lin et al., 1987). Here, crack tip blunting and cleavage under dynamic conditions are explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: (1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and (2) what happens in the dynamic "overloaded" situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed as a "dynamic cleavage test", in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, ductile crack tip behavior ultimately prevails since a single emission event can blunt and arrest the crack, preventing further cleavage. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears unrelated to dynamic effects at the crack tip. (C) 2016 Elsevier Ltd. All rights reserved
Biomechanics of predator–prey arms race in lion, zebra, cheetah and impala
The fastest and most manoeuvrable terrestrial animals are found in savannah habitats, where predators chase and capture running prey. Hunt outcome and success rate are critical to survival, so both predator and prey should evolve to be faster and/or more manoeuvrable. Here we compare locomotor characteristics in two pursuit predator–prey pairs, lion–zebra and cheetah–impala, in their natural savannah habitat in Botswana. We show that although cheetahs and impalas were universally more athletic than lions and zebras in terms of speed, acceleration and turning, within each predator–prey pair, the predators had 20% higher muscle fibre power than prey, 37% greater acceleration and 72% greater deceleration capacity than their prey. We simulated hunt dynamics with these data and showed that hunts at lower speeds enable prey to use their maximum manoeuvring capacity and favour prey survival, and that the predator needs to be more athletic than its prey to sustain a viable success rate
Forced Chemical Vapor Infiltration of Tubular Geometries: Modeling, Design, and Scale-Up
In advanced indirectly fired coal combustion systems and externally fired combined cycle concepts, ceramic heat exchangers are required to transfer heat from the hot combustion gases to the clean air that drives the gas turbines. For high efficiencies, the temperature of the turbine inlet needs to exceed 1,100 C and preferably be about 1,260 C. The heat exchangers will operate under pressure and experience thermal and mechanical stresses during heating and cooling, and some transients will be severe under upset conditions. Silicon carbide-matrix composites appear promising for such applications because of their high strength at elevated temperature, light weight, thermal and mechanical shock resistance, damage tolerance, and oxidation and corrosion resistance. The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter
The Arclight vs. traditional ophthalmoscope:a cross-over trial
BACKGROUND/OBJECTIVES: To compare skill acquisition of the new, cost-effective Arclight ophthalmoscope, with the traditional ophthalmoscope (TO), in medical students with no prior experience of ophthalmoscopy. SUBJECTS/METHODS: University of Dundee medical students took part in a cross-over trial. Students were divided into two groups and were alternately taught each device using a video tutorial. In period one, Group A was taught the TO first; Group B was taught the Arclight. They were then assessed using simulated objective, structured, clinical, examinations, examining four model heads with lettered fundal photographs of varying sizes of font. Groups crossed over following a 2-week washout period and were taught the second device and reassessed. A questionnaire was distributed to ascertain students’ opinions and preferences. RESULTS: Forty medical students participated. Overall, 92.5% of students performed better with the Arclight, irrespective of cross-over trial period. The mean difference in score in period one of the cross-over trial was 16.77 (95% CI: 11.63–21.93), with students performing better with the Arclight (p < 0.0001). The mean difference in score in period two was 8.02 (95% CI: 4.52–11.52), with students performing better with the Arclight (p < 0.0001). In addition, performance with the TO improved by 52.9% following initial exposure to the Arclight. The Arclight was the preferred device by 82.5% of students, and 82.5% of students would choose this device for future practice. CONCLUSION: Students performed better with and preferred the Arclight ophthalmoscope. The Arclight could be considered as a suitable alternative to the TO used for training medical students
Finite size effects and the order of a phase transition in fragmenting nuclear systems
We discuss the implications of finite size effects on the determination of
the order of a phase transition which may occur in infinite systems. We
introduce a specific model to which we apply different tests. They are aimed to
characterise the smoothed transition observed in a finite system. We show that
the microcanonical ensemble may be a useful framework for the determination of
the nature of such transitions.Comment: LateX, 5 pages, 5 figures; Fig. 1 change
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