This paper aims to employ a 2D thermo-mechanical multi-scale ASR model for the analysis of a concrete gravity dam in Western Switzerland. Simulation results are compared to the field measurements and ...observations. Analysis of the results reveals negligible effect of temperature variation on the ASR advancement. The difference in length between the upstream and the downstream faces is identified as the main source for the upstream drift at the level of the crest. Study of the structural effects reveals ASR-related expansion anisotropy and cracks alignment being more pronounced along the upstream part and the foundation. If the former is explained by the transmission of the self-weight, the latter is attributed to the constraining effect of the underlying rock.
•Multi-scale numerical model for ASR-affected engineering structures is presented.•ASR-expansion rate at the meso-scale is made dependent on the temperature values.•A model of concrete Salanfe dam in Switzerland is calibrated with the field measurements.•The model shows low dependence of the ASR expansion rate on the temperature variation.•Anisotropic macroscopic deformations correlate with crack patterns at the meso-scale.
Finite-element methods are used to study nonadhesive, frictionless contact between elastic solids with self-affine surfaces. We find that the total contact area rises linearly with the load at small ...loads. The mean pressure in the contact regions is independent of load and proportional to the root-mean-square slope of the surface. The constant of proportionality is nearly independent of the Poisson ratio and roughness exponent and lies between previous analytic predictions. The contact morphology is also analyzed. Connected contact regions have a fractal area and perimeter. The probability of finding a cluster of area a(c) drops as a(-tau )(c ) where tau increases with a decrease in roughness exponent. The distribution of pressures shows an exponential tail that is also found in many jammed systems. These results are contrasted to simpler models and experiments.
► Atomistic thermo-mechanics of nanomachining between diamond and copper. ► High speed MD nanomachining results in significant inertial tool force increases. ► Chip temperature increases most with ...machining velocity at low machined thicknesses. ► Chip temperature boundary layers can only persist at extremely high cutting speeds. ► There isan apparent isothermal to non-isothermal nanomachining transition at ≈50m/s.
The fabrication of high precision miniaturized components in micro- and nano-technologies requires a deep understanding of the physical mechanisms governing the nanomachining process. To aid with this need, the current article employs molecular dynamics to investigate the thermo-mechanical aspects of orthogonal nanometric machining in a copper workpiece. We study the evolution of the material removal process, the effects of machining velocity on the predicted MD response and the variation of the temperature within the chip for different machining velocities and machined thicknesses. As expected, the chip temperature rises with increasing machining velocity but it only noticeably decreases with growing machined thickness for machining velocities ⩾50m/s. The chip temperature indicates the isothermal nature of the machining process for low cutting speeds (⩽10m/s for copper) and the non-isothermal evolution of the process for high cutting speeds (⩾50m/s for copper).
This paper presents a finite element calculation of frictionless, non-adhesive, contact between a rigid plane and an elasto-plastic solid with a self-affine fractal surface. The calculations are ...conducted within an explicit dynamic Lagrangian framework. The elasto-plastic response of the material is described by a
J
2
isotropic plasticity law. Parametric studies are used to establish general relations between contact properties and key material parameters. In all cases, the contact area
A rises linearly with the applied load. The rate of increase grows as the yield stress
σ
y
decreases, scaling as a power of
σ
y
over the range typical of real materials. Results for
A from different plasticity laws and surface morphologies can all be described by a simple scaling formula. Plasticity produces qualitative changes in the distributions of local pressures in the contact and of the size of connected contact regions. The probability of large local pressures is decreased, while large clusters become more likely. Loading-unloading cycles are considered and the total plastic work is found to be nearly constant over a wide range of yield stresses.
In this paper, we present a novel finite element model to simulate alkali-silica reaction in a realistic concrete meso-structure. Application of the internal ASR loading leads to the evolution of ...multiple deviated cracks and corresponding macroscopic expansion. A particular crack-extension algorithm and a solution scheme provide numerical stability and allow to model complicated crack patterns while preserving the physics. The predictive validity of the model is demonstrated by matching an analytical solution of a loaded penny-shaped crack. The model is applied to the experimental dataset comprising the time-evolving X-ray tomograms of the ASR-affected concrete. Similar to the tomography data, the model results in an expanded concrete sample with a developed crack network. Two hypotheses on the crack loading and extension mechanisms are tested by comparing the crack statistics. Simulations with varying number of ASR sites and application of the uniaxial loading bring interesting insights. The latter concerns the role of the ASR-sites number, the individual loading amplitude and the difference in crack patterns.
A finite-element approach based on the first-order FE2 homogenisation technique is formulated to analyse the alkali-silica reaction-induced damage in concrete structures, by linking the concrete ...degradation at the macro-scale to the reaction extent at the meso-scale. At the meso-scale level, concrete is considered as a heterogeneous material consisting of aggregates embedded in a mortar matrix. The mechanical effects of the Alkali-Silica Reaction (ASR) are modelled through the application of temperature-dependent eigenstrains in several localised spots inside the aggregates, and the mechanical degradation of concrete is modelled using continuous damage model, which is capable of reproducing the complex ASR crack networks. Then, the effective stiffness tensor and the effective stress tensor for each macroscopic finite element are computed by homogenising the mechanical response of the corresponding representative volume element (RVE). Convergence between macro- and meso-scales is achieved via an iterative procedure. A 2D model of an ASR laboratory specimen is analysed as a proof of concept. The model is able to account for the loading applied at the macro-scale and the ASR-product expansion at the meso-scale. The results demonstrate that the macroscopic stress state influences the orientation of damage inside the underlying RVEs. The effective stiffness becomes anisotropic in cases where damage is aligned inside the RVE.
Context.
Classical T Tauri stars are young low-mass systems still accreting material from their disks. These systems are dynamic on timescales of hours to years. The observed variability can help us ...infer the physical processes that occur in the circumstellar environment.
Aims.
In this work, we aim at understanding the dynamics of the magnetic interaction between the star and the inner accretion disk in young stellar objects. We present the case of the young stellar system V2129 Oph, which is a well-known T Tauri star with a K5 spectral type that is located in the
ρ
Oph star formation region at a distance of 130 ± 1 pc.
Methods.
We performed a time series analysis of this star using high-resolution spectroscopic data at optical wavelengths from CFHT/ESPaDOnS and ESO/HARPS and at infrared wavelengths from CFHT/SPIRou. We also obtained simultaneous photometry from REM and ASAS-SN. The new data sets allowed us to characterize the accretion-ejection structure in this system and to investigate its evolution over a timescale of a decade via comparisons to previous observational campaigns.
Results.
We measure radial velocity variations and recover a stellar rotation period of 6.53 days. However, we do not recover the stellar rotation period in the variability of various circumstellar lines, such as H
α
and H
β
in the optical or HeI 10830 Å and Pa
β
in the infrared. Instead, we show that the optical and infrared line profile variations are consistent with a magnetospheric accretion scenario that shows variability with a period of about 6.0 days, shorter than the stellar rotation period. Additionally, we find a period of 8.5 days in H
α
and H
β
lines, probably due to a structure located beyond the corotation radius, at a distance of ∼0.09 au. We investigate whether this could be accounted for by a wind component, twisted or multiple accretion funnel flows, or an external disturbance in the inner disk.
Conclusions.
We conclude that the dynamics of the accretion-ejection process can vary significantly on a timescale of just a few years in this source, presumably reflecting the evolving magnetic field topology at the stellar surface.
This paper presents the study of nonadhesive, frictionless contact between elastic solids realized by the Discrete Element Method. This numerical method dedicated to multi-contact problems is applied ...to the field of tribology by studying the normal contact between a rigid rough surface and an elastic body modeled by spheres. A specific interparticle stiffnesses derived from homogenization techniques is implemented. From numerical tests carried out on spheres packings, we observe that the desired main macroscopic elastic constants are correctly modeled. Concerning the study of normal contact between rough surfaces, the obtained results are in accordance to existing theoretical models and numerical results from the literature, thereby demonstrating the potential of the Discrete Element Method to study the normal contact between contacting elastic bodies with rough surfaces. In particular, we recover the linear dependence of the real contact area with the normal load. In addition, we show that decreasing the surface roughness increases the average contact pressure.
Defects play a critical role in the dynamic fragmentation process of structural ceramics. Cracks initiate at seemingly random locations, propagate and coalesce to form fragments. The process is ...accompanied by stress release waves, whose influence is difficult to account for without numerical analysis. In this paper, we use a finite-element program with a cohesive fracture capability, to relate a defect distribution contained in a material with the resulting number of fragments. We show how the distribution tail, e.g. the number of large defects, and the rate at which cracks can be initiated at these sites have a critical influence on the generation of stress release waves and thus on the fragmentation process. Our numerical calculations yield a new factor, which we label communication factor, that we use to normalize the average fragment size and to define a new scaling function of material properties, defect statistics and loading rate.
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