Crack Network Research Articles

Investigation of cracking behavior of fine-grained clays exposed to different humidity environments and engineering implications based on X-ray computed tomography

Due to the humidity difference between fine-grained clay and atmosphere environment, the complex crack network is formed, which is the main channel for water storage and migration and hence poses a potential threat to engineering construction. Therefore, the appropriate characterization of cracking behavior of fine-grained clay is of great significance for exploring its microphysical properties. To this end, this work took expansive soils as research object and a series of desiccation tests and X-ray computed tomography tests were conducted to investigate the shrinkage cracking behavior. The results indicate that a higher desiccation rate records a smaller shrinkage strain, but it aggravates the cracking on a much larger scale. Especially at high desiccation rates, two-dimensional crack ratio shows a significant fluctuation. Small cracks tend to turn into large ones during desiccation, and the increasing desiccation rate primarily exerts a significant effect on the cracks with equivalent radius more than 1000 µm. The expansive soil is dominated by horizontal cracks, and a low desiccation rate mainly promotes the connection between horizontal cracks, whereas high desiccation rates enhance the connectivity of oblique cracks, which exerts an adverse effect on soil slopes. Besides, the rise in desiccation rate leads to a wealth of micro-scale pores and throats occurring in the soil. The fine-grained clays at higher desiccation rates present larger pore and throat volume, more connected pores, and higher average coordination number, thus enhancing the connectivity between pore and pore. Permeability simulation results also confirm the positive correlation between connected crack ratio and hydraulic conductivity of the soil. In terms of engineering significance, narrowing down the difference in humidity between fine-grained clays and atmosphere is the most effective measure to constrain the extension of cracks.

Poromechanical approach to blast‐induced dynamic fracture in concrete

AbstractThis study introduces a novel poromechanical approach to analyze blast‐induced dynamic fracture in concrete considering gaseous kinetics. The objective is to establish a framework for examining the fracture mechanisms during demolition. The simulation employs finite element analysis, incorporating three‐dimensional non‐orthogonal multidirectional cracks induced by gas inflation, through which the gaseous media flows too. The study investigates the principles governing crack network development and the influence of pressurized gas using a multiphase scheme and verifies its findings experimentally. It was also emphasized that benchmark blast experimental data are essential for validating the model, for practical applications in the demolition and renovation of existing structural concrete.

Development of an intelligent geographic information system for mountain roads monitoring: Ground data collection and analysis

This study examines the development of an intelligent geographic information system for mountain road monitoring. To make well-informed and timely management decisions in the planning, construction, and maintenance of mountain roads, and to anticipate potential critical situations and their effects on road conditions, it is imperative to employ modern methodologies. This study involves data collection from a specific section of a mountain road situated near Almaty, Kazakhstan. We gathered data on the road section through visual assessment, measurements of the coefficient of adhesion, and the acquisition of material samples from the structural layers of the roadbed. Based on the visual assessment and surface adhesion measurements, it became evident that the deformation of the studied road section, which includes a network of cracks and "non-standard" transverse cracks, is attributed to mountain mass shifts and water flows from slopes. Laboratory analysis of the collected samples revealed that excessive water saturation could result in cracks during freeze-thaw cycles, while a low resistance coefficient would lead to softening during humid conditions. An Intelligent Geographic Information System (IGIS) will incorporate the collected data alongside open remote sensing and weather data. The objective is to utilize this integrated system in the future for forecasting road conditions and maintenance requirements for similar roads in mountainous regions of the area. The measurements are carried out according to the state standards. That makes the system useful for government management purposes.

Analysis of microstructure of prefabricated fractured granite under thermal effects based on CT technology

In this study, Computed Tomography (CT) technology was employed to visually analyze the influence of thermal effects on the microstructure of prefabricated fractured granite. The porosity and heterogeneity of the samples were calculated using three-dimensional reconstruction techniques, and two parameters, crack length and orientation, were introduced to quantitatively evaluate the impact of prefabricated fractures on subsequent crack propagation. The results indicated that at 400 °C–500 °C, numerous microcracks were generated within the samples, but they did not interconnect. Above 600 °C, microcracks gradually propagated, forming well-connected crack networks. Due to the uneven thermal stress distribution caused by the prefabricated fractures, samples subjected to temperatures above 600 °C exhibited higher heterogeneity in the vertical direction. While in other directions, the samples gradually tended to homogenize as the temperature rose. Below 500 °C, crack length was significantly affected by the prefabricated fractures, although this influence diminished with increasing distance from the fractures. When the temperature was below 400 °C, microcracks mainly propagated along the direction of the prefabricated fractures. However, when the temperature exceeded 500 °C, microcracks began to initiate and propagate in the opposite direction of the prefabricated fractures, and anti-wing cracks started to emerge.

Acoustic Emission Measurements During a Four-Point Bending Test on a Reinforced Concrete Specimen

Acoustic emission (AE) measurements were carried out during a four-point bending test to in-vestigate the influence of the shear load-bearing capacity of reinforced concrete beams (so-called Leonhardt beam) with shear reinforcement. The results of AE measurements show that the AE activity begins immediately after starting loading. During the test approximately 3.500 AE events could be automatically located using longitudinal and transverse wave arrival times. Most AE events are in the central region of the specimen within the sensor network. Therefore, only the flexural tensile cracks are represented through AE locations. The transverse shear crack that occurred during the ultimate failure of the specimen is outside the sensor arrangement and has not been detected. Due to the localization error, the AE events are distributed in a cloud-like manner, and as a result, they do not sharply depict the macroscopic flexural tensile cracks. Ra-ther, acoustic emission indicates that a crack network of many small microscopic cracks formed in the tensile area, resulting in a very rough and fragmented crack morphology. Additional to the conventional source location, we applied the so-called collapsing method to highlight structures already inherent within the unfocused AE events clouds.

Insights into Structural Changes During Scratch Deformation of Ductile Coating Systems: Plastic Flow, Microstructural Transformations, and Crack Development

This study investigates the structural changes occurring during the scratch deformation of ductile materials, focusing on a composite aluminum (Al) coating on a steel substrate fabricated using a solid-state aluminizing technique. By analyzing the results of the scratch test with transmission electron microscopy (TEM) and focused ion beam (FIB) techniques, the research provides a detailed examination of plastic flow, microstructural transformations, and crack development. Results demonstrate that strong adhesion at the interface and similar mechanical properties between steel and Al layers facilitate co-deformation, crucial for maintaining the integrity of the composite structure. The study features the formation of a rolling-like laminated structure and significant grain refinement in both the steel and Al components, driven by plastic deformation. Observed atomic-scale intermixing and the formation of an amorphous interlayer highlight extensive material interactions during deformation. The visualization of crack progression using FIB allows for an in-depth understanding of the fracture process. Subsurface cracks initiate within the thin Al interlayer and propagate toward the surface, evolving through the coalescence of nanopores into microvoids and larger cavities, ultimately leading to crack propagation. Secondary internal cracks may arise due to stress fields generated by the primary crack, contributing to a complex network of internal cracks.

Study of Structural Defects Evolution in Fine-Grained Concrete Using Computed Tomography Methods

Introduction. When studying composite materials for construction purposes, it is needed to consider the mechanisms of formation of the structure and properties of modern concretes in the process of strength development. In studies of modern composite materials based on cement binder, there is no information about the development of structural defects and destruction of the material at the initial stages of strength development. This information can be obtained using X-ray computed tomography, a promising method of nondestructive testing of the state of the material. Therefore, the objective of this work was to study the formation and propagation of cracks in samples of fine-grained concrete with different fractional composition of sand due to natural processes of cement shrinkage, as well as the mechanics of destruction of samples of modified fine-grained concrete when applying a compressive load at the early stages of strength development. Materials and Methods. The study used fine-grained concrete mixtures of three compositions with different sand gradation. The tomography samples were made by placing fresh mixtures in polymer cylindrical containers. Tomography of the samples immediately after manufacture, as well as after 8 and 51 days, was performed in a YXLON Cheetah microfocus X-ray machine. The composition with two-fraction sand was modified by mechanical activation of the components, 20×20×20 mm cube samples were made. Further, compression tests were performed at the Instron installation after 3 and 7.5 hours, and then — tomography of the destroyed samples.Results. It was established that the destruction of contact zones depended on the ratio of the size of the fractions. In the presence of a bulk of coarse sand grains in concrete, the destruction of contact zones was more pronounced and had a main mode. When using fine or polyfraction sand, contact zones were destroyed locally and had a visually smaller area. The images of the destroyed modified sample, tested 3 hours after manufacturing, showed clear cracks and indents on the edges, which indicated the elastic-plastic nature of the destruction. In 7.5 hours, the edges of the sample upon destruction were covered with a network of small cracks; inside the sample there were also numerous cracks and microcracks, which indicated brittle fracture. Based on the obtained images of the deformed structure of modified concrete, the mechanism of transition from elastic-plastic destruction of the material to brittle one was clearly visible.Discussion and Conclusion. The studied dependences of the influence of the size of fine aggregate on the mechanisms of formation and propagation of structural defects contribute to the theory of the processes of destruction of fine-grained concretes. The results obtained prove the prospects of using X-ray computed tomography as a method of nondestructive testing of the internal structure of fine-grained concrete, including at the early stages of strength development.

Anisotropic hydraulic conductivity of as-compacted, bare and vegetated soils

Soils are generally considered anisotropic with respect to hydraulic conductivity, while the evolution of the anisotropy condition is unknown for bare and vegetated soils. Therefore, the main goal of this study is to compare the anisotropic hydraulic conductivity of as-compacted, bare and vegetated specimens. Accordingly, a series of 54 hydraulic conductivity tests was conducted in a custom-made cube triaxial permeameter. The as-compacted specimens were revealed as isotropic because the loosely packed preparation procedure resulted in a dominant flocculent structure. However, a five-fold increase in the anisotropy ratio of bare specimens was measured along the isotropic loading path because of the induced surficial degradation zone formed by irrigation and desiccation processes, as evident in preliminary observations and crack network analysis. The variations in anisotropy ratio compared against void ratio function of vegetated soil generally fall below the corresponding function of the bare soil. The function was revealed to have a crossed nature, varying from sub-isotropic to super-isotropic states, corresponding to the lower and upper bounds of 0·3 and 3, respectively. It was postulated that vegetation impacts the flow differently by reducing the potential of desiccation cracks, creating preferential flow through the propagation of primary roots and clogging flow channels by secondary roots.

Improved 3D characterization of in-situ soil desiccation cracking by multi-source data integration

Desiccation cracking is a common and natural phenomenon under a drought climate. The geometric and morphologic characteristics of the crack pattern are critical to understanding the response of soil mechanical and hydraulic properties to drought climate. It is always a big challenge to obtain the refined geometric structure of the in-situ soil desiccation crack network. This study proposes an integrated method named ERT+ for determining the three-dimensional geometry of in-situ soil desiccation crack networks by integrating multi-source data from electrical resistivity tomography (ERT), surface image analysis, and depth investigation. The proposed method was applied to three in-situ expansive soil sites with different crack geometries and soil properties. The results showed that the joint application of ERT with other investigations reduced the ambiguity of interpreting each technique independently. The crack network model characterized the three-dimensional geometric structure of the desiccation crack accurately and quantitatively, verifying the effectiveness and feasibility of the ERT+ method. Field and laboratory experiments showed that heterogeneity in soil properties resulted in different cracking morphologies (width, depth, and width-depth ratio). The crack geometric data suggested that the width-depth ratio of the crack network was related to the cracking modes and the soil properties, indicating the non-homology of different crack networks. In addition, the correction gradients calculated from the ERT+ method also varied with the cracking modes and soil properties, further suggesting the reliability and prospect of the proposed method.

Evolution of the crack patterns in nanostructured films with subsequent wetting and drying cycles

Crack patterns in coatings present various morphologies as a signature of the matter to external stresses. Brittle films generally show a network of connected cracks due to a hierarchical formation process. On the contrary, non-sequential crack growth leads to a different morphology with few junctions. The present work focuses on the evolution of both crack networks under the effect of repeated stresses. The experimental work is performed through porous thin films over subsequent wetting and drying processes. The non-connected network of cracks is investigated through nanostructured films exhibiting compliant and elastic properties. Over repeated stresses, this crack network evolves until it reaches stabilization. The stabilization appears when the cracks stop growing and a shielding effect occurs. This behaviour is compared with a more classical connected network of cracks that do not evolve in the plane under the effect of repeated processes.

Study on thermal crack characteristics of granite in Shandong Province, China under different temperatures and heating/cooling treatments

The qualitative and quantitative analysis of crack initiation characteristics, extended procedure, and crack network morphology of rock under different temperatures and heating/cooling paths was conducted in order to delve deeper into the mechanism of thermal cracking, and the analytical perspectives mainly include microstructure, thermodynamics, and mineral crystal properties. Research results indicate that 200–300 °C and 600 °C are two obvious mutation points of water-cooled samples, while air-cooled samples show mutation points around 600 °C. The initiation positions of thermal cracking under all temperature paths are relatively similar, with intergranular crack located between particles at the edge of the sample or intercrystalline cracks in the middle of feldspar or quartz aggregates. Slow heating and rapid cooling contribute to the complexity of the thermal crack network, and rapid heating can form larger sized thermal cracks. The process of thermal cracking can be divided into three stages: initiation of small cracks, joint development of main cracks and small cracks, and interconnection of cracks to form a network structure. The thermal cracking mechanism is attributed to differences in expansion and thermal conductivity among various crystals.

Visualization of hydraulic fracturing in compacted bentonite: The roles of dry density, water content, and pressurization rate

Deep geological repository is typically situated at depths ranging from several hundred to 1000 m below ground, making bentonite engineered barrier potentially vulnerable to high water pressure and even inducing hydraulic fracturing. This study conducted injection tests on compacted GMZ (Gaomiaozi) bentonite with a self-developed visualization set-up. The objective was to unveil the roles of dry density, water content, and pressurization rate in hydraulic fracturing from the perspective of fracturing macro-morphological dynamics and breakthrough characteristics. Moreover, the relationships between breakthrough characteristics and microstructure were examined by MIP (mercury intrusion porosimetry) analysis. Results showed that the fracturing dynamics were characterized by three stages: hydration, cracking, and fracturing stages. Compared to water content and pressurization rate, dry density exerted more pronounced effects on these stages. Increasing dry density can lead to an expansion of circular hydration zone, a more complex cracking network, and a change in fracturing patterns from long and clear to short and fuzzy. In terms of breakthrough characteristics, the breakthrough pressure was positively correlated with dry density and negatively correlated with water content. Interestingly, there is a good and unique logarithmic correlation between the breakthrough pressure and the ratio eM/em of inter-aggregate void ratio and intra-aggregate void ratio, regardless of dry density and water content. Within a certain range (i.e. 200-50 kPa/min), breakthrough pressure showed slight dependency on pressurization rate. Nevertheless, an extremely low pressurization rate of 20 kPa/min caused a transition for the specimen from quasi-brittle to plastic state owning to more water infiltration, thereby hindering fracture initiation and propagation.

Insight into the initiation and propagation mechanism of desiccation cracking in clayey soil from DEM simulations

Desiccation cracking, a common natural phenomenon, significantly affects the mechanical and hydraulic properties of clayey soils. This study employs 3D Discrete Element Method (DEM) to investigate the initiation and propagation mechanisms of soil desiccation cracking. By integrating water evaporation at the soil surface and water transportation between soil grains into a traditional DEM model, we unveil a sequential and hierarchical pattern in the development of desiccation cracks. Initially, micro cracks proliferate and coalesce in regions of soil defects characterized by the lowest coordination numbers, leading to the genesis of primary cracks. Subsequently, a complex network of cracks propagates, governed by the interplay between cracking and the redistribution of the tensile stress field. As desiccation advances, the detachment of soil from the bottom wall and an increase in soil strength contribute to the stabilization of the crack network. Notably, the hierarchical nature of desiccation cracking is predominantly shaped by initial soil defects and subsequently refined by stress concentration within the soil clods, correlating with changes in the length-to-height ratio of the soil clods.

Effects of vetiver root on cracking of expansive soils and its mechanistic analysis

The study investigated the reinforcing effect of vetiver root on soil by conducting outdoor planting tests and indoor root tests. The cracking indexes of soil specimens with varying root contents were analyzed, and a statistical model was established to determine the relationship between the cracking indexes, the number of dry and wet cycles, and the root content. The study revealed the crack evolution law of vetiver-reinforced expansive soil. The study explored the mechanism of the vegetation root in inhibiting the cracking of expansive soil and determined the optimal planting density of vetiver grass through outdoor planting tests. The results indicate that: The surface crack rate (CR), total crack length (CL), and crack number (CN) in the root-soil specimen exhibited exponential growth with an increase in the number of wet and dry cycles. This growth was more pronounced during the first and second cycles. The vetiver root could effectively reduce soil crack formation, and the specimen's cracking resistance is positively correlated with the root content. With the root content increased, the CR, CN, and CL decreased. The logistic model is suited to the CL of added root soil. The logistic model is more suitable for the growth model of the CR of the expansive soil with low root content, while the Boltzmann model is more suitable for the growth model of the CR of the expansive soil with high root content. Width of crack (CW) is better suited to the DoseResp growth model. The Boltzmann model is more applicable to the CN in expansive soils with low reinforcement, while the logistic growth model is more suitable for the development of CN above 0.21% root content. The development of the crack network was influenced by two key factors: the root content and the number of wet and dry cycles. Under the condition of planting roots, the development of crack networks in expansive soil differs from that of expansive soil with added roots, and there is no clear pattern to follow. The inhibitory effect of the vetiver root on cracking of expansive soil is related to the planting density of vetiver.

3D in situ observation of the capillary infiltration of molten silicon in a SiC/SiC composite by X-ray tomography

The Liquid Silicon Infiltration (LSI) process is used to decrease residual porosity of SiC/SiC composite materials. However, it is not fully mastered since the mechanisms involved at 1500 °C under high vacuum are complex to analyze, especially without direct observation. Previous work had demonstrated the feasibility of using X-ray radiography to observe the front rise of silicon in a SiC/SiC composite material during the LSI process. 2D observations, as a first approach, could give a basic understanding of the mechanisms but raised several interrogations due to the superposition of these phenomena along the thickness preventing any quantification. The setup has been improved in order to make X-ray tomography using a fully integrated DC motor in place of the rotation stage commonly used. The sets of X-ray tomographs confirm two successive fillings. First, the molten silicon rapidly and non uniformly invades the accessible intergranular micro porosity of the powder. Then, the liquid slowly fills the remaining isolated powder areas. Once the SiC matrix is fully saturated, the liquid fills the bigger porosities such as cracks and intra yarn macro porosities. In addition, this 3D analysis enabled to give a better comprehension of the non uniform wetting front in the SiC matrix powder. During the 1st step, the accessibility of the powder was known to have a major effect on the speed of progression. Also, the cracks network plays a key role in the filling of the isolated areas in the powder matrix.

Research on simulating coal crack development under high voltage electric pulse stress waves using zero thickness cohesive elements

High voltage electric pulse (HVEP) technology has demonstrated significant potential in enhancing coal seam permeability. However, there is a notable gap in numerical simulation studies that investigate the cracks development patterns in coal under the influence of this technology. In this study, a HVEP coal rock fracturing system was utilized to conduct a physical simulation experiment of coal sample electrical fragmentation. A finite-discrete element method (FDEM) model, incorporating zero-thickness cohesive elements, was developed to simulate the process of stress wave-induced crack propagation in coal, triggered by the plasma channel. The results indicate that over time, the stress wave typically exhibits an initial rapid rise followed by an oscillatory decline. The peak value of the stress wave decreases significantly with distance, stabilizing beyond a propagation distance of 10 mm. Furthermore, the fracturing efficacy of coal by HVEP stress waves is closely linked to specific characteristic parameters: within the range of 75–125 MPa, a positive correlation exists between the stress wave peak and the complexity of the coal crack network. Reducing the stress wave loading rate can increase the crack area to some extent, although excessively slow loading rates may lead to excessive energy dissipation during propagation, which is detrimental to crack expansion. Conversely, when the ratio of β to α ranges from 1 to 100, decelerating the attenuation rate of the stress wave aids in generating stress concentration within the crack zone, thereby facilitating crack propagation. The established numerical model aims to advance understanding of HVEP’s fracturing mechanisms and enhance its application in coalbed methane extraction.

A regularized variational mechanics theory for modeling the evolution of brittle crack networks in composite materials with sharp interfaces

In the design of structural materials, there is traditionally a tradeoff between achieving high strength and achieving high toughness. Nature offers creative solutions to this problem in the form of structural biomaterials (SBs), intelligent arrangements of mineral and organic phases which possess greater strength and toughness than the constituents. The micro-architecture of SBs like nacre and sea sponge spicules are characterized by weak organic interfaces between brittle mineral phases. To better understand the toughening mechanisms in SBs requires simulation techniques which can resolve arbitrary interface and bulk fracture patterns.In this work, we present a modified regularization of Variational Fracture Theory (VFT) that allows for simulation of fracture in materials and structures with weak interfaces. The core of our approach is to widen the weak interfaces on a length scale proportional to that of the diffuse damage field, and assign a reduced fracture toughness therein. We show that in 2D the modified regularized functionals Γ-converge to that for sharp cracks. The resulting thin weak interfaces have fracture toughness which depends on the bulk material fracture toughness, the widened interface fracture toughness, and the ratio of the widened interface length scale to the crack regularization length scale. We next apply our modified regularization within a computer implementation of regularized VFT, which we term RVFTI. We assess the performance of RVFTI in 2D by reproducing the effective interface fracture toughness predicted by the Γ-convergence theory and simulating crack trapping at a bi-material interface. We then use RVFTI to study toughening in SB-inspired microarchitectures, namely layered materials and materials with wavy interfaces.

Thermal shock resistance of additively manufactured alumina

AbstractThe mechanical behavior and cracking patterns of thermally‐shocked additively manufactured alumina were investigated. The flexural strength of test specimens that had been heated to temperatures ranging from 200°C to 1000°C and then rapidly quenched in water was determined at ambient temperature by four‐point bending. Results indicated that the surface cracking patterns had a multifractal structure and that an increase in the thermal shock temperature led to an increase in the density and uniformity of the crack network. The flexural strength results were analyzed with Weibull statistics, where the Weibull moduli for most of the thermal shock conditions tested were found to be statistically indistinguishable. It was also found that a significant decrease (∼50%) in flexural strength occurred for heating temperatures ≥300°C. The effect of the manufacturing method on cracking patterns is discussed, as well as the implication of the material behavior for practical applications of these materials.

Mechanical and failure characteristics of flawed granite after high-temperature cycling: An experimental study using biaxial compression

Unveiling the mechanical and failure characteristics of flawed granite subjected to high-temperature (HT) cycles is essential for assessing the stability of underground engineering. There is limited advanced knowledge of the rock performance under HT cycles even today. To that end, biaxial compression experiments with the simultaneous acoustic emission (AE)-digital image correlation (DIC) monitoring are conducted on flawed granite containing the flaw pairs, with the emphasis on the influences of HT cycles on the mechanical and failure characteristics of flawed granite. Experimental results suggest that the rock strength is increased at 400 °C, regardless of the cycle number, while it is weakened at 600 °C, especially under the thermal cycles. By the coupling analysis of acoustic-optical-mechanical data, the levels of cracking in flawed granite are revealed as the process zone nucleation and quasi-static growth, along with the macrocrack initiation, propagation, and coalescence until the eventual failure, dictated by the stress shielding effect and stress amplification effect. The splitting crack, hook crack, and far-field network crack are reported for the first time. Moreover, the flawed granite at different temperatures transitions from tensile-dominated cracks to mixed tensile-shear (T-S) cracks. Besides, thermal cycling causes rock fatigue damage, and the alternating thermal stress generated by HT cycling promotes the initiation and propagation of microcracks, leading to the development and further intensification of thermally induced fracture.

Numerical analysis of stresses on angular contact ball bearing under the static loading with respect to race thickness and housing stiffness

A 3-dimensional model of the angular contact ball bearing (ACBB) was modeled using Abaqus/standard (Dassault systems- version 2017) to investigate the influence of race thickness on the bearing performance. It was found that the ability to support higher contact stress increased with race thickness. However, large deformations were found to occur on outer race with thickness of 3.3 mm and only small deformations were observed on outer race with a thickness of 9.9 mm. The large deformations induce higher shear stresses on thin races than on thick races. These stresses cause spall growth in bearings and propagate into a network of cracks. As a result of these findings, thin races are prone to failure compared with thick races.