Atomic structure of loaded crack tip by AFM
Chen Wang1998, Scripta Materialia
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Abstract
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This work investigates the atomic structures of unloaded and loaded cracks at the (001) surface of muscovite mica using Atomic Force Microscopy (AFM). The study identifies an unperturbed atomic structure at unloaded crack tips, while a disorder zone about 8nm wide is present at the loaded crack tips, suggesting a difference in mechanical properties and atomic arrangements due to stress concentration.
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DIC Microscale Crack Tip.pdf
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Journal of the Mechanics and Physics of Solids, 1992
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Engineering Fracture Mechanics, 2015
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Scripta Metallurgica, 1988
lntroduction Dislocation emission from the tip of a propagating crack has been shown to be an important factor in determining the fracture toughness and ductile to brittle transition temperature (DBTF) of many crystalline solids (Gilman et al [1], Burns & Webb [2], St.John [3], and Brede & Haasen [4]). Emitted dislocations reduce the stress intensity factor at the crack tip and slow its motion by crack tip blunting and dislocation shielding. However, many analyses to date, most notably that of Rice & Thomson [5], have considered only dislocation nucleation on slip planes which contain the crack front. Argon [6] has described the DBTT by analyzing dislocation emission on planes inclined to the crack front for isotropic materials. The purpose of this paper is to show that for some anisotropic materials of cubic symmetry under mode I crack loading, the resolved shear stress is greater on slip planes which are inclined 45 ° to the crack front rather than on planes which contain the crack front.
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Journal of Materials Science
This paper presents a study of microscale plastic deformation at the crack tip and the effect of microstructure feature on the local deformation of aluminum specimen during fracture test. Three-point bending test of aluminum specimen was conducted inside a scanning electron microscopy (SEM) imaging system. The crack tip deformation was measured in situ utilizing SEM imaging capabilities and the digital image correlation (DIC) full-field deformation measurement technique. The microstructure feature at the crack tip was examined to understand its effect on the local deformation fields. Microscale pattern that was suitable for the DIC technique was generated on the specimen surface using sputter coating through a copper mesh before the fracture test. A series of SEM images of the specimen surface were acquired using in situ backscattered electronic imaging (BEI) mode during the test. The DIC technique was then applied to these SEM images to calculate the full-field deformation around the crack tip. The grain orientation map at the same location was obtained from electron backscattered diffraction (EBSD), which was superimposed on a DIC strain map to study the relationship between the microstructure feature and the evolution of plastic deformation at the crack tip. This approach enables to track the initiation and evolution of plastic deformation in grains adjacent to the crack tip. Furthermore, bifurcation of the crack due to intragranular and intergranular crack growth was observed. There was also localization of strain along a grain boundary ahead of and parallel to the crack after the maximum load was reached, which was a characteristic of Dugdale–Barenblatt strip-yield zone. Thus, it appears that there is a mixture of effects in the fracture process zone at the crack tip where the weaker aspects of the grain boundary controls the growth of the crack and the more ductile aspects of the grains themselves dissipate the energy and the corresponding strain level available for these processes through plastic work.
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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.
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The dislocation shielding field at a crack tip was experimentally proven at the atomic scale by measuring the local strain in front of the crack tip using high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA). Single crystalline (110) silicon wafers were employed. Cracks were introduced using a Vickers indenter at room temperature. The crack tip region was observed using HRTEM followed by strain measurements using GPA. The measured strain field at the crack tip was compressive owing to dislocation shielding, which is in good agreement with the strain field calculated from elastic theory.
Observation of Micro Crack Propagation in a Thin Material
Fracture analysis is one important role in a material characterization. Many theory and observation on the subject especially after the matter was broken or post cracked, but on the other hand, as far as author knowledge, only a few research are did in in situ observation on micro crack propagation, because of the propagation is in a micrometer scale and can't be observed using a naked eye and has difficulties to observed using standard optical microscope. In this study, different material are observed, the micro cracks propagation in Aluminum foil is observed in an in situ mode using an optical microscope during loaded in an axial tensile mode, the other is a Si thin plate was observed using TEM after compressed in axial compressed.
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Materials Science and Engineering: A
The newly introduced combination of focused ion beam technique with high resolution scanning electron microscopy is the first method to initiate artificial microcracks with idealized crack parameters in front of selected phase-and grain boundaries. The boundaries of interest are selected after a complete sample characterization by Electron Back Scatter Diffraction and a calculation of the preferred slip systems within each grain. This combination of characterizing-and manipulating techniques on a microscale enables for the first time a systematic investigation of the interaction of short cracks with microstructural barriers to check the models of Tanaka, Navarro and De Los Rios quantitatively. In this paper, different notch geometries from single lines to penny shaped surface cracks for the initiation of artificial microcracks are presented. It is shown that not only the starting point of a crack can be selected. Further, the crack propagation path can be predetermined by initiating the crack directly on a slip system with a high Schmid factor. As a first result the influence of grain boundaries with different misorientation angles on the crack propagation rate for a directionally solidified nickel-based superalloy is shown. Finally this method is discussed as a powerful technique for grain boundary engineering.
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To investigate the possibility of cavity formation in silica glass during subcritical crack growth, the topography of fracture surfaces formed in water at a crack velocity of 8.10 -11 m/s was mapped using an atomic force microscope. The objective of the study was to determine how well the two halves of a crack in silica glass matched by establishing a three dimensional quantitative comparison of the shape of the two surfaces. This procedure uses a minimization routine to correct first order and quadratic deformations between the two AFM images of the crack surfaces. A three-dimensional image representing topographical differences between the corresponding fracture surfaces followed a Gaussian law centered on zero with a standard deviation, , of 0.22 nm. Within the resolution of the technique, 6 nm within the fracture surface and ~ 0.3 nm normal to the fracture surface, no evidence for cavitation was found in silica glass.
Molecular dynamics study of crack processes associated with dislocation nucleated at the tip
1994
Atomic scale changes of structure around a crack-tip in an f.c.c, crystal under in-plane shear (mode II) loading are analyzed by molecular dynamics simulation (MD simulation), and its counterpart in the continuum crystal plasticity model is discussed. A ductile fracture process involving dislocation nucleation from the stressed tip is always observed. The nucleated dislocation is driven away from the crack and a dislocation-free zone develops in the near-tip region. Time averages of local stress in the near-tip region, both before and after dislocation nucleation, coincide well with the linear elastic prediction. The critical stress intensity factor estimated by MD simulation agrees well with Rice's theoretical prediction derived from the unstable stacking energy concept. The temperature dependence of the critical factor can be explained primarily as a thermally activated process. © 1994 -Elsevier Sequoia. All rights reserved ,%'DI 0921-5093(93)02520-D
Molecular dynamics study of dislocation nucleation from a crack tip
Physical Review B, 2005
We have performed a systematic molecular dynamics study of the competition between crack growth and dislocation emission from a crack tip. Two types of boundary conditions are adopted: either planar extension or boundary displacements according to the anisotropic mode-I asymptotic continuum solution. The effects of temperature, loading rate, crystal orientation, sharpness of the crack tip, atomic potential, and system size are investigated. Depending on the crystal orientation, dislocation nucleation can be driven either by the strain or by concerted fluctuations at the crack tip. In the latter case, crystal orientation and temperature have the largest influence on the process of dislocation nucleation.
The effect of thermal activation on dislocation processes at an atomistic crack tip
1999
The effects of thermal activation on the dislocation emission from an atomistic crack tip are discussed. Molecular dynamics simulations at different constant temperatures are carried out to investigate the thermal effects. The simulated results show that the processes of the partial dislocation generation and emission are temperature dependent. A s the temperature increases, the incipient duration of the partial dislocation nucleation becomes longer, the critical stress intensity factor for partial dislocation emission is reduced and, at the same loading level, more dislocations are emitted. The dislocation velocity moving away from the crack tip and the separations of partial dislocations are apparently not temperature dependent. The simulated results also show that, a s the temperature increases, the stress distribution along the crack increases slightly. Therefore stress softening at the crack tip induced by thermal activation does not exist in the present simulation. A simple model is proposed to evaluate the relation of the critical stress intensity factor versus temperature. The obtained relation is in good agreement with our molecular dynamics results.
Crack initiation site at the interface between a nano-component and substrate
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