Boundary Fracture Research Articles

Natural fractures in deep tight gas sandstone reservoirs in the thrust belt of the southern Junggar Basin, northwestern China

The development of natural fractures is a significant characteristic of the Jurassic deep tight sandstone reservoirs in the thrust belt of the southern Junggar Basin, and these reservoirs have a great potential for natural gas resources. Based on the analyses of outcrops, cores, thin sections, and other laboratory data, natural fractures in these reservoirs are mainly tectonic ones, which appear in groups and vary in scale, dip angle, and density. We have classified fractures in thin sections into intragranular, grain boundary, and transgranular ones depending on their relationship with minerals grains. Almost 58% of the whole fracture population is opening-mode fractures, and calcite is the main filling mineral for the remaining ones. Fracture apertures vary based on their types, where transgranular fractures are the widest, followed by grain boundary and intragranular ones. Lithology, rock mechanical mechanics layers, and structures control the development of natural fractures. Fractures are more frequent in siltstone and fine sandstone. Sandstones with larger mineral grains are more likely to develop grain boundary and intragranular fractures. Intralayer fractures are the dominant ones, which intersect the rock mechanics interface at high angles or perpendicularly. The linear density of these fractures decreases when the thickness of the rock-mechanics layer increases. Furthermore, fractures have a higher degree of development in the hanging wall of the faults, with the degree decreasing when the distance from the fault plane increases. Additionally, the development degree of fractures in the damage zone is better than the adjacent rocks, and the width of damage zones is a function of the amount of fault displacement.

Periodic formations of deformation twins and dislocation arrays via grain boundary sliding at serrated grain boundary in cold-shear-strained AZ31 alloy

It is well known that grain boundary sliding (GBS) provides stress concentration and initiates grain boundary fracture at the serrated grain boundary or grain triple junction during high-temperature creep. Recent reports on GBS occurring during deformation at room temperature suggest that stress concentration may occur due to GBS, which has a stress component parallel to the grain boundary plane, even in cold working. To identify if GBS can generate stress concentration and related defects at the grain boundary at room temperature, we experimentally investigated periodic defect formation along the serrated grain boundary during cold shear deformation. To obtain defect-free serrated grain boundary structure, optimal conditions of hot-torsion and annealing were determined. Cylindrical torsion specimens were then machined into notched tensile specimens, which provide the advantage of directly observing the GBS and lattice curvature. Extensive observations and orientation analysis on periodic defect formation along the serrated grain boundary suggest that GBS at room temperature can periodically nucleate twinning as well as dislocation arrays. Crystal plasticity simulation proved that serrated GB generates non-rotating region and split stress distribution with intensified value. Two stress concentration mechanisms during the accommodation of GBS at the serrated grain boundary were proposed: direct transfer from rotational stress and indirectly formed tensile component, namely “cleavage stress.” While direct energy transfer was likely to follow the local Schmid's law for direction of the grain rotation, cleavage accommodation represented geometrically necessary characteristics.

Effects of methanol and acetone as mutual solvents on wettability alteration of carbonate reservoir rock and imbibition of carbonated seawater

The enhanced oil recovery (EOR) methods combination is a way to simultaneously utilize the mechanisms of the combined methods. Besides, in chemical water injection, additives controlling the mechanisms by their mutual interactions also exert other effects on the system, known as synergistic effects. In the current study, acetone and methanol mutual solvents were utilized to enrich carbonated water and their effects on the mechanisms of carbonate reservoir rock wettability alteration and oil production in the imbibition process have been investigated at different conditions of salinity, the ratio of solvent to water and pressure. Also, various boundary conditions and fracture effects were considered in the imbibition experiments. Imbibition fluid optimization was performed based on the contact angle tests and reservoir rock wettability alteration. Seawater was used as the base fluid at various initial and diluted concentrations. According to the results, the contact angle magnitude depends on the parameters of salinity, the volume percent of the mutual solvents and the pressure so that as the solvents volume and pressure increase and the salinity decreases, the contact angle decreases when carbon dioxide (CO2) is present but in absence of CO2, the contact angle decreases with increasing base fluid salinity. The lowest contact angle values for methanol and acetone solvents were recorded at 10.342 MPa, initial seawater salinity, 15% volumetric solvent, and a constant temperature 75 °C in absence of dissolved CO2 at 60.95° and 58.43°, respectively. The lowest contact angle values were obtained in the presence of dissolved CO2 for methanol and acetone solvents at 15% volumetric pressure at 10.342 MPa, salinity resulting from 20 times seawater dilution and at a temperature of 75 °C at 36.31° and 32.92°. Final oil production of 66.29%, 28.44%, 85.42% and 91.38% of primary oil saturation for the imbibition of carbonated water containing 15%Vol of methanol and final oil production of 70.32%, 32.10%, 90.60% and 94.09% of primary oil saturation for the imbibition of carbonated water containing 15%Vol acetone were obtained which were performed as one-dimensional co-current spontaneous imbibition (COCSI), one-dimensional counter-current spontaneous imbibition (COUCSI), multi-dimensional COUCSI and imbibition in the fractured plug, respectively.

Influence of grain boundary and grain size on the mechanical properties of polycrystalline ceramics: Grain‐scale simulations

AbstractThe Orowan‐Petch relation is a famous model to describe the strength of polycrystalline ceramics covering a wide range of grain sizes. However, it becomes difficult to explain the strength trend when the grain size decreases to the sub‐microscale or nanoscale. This is because some microstructural parameters (such as grain size, grain boundary fracture energy, and grain boundary defects) vary with different processing technologies, and their coupling effects on mechanical properties are still unclear. In this study, a finite element method (FEM) was applied to investigate the dependence of mechanical properties, such as strength and damage resistance, on the abovementioned microstructural parameters on example of alumina. The numerical results show that the grain boundary energy is weakly coupled with the grain size and grain boundary defects. The grain size and grain boundary are intercoupling, which affects mechanical properties. The mechanical properties could be improved by increasing the grain boundary fracture energy and decreasing the grain size and the grain boundary defect density.

Hydrogen-assisted fatigue crack-propagation in a Ni-based superalloy 718, revealed via crack-path crystallography and deformation microstructures

Fatigue crack-growth (FCG) of Ni-based superalloy 718 was investigated under gaseous hydrogen environment (external hydrogen) and uniformly pre-charged state (internal hydrogen). Under external hydrogen, intergranular fracture predominated, whereas dislocation slip-band or twin boundary fracture were prevalent under internal hydrogen. This failure mode divergence encompassed unique characteristics of macroscale FCG response, leading to both cycle- and time-dependent cracking. The intergranular cracking was ascribed to short-circuit diffusion of hydrogen along grain boundaries. Meanwhile, the material’s inherently inhomogeneous deformation mode exerts harmfulness when hydrogen was uniformly distributed inside the specimen, causing slip-bands or twin boundaries to become the weakest links for fracture.

Mechanical Properties and Shape Memory Characteristics of NiAl Alloys by Powder Metallurgy

The composition of martensite transformation in NiAl alloy is determined using pure nickel and aluminum powder by vacuum hot press powder metallurgy, which is a composition of martensitic transformation, and the characteristics of martensitic transformation and microstructure of sintered NiAl alloys are investigated. The produced sintered alloys are presintered and hot pressed in vacuum; after homogenizing heat treatment at 1,273 K for 86.4 ks, they are water-cooled to produce NiAl sintered alloys having relative density of 99 % or more. As a result of observations of the microstructure of the sintered NiAl alloy specimens quenched in ice water after homogenization treatment at 1,273 K, it is found that specimens of all compositions consisted of two phases and voids. In addition, it is found that martensite transformation did not occur because surface fluctuation shapes did not appear inside the crystal grains with quenching at 1,273 K. As a result of examining the relationship between the density and composition after martensitic transformation of the sintered alloys, the density after transformation is found to have increased by about 1 % compared to before the transformation. As a result of examining the relationship between the hardness (Hv) at room temperature and the composition of the matrix phase and the martensite phase, the hardness of the martensite phase is found to be smaller than that of the matrix phase. As a result of examining the relationship between the temperature at which the shape recovery is completed by heating and the composition, the shape recovery temperature is found to decrease almost linearly as the Al concentration increases, and the gradient is about -160 K/at% Al. After quenching the sintered NiAl alloys of the 37 at%Al into martensite, specimens fractured by three-point bending at room temperature are observed by SEM and, as a result, some grain boundary fractures are observed on the fracture surface, and mainly intergranular cleavage fractures.

Effects of heat treatment and zirconium addition on grain boundary fracture of Al-5%Mg alloys with sodium

Effects of heat treatment and zirconium addition on grain boundary fracture of Al-5%Mg alloys with sodium

TiB2-reinforced B4C composites produced by reaction sintering at high-pressure and high temperature

ABSTRACTObtaining both high hardness and toughness is a challenge in B4C-nano-adhesive composites. Solving the inhomogeneous distribution of nano-adhesives in B4C and forming the chemical bonding at grain boundary is an effective method. Here, we reported that the uniform distribution of titanium diboride (TiB2)-reinforced B4C composites synthesized by high-pressure and high temperature (HPHT). It is found that HPHT sintering can effectively inhibit the grain growth and increase the relative density. Moreover, HPHT sintering can cross high reaction energy barrier and effectively promote the formation of chemical bonding at grain boundary between B4C and TiB2. The optimal hardness and toughness value reach 30.0 ± 0.9 GPa and 7.87 MPa·m1/2, respectively. The improvement of hardness and toughness in the final products are ascribed to the strengthening of nanoTiB2 connection of B4C boundary and intergranular fracture mechanism. This work suggests a new way to achieve the uniform distribution of nanoTiB2 in B4C and form the chemical bonding at grain boundary, which is of great significance to the further development of TiB2-reinforced B4C composites with excellent mechanical properties.

Grain Boundary and Lattice Fracture Toughness of UO2 Measured Using Small-Scale Mechanics

Small-scale mechanical testing has been used to isolate the properties of individual grain boundaries and regions of the lattice in undoped UO2 polycrystals. Notched microcantilever specimens were prepared by focused ion beam milling and tested in bending. Fracture toughness values were calculated from finite element analysis of the sample geometry at fracture load using the J-integral method. Lattice and grain boundary fracture toughness values of 1.17 ± 0.16 MPa m1/2 and 1.08 ± 0.08 MPa m1/2, respectively, were obtained. The similarity between the lattice and grain boundary fracture toughness values is consistent with the preference for transgranular room-temperature fracture in UO2. The grain boundary and lattice fracture toughness values also agree well with the toughness of dense polycrystals, which suggests that microstructural-scale toughening mechanisms are limited in such samples.

Convolution neural network based on fusion parallel multiscale features for segmenting fractures in coal-rock images

The coal-rock fractures formed by natural geological evolution are complex and the shapes are not fixed, which makes it difficult to manually define the features of coal-rock fractures. Based on the full convolutional neural network U-net, we propose a convolutional neural network with fusion parallel multiscale features (FPMF-U-net). The FPMF-U-net uses two feature extraction networks in parallel to extract multiscale features. The feature fusion layer in this network is responsible for weighted fusion of the shallow high-resolution features and the deep abstract features. Average pooling operator is used in the network to avoid loss of weak boundary fractures caused by max pooling. The FPMF-U-net can automatically extract fracture features and segment coal-rock fractures from images. According to the three-dimensional gradual change feature of the fracture shapes between adjacent images, the segmentation results of the FPMF-U-net are further optimized. Due to the lack of training samples of the fracture, we use a data augmentation technique to increase the number of training samples. At the same time, the transfer learning method is used to improve the convergence speed of the FPMF-U-net. The experimental results show that the proposed FPMF-U-net has a good effect on the segmentation of coal-rock fractures.

The contrasting fracture behaviour of twin boundaries and general boundaries – A first principles study based on experimental observation

Experimental observations of grain boundary fracture behavior have been used as exemplars, and boundaries with the same crystallography as the experiments were developed. These boundaries were non-symmetrical, and this necessitated the first ever study of non-symmetrical boundaries in magnesium using density functional theory. The broad agreement of the calculated boundary cohesion values with the experimental observations of fracture behavior showed the simulations to be good approximations of real behavior, and from this point the simulations were further interrogated to understand the differences between the boundary types and solute species. Solutes with both larger and smaller radii than magnesium had a preference for segregation to the grain boundary. The boundary cohesion was examined by the parameter known as embrittlement potency, and it was found that solutes smaller than magnesium had a toughening effect, while those solutes larger than magnesium had more tendency to embrittle. The two boundaries studied in most detail, the {101‾2} twin boundary, and a general grain boundary observed experimentally, showed different cohesive behaviors. Although the symmetrical twin boundary behaved in a similar manner to previous reports, the non-symmetrical boundary showed more complex cohesive behavior, highlighting the importance of studying the non-symmetrical boundaries that predominate real materials.

Effects of tungsten disulfide on microstructure and mechanical properties of AZ91 magnesium alloy manufactured by stir casting

In this work, AZ91 magnesium alloy reinforced with a low fraction (0–1) wt.% tungsten disulfide(WS2) microparticles have been fabricated using stir casting method. Effects of WS2 microparticles on mechanical properties, microstructure, and their correlation have been analyzed. The microstructure of composites has been investigated by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffractometer. Results indicate that density, hardness, ultimate tensile strength (UTS), and tensile ductility have a direct relation with reinforcement (WS2) contents and has optimal value for 0.6 wt% WS2/AZ91 composites then slightly decrease afterward. The UTS and hardness of 0.6 wt%WS2/AZ91 were increased by 23.23% and 10.65% respectively with respect to monolithic AZ91. The ultimate compressive strength(UCS) is directly related with WS2 concentration and 10.07% enhancement in UCS for 1 wt%WS2/AZ91 composites have been noted. The trend is interlinked with density and hardness of composites. SEM images depict, WS2 micro-particles suppress β-Mg17Al12 precipitates, refine the microstructure and enhance the grain size resulting in an increase in mechanical properties. The improvement in microstructure and mechanical properties is attributed to the dispersion of WS2 particles which hinder the grain growth. The increase in porosity has causes the decrease in mechanical behavior of 1 wt%WS2/AZ91. Twin boundary fracture has been found major fracture phenomenon in fractured WS2/AZ91 surfaces.

Coupled chemo-mechanical modeling of fracture in polycrystalline cathode for lithium-ion battery

A thermodynamically consistent multi-physics framework has been developed to understand the chemo-mechanical interplay towards fracture behavior of polycrystalline microstructure along with the current collector. Adopting non-equilibrium thermodynamics, and considering bulk and interfaces as separate systems, the coupled governing equations are derived. The proposed framework accounts the plastic deformation of the host lattice and the substrate. A chemo-mechanical cohesive zone model (CZM) has been formulated to simultaneously capture the decohesion and transport across grain boundaries, while classical CZM is used between the cathode/substrate interface. Using numerical framework, grain-boundary width is initially calibrated to ensure transport of Li-ions through intact grain boundaries. The proposed numerical scheme has been verified with semi-analytical model for cylindrical cathode particle in the presence of an interface. Further, thin film polycrystalline cathode along with current collector has been examined to explore the role of various interactions (one way or two way) between the diffusion and stresses on the emergent voltage profile for an electrochemical cycle. In particular, the chemical and mechanical driving forces have been investigated together with evolution of grain boundary fracture and their consequences on the insertion/extraction of Li-ions from the host lattices. In addition, we address the mechanical failure and attendant voltage profiles for various microstructural and electrochemical parameters (i.e., grain size and charge rate). The present framework can provide better design perspective for the development of heterogeneous electrode system with improved mechanical integrity under fully coupled chemo-mechanical environment.

Microstructural Influences on Fracture at Prior Austenite Grain Boundaries in Dual-Phase Steels.

Dual phase (DP) steels provide good strength and ductility properties. Nevertheless, their forming capability is limited due to the damage characteristics of their constituting microstructural phases and interfaces. In this work, a specific type of interface is analysed, i.e., prior austenite grain boundaries (PAGBs). In the literature, prior austenite grain boundary fracture has been reported as an important damage mechanism of DP-steels. The influence of the morphology of phase boundaries near the PAGB and the role of the martensite substructure in the vicinity of a PAGB on damage initiation is analysed. The experimentally observed preferred sites of crack nucleation along the PAGB are assessed and clarified. A finite strain rate dependent crystal plasticity model accounting for the anisotropic elasto-plasticity of martensite (and also ferrite) was applied to an idealized volume element approximating a typical small-scale PAGB microstructure. The boundary value problem is solved using a fast Fourier transform (FFT) based spectral solver. The role of crystallography and geometrical features within the volume element is studied using simulations. Results are discussed considering possibly dominant regimes of elasticity and plasticity.

Electrochemical-Shock Resistant Single-Crystal Ni-Rich Layered Cathode Materials

Ni-rich layered oxides LiNi x M1 -x O2 (x ≥ 0.6, M = combinations of Co, Mn and/or Al) are technologically important cathode materials for lithium-ion batteries due to their high energy density, good power capability, and lower material cost than LiCoO2. These materials are usually prepared by a co-precipitation and calcination method that produces micron-sized polycrystalline (PC) particles consisting of densely packed nanoscale grains. The unique microstructure and anisotropic Vegard coefficient of these materials make them vulnerable to grain boundary fracture (“electrochemical shock”), a major cause for battery impedance growth and performance degradation. We have recently developed a scalable method to prepare a variety of single-crystal (SC) Ni-rich cathode materials, such as LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.6Mn0.2Co0.2O2, and LiNi0.8Co0.15Al0.05O2. SC-NMC622 outperforms the commercially available PC-NMC622 in terms of cycling stability, rate performance, and thermal stability. Post-cycling structural characterizations reveal that the SC-NMC622 do not crack even when charged to 4.9 V vs Li+/Li and its surface undergoes less phase transformation than PC-NMC622 during cycling in a normal voltage range (4.3-2.8 V). Single-crystal Ni-rich cathode materials offer hope for fabricating mechanically reliable solid-state batteries and provide a unique platform for studying doping and surface modification methods to further improve the durability of Ni-rich cathode materials.

On the effect of grain structure in micro-cutting of polycrystalline aluminate magnesium spinel (PAMS) crystals

This paper studies the anisotropic effect induced by the grain structure of polycrystalline aluminate magnesium spinel (PAMS) on the mechanism of surface generation in the micro-cutting process. The critical depth of cut for ductile–brittle transition (DBT) of PAMS is estimated from 81 to 163 nm by plunge-cutting tests. In the cutting tests with constant cutting depths from 250 nm to 1.5 µm, an apparent regional distribution of crack features is clearly observed and it implies that the randomly oriented crystal grains with different slip systems play an important role in brittle-regime surface generation and fractures evolution. An effective way to obtain a ductile-regime machined surface is to decrease the cutting depth to 50 nm, and a flawless surface can be obtained when the cutting depth is as shallow as 30 nm. With such a low cutting depth, the material is mainly removed by the ploughing effect. Finite element method (FEM) model has been developed to simulate the cutting process crossing a grain boundary of two adjacent grains with different anisotropy material properties. The simulation result supports that the subsurface damage and boundary fractures can be evoked ahead of the actual material removal process by the cutting stress propagating from the precedingly machined grain. Different crack-patterns are shown on the finished surfaces of the two adjacent grains of different orientations.

Direct observation of atomic-scale fracture path within ceramic grain boundary core

In fracture processes, grain boundaries behave as preferential paths for crack propagation. These grain boundary fractures proceed by the atomic-bond rupture within the grain boundary cores, and thus grain boundary structures have crucial influence on the fracture properties. However, the relationship between grain boundary structures and atomic fracture processes has been a matter of conjecture, especially in the case of dopant-segregated grain boundaries which have complicated local structures and chemistries. Here, we determine the atomic-bond breaking path within a dopant-segregated Al2O3 grain boundary core, via atomic-scale observations of the as-fractured surface and the crack tip introduced by in situ nanoindentation experiments inside a transmission electron microscope. Our observations show that the atomic fracture path is selected to produce less coordination-deficient oxygen polyhedra of dopant cations, which is rationalised using first-principles calculations. The present findings indicate that the atomic coordination geometry at the grain boundary core affects the fracture processes.

A crack-location correction for T0 analysis of an alloy 52 dissimilar metal weld

For narrow-gap Alloy 52 dissimilar metal welds, the fusion boundary between the ferritic pressure vessel steel and austenitic Ni-base alloy weld metal is the weakest region from fracture mechanical perspective. Producing of the pre-crack for fracture toughness test specimens in the fusion boundary region to determine T0 is challenging. For pre-cracks further away from the fusion boundary, the brittle fracture initiation does not necessarily occur on the initial crack plane, but rather in the fusion boundary region. This offset in initiation site can increase the measured fracture toughness leading to a nonconservative T0 estimate. In this study, a method is proposed for correcting the fracture toughness for crack location and obtaining a lower boundary fracture toughness curve. The crack-location correction shows promise for analysing T0 of specimens with pre-cracks systematically at a significant distance from the weakest region, but with brittle fracture initiation still occurring at the weakest location.

Ductility enhancement of tungsten after plastic deformation

An unusual room temperature mechanical behavior of pure tungsten is investigated by focusing on three specimens prepared with different microstructures: as-received (hot-rolled), recrystallized, and cold-rolled specimens. Contrary to ordinary expectations in metallic materials, only the cold-rolled specimen exhibits significant plastic deformation during tensile testing, with improved strength and ductility, while the recrystallized and the as-received specimens fail in the elastic region. We further provide an explanation of such characteristics by systematically utilizing experimental and theoretical analysis at a small scale: nano-indentation tests clarify the inherent mechanical response inside grains and provide a statistical distribution of the maximum shear stress during pop-in events, corresponding to onset of plastic yielding; atomistic simulations provide information on the overall fracture strength of various grain boundaries. By comparing the maximum shear stress and the grain boundary fracture stress, we could explain that the distinctive plasticity in the cold-rolled specimen is caused by the movement of pre-existing dislocations before grain boundary fracture. This contrasts the recrystallized and as-received specimens, where grain boundary fracture occurs prior to the nucleation of dislocations or activation of dislocation sources.

The effects of the boundary layer and fracture networks on the water huff-n-puff process of tight oil reservoirs

Application of multi-fractured horizontal wells has led to significant progress in the development of tight oil reservoirs. However, oil production rapidly decreases and the estimated ultimate recovery is still low when the well is in production and approaching depletion. Water huff-n-puff is regarded as a useful measure to improve oil recovery due to oil-water imbibition and drainage. Additionally, complex fracture networks consisting of natural fractures (NF) and hydraulic fractures can significantly enhance oil recovery in the process of water huff-n-puff after multi-stage hydraulic fracturing. However, there is a non-flowing thin film on the solid-liquid wall surface in the pore-throats of the matrix, and the so-called boundary layer effect (BLE) is quite significant at the nano-scale. This effect reduces the effective flow space and effective permeability of the reservoir, leading to production damage. To date, few numerical simulation studies have considered the impact of the BLE and complex fracture networks in water huff-n-puff process simulation studies. In this study, the process of water huff-n-puff is evaluated considering complex fracture networks and formation damage in a matrix. The embedded discrete fracture model (EDMF), validated by local grid refinement (LGR), is applied to describe the distribution of hydraulic fractures and NF. The well performance considering the BLE is analyzed by a vectorization programming. The results show that the BLE reduces production and accelerates the production decline, so the BLE should be considered in reservoir simulations and productivity evaluations of water huff-n-puff processes. A certain degree of natural fracture development is conducive to improving the sweep efficiency of the water huff-n-puff, which could significantly improve oil recovery compared to depletion. By considering the impact of the BLE in water huff-n-puff simulations in tight oil reservoirs, it is possible to accurately evaluate the well performance and provide a theoretical basis for the optimization of water huff-n-puff.