Atomistic aspects of dynamic fracture in a variety of brittle crystalline, amorphous, nanophase, and nanocomposite materials are reviewed. Molecular dynamics (MD) simulations, ranging from a million to 1.5 billion atoms, are performed on massively parallel computers using highly efficient multiresolution algorithms. These simulations shed new light on (a) branching, deflection, and arrest of cracks; (b) growth of nanoscale pores ahead of the crack and how pores coalesce with the crack to cause fracture; and (c) the influence of these mechanisms on the morphology of fracture surfaces. Recent advances in novel multiscale simulation schemes combining quantum mechanical, molecular dynamics, and finite-element approaches and the use of these hybrid approaches in the study of crack propagation are also discussed.

International audienceThis paper is devoted to a comparison of experimental, simulation, and theoretical results on the density of SiO 2 – B 2 O 3 –Na 2 O glasses. It is found that theoretical and simulation densities do compare favorably with experimental values yet simulations give a better estimate of the density of the samples. Furthermore, the structural make-up (i.e. types of borate and silicate units) of the ternary glasses and the volume of the elementary units have also been investigated with simulations and compared to theory

International audienceThis study investigates the mechanical response of sodium borosilicate (SBN) glasses as a function of their chemical composition. Vickers's indentation tests provide an estimate of the material hardness (H V) and indentation fracture toughness (K C VIF) plus the amount of densification/shear flow processes. Sodium content significantly impacts the glass behavior under a sharp indenter. Low sodium glasses maintain high connected networks and low Poisson's ratios (ν). This entails significant densification processes during deformation. Conversely, glasses with high sodium content, i.e. large ν, partake in a more depolymerized network favoring deformation by shear flow. As a consequence , indentation patterns differ depending on the processes occurring. Densification processes appear to hinder the formation of halfpenny median–radial cracks. Increasing ν favors shear flow and residual stresses enhance the development of halfpenny median–radial cracks. Hence, K C VIF decreases linearly with ν

Amorphous silica density at ambient pressure is known to depend on thermal history (through the quenching rate) but also, at room temperature, on the maximum pressure applied in the past. Here we show that beyond density, a mechanical loading can endow the structure with an orientational order. Molecular dynamics simulations show evidence that amorphous silica develops a permanent anisotropic structure after extended shear plastic flow. This anisotropy which survives for an unstressed specimen is revealed markedly by the fabric tensor computed over the Si-O-Si orientations, albeit the SiO4 tetrahedra microstructure remains mostly unaltered.PACS numbers: 62.20.F, 81.05.Kf Plasticity of amorphous media, which can be easily evidenced via indentation or scratch tests [1], has a very different nature from its counterpart for crystalline media, since no elementary entities such as dislocations whose evolution controls plastic flow can be easily defined [2,3]. The current view is that spatially distributed local restructuring rather than extended defect motion (such as dislocation) are responsible for irreversible strains in amorphous materials [4,5]. At a very local scale, under load, a small group of atoms (called a Transformation Zone or TZ) may undergo rearrangements, a change of conformation eventually affecting the topology of the atomic bonds which will contribute to an elementary increment in irreversible strain. Although a complete description of these TZ is extremely complex, and cannot be cast into simple categories, a statistical analysis capturing the key properties of these zones is an attractive route for relating the macroscopic mechanical behavior to the underlying microstructural counterpart [4].In contrast with dislocations which naturally lead to isochoric plastic deformation, transformation zones may densify or dilate as they rearrange. Indeed at a macroscopic scale, plasticity of silicate glasses is known to exhibit permanent densification [6,7] from a few percents for soda-lime glasses [8] to values as large as 20% in the extreme case of amorphous silica [9,10]. This densification naturally affects shear plasticity, and hence pressure and shear stress are to be coupled in the yield criterion of amorphous silica [11].Plasticity of structural glasses is furthermore characterized by a significant hardening behavior [9,10]. The yield surface evolves with the mechanical loading. This means in particular that, when applying stress in a given direction (pure shear, pure hydrostatic pressure, etc) the value of the elastic limit depends on the history of the loading.To account for this dependence on mechanical history a proper description of plasticity thus requires the use of additional internal variables. The first one is obviously density and indeed a recent study of densification with pressure allows one to characterize the density hardening of silica [10]. The necessity to include more internal variables than the mere density is a difficult question to address. Experimentally, plasticity of amorphous...

We report here atomic force microscopy experiments designed to uncover the nature of failure mechanisms occuring within the process zone at the tip of a crack propagating into a silica glass specimen under stress corrosion. The crack propagates through the growth and coalescence of nanoscale damage spots. This cavitation process is shown to be the key mechanism responsible for damage spreading within the process zone. The possible origin of the nucleation of cavities, as well as the implications on the selection of both the cavity size at coalescence and the process zone extension are finally discussed.

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A scientific hurdle in manufacturing solid films by drying colloidal layers is preventing them from fracturing. This paper examines how the drying rate of colloidal liquids influences the particle packing at the nanoscale in correlation with the crack patterns observed at the macroscale. Increasing the drying rate results in more ordered, denser solid structures, and the dried samples have more cracks.Yet, introducing a holding period (at a prescribed point) during the drying protocol results in a more disordered solid structure with significantly less cracks. To interpret these observations, this paper conjectures that a longer drying protocol favors the formation of aggregates. It is further argued that the number and size of the aggregates increase as the drying rate decreases. This results in the formation of a more disordered, porous film from the viewpoint of the particle packing, and a more resistant film, i.e. less cracks, from the macroscale viewpoint.

The roughness of fracture surfaces exhibits self-affinity for a wide variety of materials and loading conditions. The universality and the range of scales over which this regime extends are still debated. The topography of these surfaces is however often investigated with a finite contact probe. In this case, we show that the correlation function of the roughness can only be measured down to a length scale Deltax{c} which depends on the probe size R, the Hurst exponent zeta of the surface and its topothesy l, and exhibits spurious behavior at smaller scales. First, we derive the dependence of Deltax{c} on these parameters from a simple scaling argument. Then, we verify this dependence numerically. Finally, we establish the relevance of this analysis from AFM measurements on an experimental glass fracture surface and provide a metrological procedure for roughness measurements.