Dislocation Crack Research Articles

Correlation between Surface Integrity and Low Cycle Fatigue Life of Machined Inconel 718

This work aims to improve the surface integrity and LCF life of machined Inconel 718. The correlation between the LCF life of Inconel 718 and various states of machined surface integrity is explored. In this paper, the surface integrity of Inconel 718 specimens is enhanced by low-plasticity burnishing (LPB). The LCF life of specimens with different surface integrity is predicted using a micro–macro finite element method (FEM). Firstly, the LCF specimens with different surface integrity are machined by turning (turned specimen), polishing (matrix specimen), and LPB process (LPBed specimen). Secondly, the LCF experiment is carried out to reveal the effect of surface integrity on LCF life. Finally, the LCF micro–macro FEM model is proposed to predict the LCF of machined Inconel 718 specimens. The representative volume element (RVE) model is established based on the measured surface integrity and microstructure of Inconel 718 specimens. The effect of surface integrity on LCF life is transformed to equivalent load. The micro–macro FEM model combined with Tanaka–Mura dislocation crack initiation theory and extended finite element method (XFEM) is applied to predict the LCF life of the machined specimens. The study results show that the LCF life of LPBed specimens can be improved by 90.5% and 36.1% compared with that of turned specimens and polished matrix specimens, respectively. The errors between FEM prediction results and experimental results are 13.1%, 9.2%, and 12.2%, respectively. The proposed micro–macro FEM model could be utilized to predict the LCF life of Inconel 718 with different surface integrities, and to apply the LCF life prediction further in industry.

Crystal crack dislocation model and micro-crack nucleation criterion in the hydrogen environment

Based on the theory of fracture mechanics, a hydrogen brittleness model of microcrystalline Al materials is proposed by using the distributed dislocation method. The distinguishing feature of this study is that the model can be used to clarify the dependence of hydrogen atoms at different positions and different numbers of hydrogen atoms on dislocation emission in the initial stage of corrosion fracture. At the same time, the mechanism of crack nucleation at the grain boundary in the hydrogen environment is studied. The results indicate that hydrogen can facilitate the emission, proliferation and motion of dislocations at the crack tip, and increase the accumulation energy of dislocations at the grain boundary, making it easier to generate micro-cracks. In the corrosive environment with hydrogen, a large number of hydrogen atom segregation at the grain boundary reduces the surface energy, and cracks are more likely to occur, and the length of microcracks is longer than that in the absence of hydrogen. These studies are helpful to deepen the understanding of the hydrogen embrittlement of materials.

Crystal crack dislocation model in the hydrogen environment

A new hydrogen embrittlement fracture model is proposed based on the discrete distributed dislocation method. The effect of hydrogen atoms distribution at the crack tip on dislocation emission before micro-crack nucleation in the early stage of corrosion fracture is investigated. The results show that hydrogen can promote the emission, diffusion and movement of dislocations, increasing the plastic zone at the crack tip. In the hydrogen environment, the critical stress intensity factor of the crystalline material decreases by a maximum of about 11%. Within the theoretical framework of this model, it is believed that hydrogen atoms accumulated closest to the crack tip will promote the emission of dislocations from the crack tip, thereby suppressing passivation of the crack while promoting brittle fracture of metallic materials. These results are helpful to analyze the hydrogen embrittlement fracture behavior of the material.

Interaction between Screw Dislocation and Interfacial Crack in Fine-Grained Piezoelectric Coatings under Steady-State Thermal Loading

The mechanical behavior of fine-grained piezoelectric/substrate structure with screw dislocation and interface edge crack under the coupling action of heat, force and electricity are studied. Using the mapping function method, firstly, the finite area plane is transformed into the right semi-infinite plane, then the expression of the temperature field is given with the help of the complex function, and then the temperature field of the problem is achieved. By constructing the general solution of the governing equation with temperature function, the analytical expression of the image force is derived. Finally, the effects of material parameters, temperature gradient, coating thickness and crack size on image force are analyzed by numerical examples. The results show that the temperature gradient has a very significant effect on the image force, and thicker coating is conducive to the stability of dislocation and interface crack.

The role of non-equilibrium grain boundary in micro-deformation and failure mechanisms of Bicrystal structural tungsten

The molecular dynamics (MD) simulation is performed to obtain the mechanical properties and plastic deformation mechanism of bicrystal structural tungsten (BS W) with equilibrium grain boundaries (EGBs) and non-equilibrium grain boundaries (NEGBs) under tension at different strain rates. The BS W with NEGBs has shown excellent comprehensive mechanical properties, and its yield strength is only about 1.5 Gpa (~10%) lower than that of the BS W with EGBs. The yield strain of BS W with NEGBS is about double than that with EGBs. The micro-deformation mechanisms for BS W with NEGBs under tension are dislocation emission and movement, GBs interaction, and phase transition. However, the failure modes of BS W with EGBs are intergranular brittle fracture of crack near the GBs, and the strain rate has little effect on the fracture. The phase transition mechanism for BS W with NEGBs under the tension loading strain rate of 0.0001 ps−1 is atomic slippage, but that of 0.001 ps−1 is stress concentration induced quasi-reconstructive phase transition. At the loading strain rate of 0.0001 ps−1, dislocation emission and movement to the GBs would form the non-slippable crack dislocations for crack initiation. However, at the loading strain rate of 0.001 ps−1, the crack propagation is considered as a zigzag-shaped form on account that the phase transition induced stress relaxation. The NEGBs could improve plasticity without sacrificing strength, providing a theoretical method for nano-structure design, material modification, and sustainable material plain.

Base friction test on unloading deformation mechanism of soft foundation waste dump under gravity

Abstract Problems of waste dumps induced by foundation weaken are quite common. A series of base friction tests were conducted in order to reveal the unloading deformation mechanism of soft foundation waste dumps. The displacement histories of waste dumps were measured by a speckle analysis and displacement measurement system and compared with hard foundation waste dumps. The test results showed that: (1) the stability of a soft foundation waste dump is obviously worse than that of a hard foundation waste dump. (2) The deformation and failure mode of a hard foundation waste dump shows a collapse-traction-push feature, and it starts from the low-grade slope and develops towards the high-grade slope. (3) The deformation and failure mode of a soft foundation waste dump is mainly controlled by the soft foundation. The non-uniform unloading settlement between front and rear parts of the waste dump results in the cyclic subsidence-crack-slip, until all the slopes above the soft foundation slide. (4) Cracks in a soft foundation waste dump first initiate at the slope bottom and propagate upwards until penetrate the whole slope, but the generation and extension pattern of cracks is opposite in a hard foundation waste dump. Cracks in a soft foundation waste dump are dislocation cracks; however, cracks in a hard foundation waste dump are tension cracks. (5) The reinforcement measures such as dynamic consolidation and borehole grouting are recommended for a soft foundation waste dump, and a drainage system could be constructed to discharge water from the dump site. These results can provide reference for further research of unloading deformation of soft foundation waste dumps under gravity.

Exact Solutions for Interaction of Parallel Screw Dislocations with a Wedge Crack in One-Dimensional Hexagonal Quasicrystal with Piezoelectric Effects

The purpose of this paper is to consider the interaction between many parallel dislocations and a wedge-shaped crack and their collective response to the external applied generalized stress in one-dimensional hexagonal piezoelectric quasicrystal, employing the complex variable function theory and the conformal transformation method; the problem for the crack is reduced to the solution of singular integral equations, which can be further reduced to solving Riemann–Hilbert boundary value problems. The analytical solutions of the generalized stress field are obtained. The dislocations are subjected to the phonon field line force, phason field line force, and line charge at the core. The positions of the dislocations are arbitrary, but the dislocation distribution is additive. The dislocation is not only subjected to the external stress and the internal stress generated by the crack, but also to the force exerted on it by other dislocations. The closed-form solutions are obtained for field intensity factors and the image force on a screw dislocation in the presence of a wedge-shaped crack and a collection of other dislocations. Numerical examples are provided to show the effects of wedge angle, dislocation position, dislocation distribution containing symmetric configurations and dislocation quantities on the field intensity factors, energy release rate, and image force acting on the dislocation. The principal new physical results obtained here are (1) the phonon stress, phason stress, and electric displacement singularity occur at the crack tip and dislocations cores, (2) the increasing number of dislocations always accelerates the crack propagation, (3) the effect of wedge angle on crack propagation is related to the distribution of dislocations, and (4) the results of the image force on the dislocation indicate that the dislocations can either be attracted or rejected and reach stable positions eventually.

Theoretical framework for predicting solute concentrations and solute-induced stresses in finite volumes with arbitrary elastic fields

A theoretical model for computing the interstitial solute concentration and the interstitial solute-induced stress field in a three-dimensional finite medium with any arbitrary elastic fields was developed. This model can be directly incorporated into two-dimensional or three-dimensional discrete dislocation dynamics simulations, continuum dislocation dynamics simulations, or crystal plasticity simulations. Using this model, it is shown that a nano-hydride can form in the tensile region below a dissociated edge dislocation at hydrogen concentration as low as χ0=5×10−5, and its formation induces a localized hydrogen elastic shielding effect that leads to a lower stacking fault width for the edge dislocation. Additionally, the model also predicts the segregation of hydrogen at Σ109(13 7 0)/33.4∘ symmetric tilt grain boundary dislocations. This segregation strongly alters the magnitude of the shear stresses at the grain boundary, which can subsequently alter dislocation-grain boundary interactions and dislocation slip transmissions across the grain boundary. Moreover, the model also predicts that the hydrogen concentration at a mode-I central crack tip increases with increasing external loading, higher intrinsic hydrogen concentration, and/or larger crack lengths. Finally, linearized approximate closed-form solutions for the solute concentration and the interstitial solute-induced stress field were also developed. These approximate solutions can effectively reduce the computation cost to assess the concentration and stress field in the presence of solutes. These approximate solutions are also shown to be a good approximation when the positions of interest are several nanometers away (i.e. long-ranged elastic interactions) from stress singularities (e.g. dislocation core and crack tip), for low solute concentrations, and/or at high temperatures.

Fatigue damage study of cold metal transition fusion-brazed aluminium/steel dissimilar joints

ABSTRACTAn aluminium/steel CMT-welded joint is mainly composed of fusion zone (FZ), interfacial intermetallic compound (IMC) zone, and heat-affected zone (HAZ). IMC and FZ are the weakest link in the welded joint. The fatigue fracture indicates that the lattice distortion caused by the lattice mismatch of Fe2Al5 and Fe is more likely to engender dislocation slipping and cracks easily nucleate during the fatigue testing. The hysteresis loop is shifted to the right, which indicating that the specimen experiences a ratcheting effect. In the high-stress condition, the cyclic hardening induced by the ratcheting strain is much stronger than that in the lower stress condition, which improves the fatigue life of the materials.

Analyzing dynamic disturbance fragmentation mechanism of surrounding rock in roadway roof

In this paper, a numerical simulation is conducted to analyze an engineering practice pertaining to the in panel 2347, the discussion is focused on the following aspects: the influence of fault mining activation on fragmentation of surrounding rock in roof, surrounding rock failure of unsupported roadway under static load alone, surrounding rock failure of unsupported roadway under dynamic load and static load combination, damage of surrounding rock in supporting roadway under dynamic load and static load combination. The results show that, for the roadway surrounding rock with obvious dynamic disturbance, the compressive stress value can eliminate the influence of the tensile stress wave generated by the reflection of the compressive stress disturbance wave on the roadway wall. In the roadway support, it should provide a certain compressive stress to the surrounding rock of the surrounding roadway wall. An anchor support surrounding rock can significantly inhibit the tensile crack, shear the crack expansion and dislocation slip of coal gang, and it can also alleviate the tensile, compressive and shear failure of the roof carbonaceous mudstone. Because the dynamic load has a significant damage to the carbonaceous mudstone between roof anchors, it is necessary to reduce the anchor spacing and row spacing or enhance the stiffness and active stress of the protective surface member.

Microcrack under internal pressure at dislocation defect

By solving the problem of the theory of elasticity for a boundary dislocation with a cavity on its continuation, a stress-strain state in a solid crystalline body is determined. The correlation for calculating the energy of a body with a dislocation crack under pressure is obtained. The geometric parameters of a crack-like dislocation cavity are calculated. The values of equilibrium and non-equilibrium crack lengths are established on this basis. Also the value of the critical pressure at which the dislocation crack start begins is received. Stress intensity factors for the dislocation crack are obtained and calculated. Physical essence of fracture toughness is established using stress intensity factors.

Effects of Grain Size and Twin Layer Thickness on Crack Initiation at Twin Boundaries.

A theoretical model to explore the effect on crack initiation of nanotwinned materials was proposed based on the accumulation of dislocations at twin boundaries. First, a critical cracking initiation condition was established considering the number of dislocations pill-up at TBs, grain size and twin layer thickness, and a semi-quantitative relationship between the crystallographic orientation and the stacking fault energy was built. In addition, the number of dislocations pill-up was described by introducing the theory of strain gradient. Based on this model, the effects of grain size and twin lamellae thickness on dislocation density and crack initiation at twin boundaries were also discussed. The simulation results demonstrated that the crack initiation resistance can be improved by decreasing the grain size and increasing the twin lamellae, which keeps in agreement with recent experimental findings reported in the literature.

Stress state of a piecewise uniform layered space with doubly periodic internal cracks

The present paper deals with the stress state of a piecewise homogeneous plane formed by alternation junction of two distinct strips of equal height manufactured of different materials. There is a doubly periodic system of cracks on the plane. The governing system of singular integral equations of the first kind for the density of the crack dislocation is derived. The solution of the problem in the case where only one of the repeated strips contains one doubly-periodic crack is obtained by the method of mechanical quadratures.

A Cumulative Approach to Common Neighborhood Analysis for Crystalline Structure Characterization in Atomistic Simulations

Crystalline characterization poses a challenge when atomic deformation and phase transformation are occurring in an atomic simulation. Surface features are also of great importance to discern bulk features from surface properties and surface science. Embedded atomic features are conventionally characterized by high value parameters to distinguish them from perfect crystalline phase to extract crack tips, dislocations, surfaces and other crystalline features. Common neighborhood analysis has been presented as an adjusted method to characterize those features with enhanced formulation to the current framework to characterize non-monoatomic interactions. The present work introduces a novel approach to characterize crystalline structures by means of cumulative common neighborhood parameterization (CCNP) for arbitrary structures. The method is compared with centrosymmetry parameter CSP and common neighborhood parameter CNA. The results showed a better performance in discerning surface and bulk features with a high visible range between bulk, surface and other atoms. The method was also extended to characterize a complex P42/mnm space group, non-monoatomic crystal with no common first nearest neighbors.

Characterizing post-construction settlement of the Masjed-Soleyman embankment dam, Southwest Iran, using TerraSAR-X SpotLight radar imagery

Geodetic monitoring of infrastructures is one of the key tasks in surveying and engineering geology. Systematic monitoring and assessment of the exterior deformation of embankment dams for safety analysis are often difficult when using classical surveying techniques due to time-consuming surveying procedures and high labour costs. Modern remote sensing techniques play an important role in efficiently assessing deformation: changes in the geometry, position and orientation of dams. In this study, we present the feasibility of effective post-construction deformation monitoring of the Masjed-Soleyman dam in Iran using high-resolution (∼1m) synthetic aperture radar (SAR) imagery in SpotLight (SL) mode from the X-band TerraSAR-X (TSX) satellite. This dam has been monitored over the last 15years using classical surveying techniques, which provide horizontal and vertical deformation measurements of the structure. We show that high-resolution X-band SAR data provide a much more detailed identification of dam deformation in the crest and downstream that is not possible to infer from classical surveying techniques with few sparse geodetic monuments. High-resolution TSX data reveal that the dam is currently subject to two different deformation regimes: one is related to the crest and its adjacent area downstream, with a maximum rate of deformation of approximately 13cm/yr in the radar line-of-sight (LOS). The other is related to the lower part of the downstream, with a maximum LOS velocity of 7cm/yr. The effect of this centimetre displacement rate has been shown through several damage features on the dam body, including minor to large dislocation cracks on the crest and a significant deformation zone on the downstream slope.

Energy of Deformation of an Elastic Body Containing a Microcrack Under Pressure

We determine complex potentials and the corresponding stress-strain state of a body with dislocation crack under a given pressure acting in the crack. We find the geometric parameters of a cracklike dislocation cavity and deduce relations for finding the energy of the body containing a crack of this kind. On this basis, we establish the lengths of equilibrium and nonequilibrium cracks and determine the critical pressure of crack initiation.

The Fabrication and Properties of the Squeeze-Cast TiN/Al Composites

Co-continuous TiN/Al composites with different volume fractions of Al phase have been fabricated by the squeeze-casting method. TiN porous ceramics with different porosities were fabricated through carbothermal reduction by changing the content of TiN and were used as preforms. The outstanding mechanical properties were attributed to the absence of excessive interface reaction between TiN and Al for the co-continuous TiN/Al composites. With the increase of Al content in the composites, the flexural strength and the microhardness decreased, and the fracture toughness increased. The strengthening and toughening mechanism of composites included dislocation strengthening, ductile rupture, crack deflection, and secondary cracks.

Evaluation of the Potential Energy and Geometric Sizes of a Dislocation Crack

We study a dislocation crack in which a displacement jump is specified at one of its tips but the faces are closed at the other tip. The physical meaning of the problem is specified from the positions of fracture mechanics for a dislocation crack, which is associated with a mathematical model of a semiinfinite defect in solid body with an introduced extra plane of given thickness and a cut on its continuation. Within the framework of this model, we deduce an integral equation and find its solution, which enables us to establish the stress-strain state near the dislocation crack. As a result, we determine the geometric parameters and find the energy of a body weakened by the dislocation crack.

EFFECTS OF GRAIN REFINEMENT ON CREEP PROPERTIES OF K417G SUPERALLOY

Grain size is one of the most important parameters which affect the mechanical properties of cast polycrystalline superalloys. To study the effect of grain refinement on the creep behaviors of K417 G superalloy, the creep behaviors of K417 G superalloy with four grain sizes were investigated at 760 ℃/645 MPa, 900 ℃/315 MPa and 950 ℃/235 MPa. The longitudinal section of the fracture surface, crack propagation path, dislocation structure and plastic deformation distribution in the vicinity of the cracks were investigated by using SEM, TEM and EBSD techniques, thus the deformation mechanism and effect of grain refinement on the creep properties of K417 G superalloy were determined under different creep conditions. The results showed that the effects of grain refinement on the creep property of the alloy varied with the temperatures and stress. At 760 ℃/645 MPa, grain refinement improved the creep life and reduced the steady-state deformation rate of the alloy. The creep deformation was dominated by intragranular deformation. At 900 ℃/315 MPa, as grain size decreased, the creep life increased firstly and then decreased, while the steady-state deformation rate decreased firstly and then increased. The creep deformation showed a competitive effect of intragranular deformation and grain boundary sliding. At 950 ℃/235 MPa, the creep life decreased and the steady- state deformation rate increased with the decrease of the grain size. Grain boundary sliding was the main deformation mode. At the same time, grain refinement could cause a refinement of the dendrite and carbide of the alloy, which would also affect the creep behavior of the alloy to a small extent. The TEM observation showed that at 760 ℃/645 MPa, the dislocations interacted with g' particles through shearing mechanism and no dislocation network was found in the matrix. While at 900 ℃/315 MPa and 950 ℃/235 MPa, the dislocations crossed the g' particles through Orowan bypass mechanism, dislocation network formed in the matrix, and M23C6 precipitated in the interior of the grains, which had a [001]M23C6∥[001]Ma tr ixorientation relationship between the M23C6 precipitates and matrix.

Elastic interaction between edge dislocation, concentrated force, point heat source and edge crack in a semi-infinite plane

We investigate the interaction between edge crack and edge dislocation as well as concentrated force and point heat source. The stress intensity factors at the edge crack tip and the image forces acting on the edge dislocation are calculated. The influence of the concentrated force, point heat source and edge dislocation on the shielding and anti-shielding effects to edge crack as well as the glide and climb forces acting on the edge dislocation is examined in detail. The results indicate that the shielding and anti-shielding effects increase acutely with the increment of concentrated force and point heat source. In addition, the glide and climb forces increase acutely with the decrement of the distance between dislocation and crack tip.