The Coupled Atomistic/Discrete-Dislocation (CADD) method is a concurrent multiscale technique that couples atomistic and discrete dislocation domains with the ability to pass dislocations seamlessly ...between domains. CADD has been demonstrated only in 2d plane-strain problems, for which each individual dislocation is either entirely atomistic or entirely discrete. Here, a full 3d implementation of CADD is presented, with emphasis on the algorithms for handling the description of dislocation lines that span both atomistic and continuum domains, so-called hybrid dislocations. The key new features of the method for 3d are (i) the use of an atomistic template of the dislocation core structure to transmit the proper atomistic environment of a continuum dislocation to the atomistic domain for hybrid dislocations and (ii) a staggered solution procedure enabling evolution of the hybrid dislocations. The method naturally requires calibration of discrete-dislocation Peierls stresses and mobilities to their atomistic values, implementation of a dislocation detection algorithm to identify atomistic dislocations, and computation of continuum dislocation displacement fields that provide boundary conditions for the atomistic problem. The method is implemented using the atomistic code LAMMPS and the discrete dislocation code ParaDiS within the LibMultiscale environment developed by the lead authors, and so has all the advantages of these widely-used high-performance open-source codes. Validation and application of CADD-3d are presented in companion papers.
This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among ...the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
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Molecular simulations using the quasicontinuum method are performed to understand the mechanical response at the nanoscale of grain boundaries (GBs) under simple shear. The energetics and mechanical ...strength of 18 Σ 〈1
1
0〉 symmetric tilt GBs and two Σ 〈1
1
0〉 asymmetric tilt GBs are investigated in Cu and Al. Special emphasis is placed on the evolution of far-field shear stresses under applied strain and related deformation mechanisms at zero temperature. The deformation of the boundaries is found to operate by three modes depending on the GB equilibrium configuration: GB sliding by uncorrelated atomic shuffling, nucleation of partial dislocations from the interface to the grains, and GB migration. This investigation shows that (1) the GB energy alone cannot be used as a relevant parameter to predict the sliding of nanoscale high-angle boundaries when no thermally activated mechanisms are involved; (2) the E structural unit present in the period of Σ tilt GBs is found to be responsible for the onset of sliding by atomic shuffling; (3) GB sliding strength in the athermal limit shows slight variations between the different interface configurations, but has no apparent correlation with the GB structure; (4) the metal potential plays a determinant role in the relaxation of stress after sliding, but does not influence the GB sliding strength; here it is suggested that the metal potential has a stronger impact on crystal slip than on the intrinsic interface behavior. These findings provide additional insights on the role of GB structure in the deformation processes of nanocrystalline metals.
We investigate the dynamic behavior of concrete in relation to its composition within a computational framework (FEM). Concrete is modeled using a meso-mechanical approach in which aggregates and ...mortar are represented explicitly. Both continuum phases are considered to behave elastically, while nucleation, coalescence and propagation of cracks are modeled using the cohesive-element approach.
In order to understand the loading-rate sensitivity of concrete, we simulate direct tensile-tests for strain rates ranging 1–1000
s
−1. We investigate the influence of aggregate properties (internal ordering, size distribution and toughness) on peak strength and dissipated fracture energy. We show that a rate independent constitutive law captures the general increase of peak strength with strain rate. However, a phenomenological rate-dependent cohesive law is needed to obtain a better agreement with experiments. Furthermore, at low rates, peak strength is sensitive to the inclusions' toughness, while the matrix dominates the mechanical behavior at high rates.
A methodology for coupling a fully atomistic domain to a surrounding domain described by discrete dislocation plasticity, including the treatment of hybrid dislocation lines that span between the two ...domains, was presented in the first paper of this series (Anciaux et al., 2017). Here, key features of the methodology are assessed quantitatively within a quasi-static framework at 0 K. To avoid solving an expensive but standard complementary problem for the atomistic/continuum coupling of mechanical fields, which is not essential to the key features of the method, a simplified model for obtaining accurate stress and displacement fields is introduced and validated. The test problem consists of the bow-out of a single dislocation in a semi-periodic box under an applied shear stress, and excellent results are obtained in comparison to fully-atomistic solutions of the same problem.
•Fragmentation driven by eigenstresses in tempered glass plates is simulated with FEM.•Cohesive method permits to model the complex crack pattern resulting from fragmentation.•The model is validated ...against experimental results taken from the literature.•The number of fragments increases by decreasing the plate thickness.•The analytical models are improved based on the results of the numerical simulations.
Tempered glass panes are subjected to high eigenstresses that induce a state of compression along the surfaces and a state of tension in the inner part. Whenever a crack reaches the tensile region, it rapidly propagates and branches in all directions driven by the eigenstress. These mechanisms induce dynamic fragmentation. The present work contains a numerical investigation of this phenomenon on panes with different thicknesses, using massively parallel simulation based on FEM with the dynamic insertion of cohesive elements. Simulations are first validated by comparing the obtained number of fragments with experimental data. Then, the resulting energy fields are examined and they show that the dissipated energy is significantly underestimated by the existing analytical models. Finally, an extended analytical model that includes the influence of the plate thickness is proposed to correctly estimate the number of fragments for high eigenstresses.
The influence of Eigenstresses due to drying shrinkage on the development of residual deformations characterizing the tensile fatigue behavior of concrete is analyzed. During the loading phase the ...Eigenstresses are locally released around the cracks inducing a mismatch between the crack surfaces which inhibits a perfect crack re-closure. The analysis is performed by means of a 2D mesoscale implicit finite-element model. The shrinkage strain is first applied determining the development of a diffused micro-damage and then quasi-static loading–unloading tests are simulated. Different microstructures and different values of shrinkage strain are considered. The results show that the presence of residual stresses increases the amount of total dissipated energy and naturally leads to the development of residual deformations. However, the obtained values are only a portion of the residual deformations experimentally measured. The possible concomitant effect of another mechanism, namely the formation of debris at a small scale, is therefore discussed.
Volume increase between the reactants and the products of alkali-silica reaction could reach up to 100%. Taking place inside the aggregates, ASR imposes internal pressure on the surrounding material. ...In the current paper, the possibility of crack growth due to such internal loading is studied. This study is done by employing a semi-analytical mechanical model comprising an elastic solution to a well-known Eshelby problem and a linear elastic fracture mechanics solution to a ring-shaped crack encircling a spheroidal inclusion. The proposed method implies the presence of pre-existing micro-fissures within the aggregate.
The study reveals the dependence of the crack growing potential on the spheroid's shape: the larger the ASR pocket - the longer crack opens. The two most critical shapes, causing the highest stress intensity factor and developing the longest crack, are a sphere and a spheroid with a 1/4 aspect ratio respectively. The size analysis of the problem suggests a critical spheroid's radius below which no crack growth is expected. For a chosen material properties and expansion value, such radius lies in the range between 0.1μm and 1μm. Independently of the expansion value and the shape of the pocket of the ASR product, the developed crack length has a power-law dependence on the size of a spheroid.
All the theoretical predictions are confirmed by a numerical model based on the combination of the finite element method and the cohesive zone model.
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