Molecular dynamics of cracks (original) (raw)
Related papers
Effects of atoms on brittle fracture
International Journal of Fracture, 2004
This article aims to answer two related sets of questions. First: in principle, how large an effect can structure at the atomic scale have upon the fracture of two macroscopically identical samples? The answer to this question is that the effects can be very large. Perfectly sharp cracks can be pinned and stationary under loading conditions that put them far beyond the Griffith point. Crack paths need not obey the rule K I I = 0. Crack speeds can vary from zero to the Rayleigh wave speed under identical loading conditions but depending upon microscopic rules. These conclusions are obtained from simple solvable models, and from techniques that make it possible to extrapolate reliably from small numerical calculations to the macroscopic limit. These techniques are described in some detail. Second: in practice, should any of these effects be visible in real laboratory samples? The answer to this second question is less clear. The qualitative phenomena exhibited by simple models are observed routinely in the fracture of brittle crystals. However, the correspondence between computations in perfect two-dimensional numerical samples at zero temperature and imperfect three-dimensional laboratory specimens at nonzero temperature is not simple. This paper reports on computations involving nonzero temperature, and irregular crack motion that indicate both strengths and weaknesses of two-dimensional microscopic modeling.
Fracture toughness anomalies: Viewpoint of topological constraint theory
Acta Materialia, 2016
The relationship between composition, structure, and resistance to fracture remains poorly understood. Here, based on molecular dynamics simulations, we report that sodium silicate glasses and calcium-silicate-hydrates feature an anomalous maximum in fracture toughness. In the framework of topological constraint theory, this anomaly is correlated to a flexible-to-rigid transition, driven by pressure or composition for sodium silicate and calcium-silicate-hydrates, respectively. This topological transition, observed for an isostatic network, is also shown to correspond to a ductile-to-brittle transition. At this state, the network is rigid but free of eigen-stress and features stress relaxation through crack blunting, resulting in optimal resistance to fracture. Our topological approach could therefore enable the computational design of tough inorganic solids,
On the Fracture Toughness of Advanced Materials
Advanced Materials, 2009
Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. In the first instance, such resistance to fracture is a function of bonding and crystal structure (or lack thereof), but can be developed through the design of appropriate nano/microstructures. However, the creation of tough microstructures in structural materials, i.e., metals, polymers, ceramics and their composites, is invariably a compromise between resistance to intrinsic damage mechanisms ahead of the tip of a crack (intrinsic toughening) and the formation of crack-tip shielding mechanisms which principally act behind the tip to reduce the effective "crack-driving force" (extrinsic toughening).
Low-speed fracture instabilities in a brittle crystal
Nature, 2008
When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack 1 . The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen 2 . The way the crack propagates, however, is entirely determined by atomic-scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front 1,3 . The physical interplay of multiple length scales makes brittle fracture a complex 'multi-scale' phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of microdefects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known 1 , and significant advances have been made recently in understanding their origin 4,5 . Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack-tip reconstruction that makes low-speed crack propagation unstable on the (111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto (111) planes, as previously observed experimentally 6,7 .
Thresholds and reversibility in brittle cracks: An atomistic surface force model
Journal of Materials Science, 1987
A new picture of environmentally-enhanced fracture in highly brittle solids is presented. It is asserted that the fundamental relations for crack growth are uniquely expressible in terms of the surface force functions that govern the interactions between separating walls in an intrusive medium. These functions are the same, in principle, as those measured directly in the newest submolecular-precision microbalance devices. A fracture mechanics model, based on a modification of the Barenblatt cohesive zone concept, provides the necessary framework for formalizing this link between crack relations and surface force functions. The essence of the modification is the incorporation of an element of discreteness into the surface force function, to allow for geometrical constraints associated with the accommodation of intruding molecules at the crack walls. The model accounts naturally for the existence of zero-velocity thresholds; further, it explains observed shifts in these thresholds in cyclic load-unload-reload experiments, specifically the reduction in applied loading needed to propagate cracks through healed as compared to virgin interfaces. The threshold configurations emerge as thermodynamic equilibrium states, definable in terms of interfacial surface energies. Crack velocity data for cyclic loading in mica, fused silica and sapphire are presented in support of the model. Detailed considerations of the theoretical crack profiles in these three materials, with particular attention to the atomic structure of the "lattice" (elastic sphere approximation) at the interfaces, shows that intruding molecules must encounter significant diffusion barriers as they penetrate toward the tip region. It is concluded that such diffusion barriers control the fracture kinetics at low driving forces. At threshold the barriers become so large that the molecules can no longer penetrate to the tip region. This leads to a crucial prediction of our thesis, that the cohesive Zone consists of two distinct parts: a "protected" primary zone adjacent to the tip, where intrinsic binding forces operate without influence from environmental influences; and a "reactive" secondary zone more remote from the tip, where extrinsic interactions with intruding chemical species are confined. The prevailing view of chemically enhanced brittle fracture, that crack velocity relations are determined by a concerted reaction with reactive species at a single line of crack-tip bonds, is seen as a limiting case of our model, operative at driving forces well above the threshold level. The new description offers the potential for using brittle fracture as a tool for investigating surface forces themselves.
Atomic theory of Fracture and Quantization of Fracture Toughness
This research work is an atomic theory of fracture and quantization of Kic Fracture toughness. Especially in ceramics. It shows the atomic level aspects of fracture process from the stress intensity factor or fracture toughness, KIc. The crystalline structure, the atomic positions and lattice points, and how nanomaterials show atomic level fracture process as well as nanoceramics exhibit Quantization of fracture toughness and other nanomaterials show higher stress intensity factor, KIc than microsize equivalents of those nanomaterials. This is a deep treatment of the fracture process with a survey of present status of fracture, the application of the fundamentals of fracture toughness for the atomic theory of fracture, the data evidence for confirmation of the theory and some extension for its applications in biomaterials, electronic materials and cutting tools for manufacturing. This is a rigorous and clear treatment of the atomic theory of fracture.
Journal of Physics and Chemistry of Solids, 1987
This article first presents introductory material which should make it possible for the person unfamiliar with fracture to read the papers of this series. Then material of a basic physical nature regarding cracks in materials is presented. Emphasis is placed on the effects of chemical attack of bonds at a crack tip, and on the basic physical cause for a material to exhibit a tough (desirable) or a brittle (undesirable) overall aspect.
International Journal of Fracture, 2015
Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crackmicrostructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in
Universal Aspects of Dynamic Fracture in Brittle Materials
Experimental Chaos, 2004
We present an experimental study of the dynamics of rapid tensile fracture in brittle amorphous materials. We first compare the dynamic behavior of "standard" brittle materials (e.g. glass) with the corresponding features observed in "model" materials, polyacrylamide gels, in which the relevant sound speeds can be reduced by 2-3 orders of magnitude. The results of this comparison indicate universality in many aspects of dynamic fracture in which these highly different types of materials exhibit identical behavior. Observed characteristic features include the existence of a critical velocity beyond which frustrated crack branching occurs 1, 2 and the profile of the micro-branches formed. We then go on to examine the behavior of the leading edge of the propagating crack, when this 1D "crack front" is locally perturbed by either an externally introduced inclusion or, dynamically, by the generation of a micro-branch. Comparison of the behavior of the excited fronts in both gels and in soda-lime glass reveals that, once again, many aspects of the dynamics of these excited fronts in both materials are identical. These include both the appearance and character of crack front inertia and the generation of "Front Waves", which are coherent localized waves 3-6 which propagate along the crack front. Crack front inertia is embodied by the appearance of a "memory" of the crack front 7,8 , which is absent in standard 2D descriptions of fracture. The universality of these unexpected inertial effects suggests that a qualitatively new 3D description of the fracture process is needed, when the translational invariance of an unperturbed crack front is broken.