Cracking Characteristics Research Articles

Advanced Parallel Laser Line-Camera System for Automatic and Precise Crack Width Measurement

Accurate quantification of cracks is crucial in evaluating building conditions. However, current non-contact measurement technologies encounter challenges in measuring crack widths, particularly in hard-to-reach areas. This paper presents a novel portable parallel laser line-camera system that is specifically designed for automatic and precise measurement of crack widths from different angles and over long distances. The system is composed of a digital camera, parallel line laser diodes, and rigid positioning arms, forming a triangulation-based configuration. A newly developed image processing algorithm is at the core of this innovation, allowing for the extraction of accurate pixel scale information from laser strips, thus overcoming the challenge of pixel distortion in non-perpendicular photography. The U-Net algorithm is utilized to efficiently identify crack regions, followed by the Canny algorithm for meticulous extraction of crack features. By utilizing the pixel scale data obtained from the laser strips, the actual width of cracks can be accurately determined. The system's efficacy and precision have been validated through laboratory and field tests. This automatic, precise, and portable solution significantly advances the field of on-site crack measurement, offering substantial potential for practical applications in building condition assessments.

Bridging Convolutional Neural Networks and Transformers for Efficient Crack Detection in Concrete Building Structures.

Detecting cracks in building structures is an essential practice that ensures safety, promotes longevity, and maintains the economic value of the built environment. In the past, machine learning (ML) and deep learning (DL) techniques have been used to enhance classification accuracy. However, the conventional CNN (convolutional neural network) methods incur high computational costs owing to their extensive number of trainable parameters and tend to extract only high-dimensional shallow features that may not comprehensively represent crack characteristics. We proposed a novel convolution and composite attention transformer network (CCTNet) model to address these issues. CCTNet enhances crack identification by processing more input pixels and combining convolution channel attention with window-based self-attention mechanisms. This dual approach aims to leverage the localized feature extraction capabilities of CNNs with the global contextual understanding afforded by self-attention mechanisms. Additionally, we applied an improved cross-attention module within CCTNet to increase the interaction and integration of features across adjacent windows. The performance of CCTNet on the Historical Building Crack2019, SDTNET2018, and proposed DS3 has a precision of 98.60%, 98.93%, and 99.33%, respectively. Furthermore, the training validation loss of the proposed model is close to zero. In addition, the AUC (area under the curve) is 0.99 and 0.98 for the Historical Building Crack2019 and SDTNET2018, respectively. CCTNet not only outperforms existing methodologies but also sets a new standard for the accurate, efficient, and reliable detection of cracks in building structures.

Bending Fatigue Properties of Ultra-High Toughness Cementitious Composite (UHTCC).

Ultra-High Toughness Cementitious Composite (UHTCC) represents a composite material meticulously engineered on the foundation of micromechanical principles. The multi-crack cracking and strain-hardening characteristics of UHTCC enable it to be applied to orthotropic steel decks to control the crack width. Different from most studies which only focus on hybrid fiber or fatigue characteristics, this paper studies the influence of hybrid fiber content on static mechanical properties, flexural toughness, and flexural fatigue characteristics of UHTCC under different stress levels. The compressive and flexural strength, bending toughness, and fatigue damage of UHTCC under different fiber ratios were compared, and the fatigue properties of hybrid fiber UHTCC were verified. The results reveal that hybrid fiber exerts a more pronounced effect on toughness, augmenting the maximum folding ratio by 23.7%. Single-doped steel fiber UHTCC exhibits a characteristic strain-softening phenomenon attributable to inadequate fiber content, whereas the bending toughness index of hybrid fiber UHTCC surpasses that of SF1.5P0 by 18.6%. Under low-stress conditions, UHTCC demonstrates a nearly threefold increase in bending fatigue life with a mere 1% steel fiber content, while the influence of polyvinyl alcohol (PVA) fiber on fatigue life is more significant: with an increase of only 1/5 volume content, the fatigue life increased by 29.8%, reaching a maximum increase of 43.2% at 1/4 volume content. Furthermore, the fatigue damage accumulation curve of UHTCC follows a three-stage inverted S-shaped trajectory. The inclusion of PVA fiber facilitates early initiation of stable cracking during the fatigue failure process, thereby advancing the entire strain stability development stage and mitigating external load forces through the proliferation of micro-cracks. Consequently, compared to SF1P0, the ε0 of SF1P5 experiences a significant increase, reaching 143.43%.

Investigation of damage and crack characteristics of epoxy modified recycled asphalt mixtures based on a three-dimensional meso-heterogeneous model

Epoxy asphalt has excellent fatigue resistance, making it a valuable addition to recycled asphalt mixtures. However, the lack of sufficient understanding of the cracking mechanism in epoxy-modified recycled asphalt mixtures (EMRAM) hinders its widespread application. The purpose of this paper was to investigate the damage and crack characteristics of EMRAM based on a three-dimensional meso-heterogeneous model. The coarse aggregates were first randomly generated using the secondary Delaunay algorithm. Meanwhile, the fracture parameters of each component material and asphalt-aggregate interface in EMRAM were tested through three-point bending tests and direct shear tests, respectively. Cohesive zone model elements were then inserted at the aggregates, asphalt mastic, and interfaces between different materials to establish a three-dimensional meso-heterogeneous model of EMRAM. Finally, based on the established model, the damage degree of each composition material in EMRAM was quantitatively analyzed to identify the vulnerable aspects of EMRAM during the crack propagation stage. Optimization strategies were proposed accordingly. The results demonstrated that the model established in this study can accurately represent the meso-heterogeneous of EMRAM and effectively simulate its cracking behavior. EMRAM was more susceptible to developing internal damage within the aggregates, followed by relatively noticeable damage at the interface and the internal regions epoxy-modified fine aggregate mastic. The cracking resistance of EMRAM showed a trend of initially increasing and then decreasing with an increase in the volume fraction of coarse aggregates with a size above 4.75 mm. The optimal range for the volume fraction of coarse aggregates in EMRAM was within 35–44 %. The fracture energy of EMRAM was improved by more than 30 % after gradation optimization compared to EMRAM with gradation at the median value of AC20. Within the designed service life, EMRAM durable pavement can save at least 30 % more in economic costs compared to ordinary asphalt concrete pavement.

Fatigue performance analysis of fine aggregate matrix using a newly designed experimental strategy and viscoelastic continuum damage theory

Fine aggregate matrix (FAM), as the matrix phase in asphalt concrete (AC), significantly affects the fatigue behavior of AC. To accurately assess the mechanical properties of FAM, a newly designed experimental strategy for FAM testing was developed, and the viscoelastic continuum damage theory (VECD) theory was applied to analyze FAM’s fatigue cracking characteristics. In this study, a dumbbell-shaped geometry for dynamic shear rheometer testing was designed and verified through the FE-aided method. Subsequently, three AC mixtures’ FAM specimens with this special geometry were fabricated for the frequency sweep and linear amplitude sweep tests. Results showed that the specially designed specimens effectively capture the viscoelastic and fatigue properties of FAM with high replicability. Analyses based on the VECD theory indicated that FAM of porous asphalt (FAM(PA13)), featuring a higher asphalt content, exhibits a significant reduction in pseudo stiffness with increasing damage at the initial stage, but the reduction rate diminishes as damage progresses when compared to the other two FAMs. It was speculated that the higher aggregate content in FAM of dense-graded AC mixture (FAM(AC20) induces stress concentrations in the asphalt mastic near the protrusion areas of aggregates, thereby rendering the sample more susceptible to damage. The proposed methods will be readily extended to characterize other mechanical properties of FAM.

Enhanced Video-Level Anomaly Feature Detection for Nuclear Power Plant Component Inspections Using the Latency Mechanism

The conditions of various nuclear power plant facilities are regularly examined through manual inspections. Remote visual inspection is commonly applied and requires engineers to watch lengthy inspection footage and seek anomaly features therein. This is a labor-intensive process as anomaly features of interest usually only appear in very short segments of the original whole video. Therefore, an automated anomaly detection system is preferred to lessen the intensive labor cost in the inspection process. The detection process could also benefit from useful information that could potentially contribute to addressing reasoning traceability. With a well-prepared training data set of the anomaly feature, a convolutional neural network (CNN) can be developed to automatically detect anomaly indications in the inspection video. However, false-positive detections may occur and can be difficult to remove without seeking manual verification. To overcome this problem, we present a new automated video-level anomaly detection framework that utilizes the latency mechanism to effectively lessen false-positive occurrences, and therefore, increase detection accuracy. In this framework, a CNN-based anomaly classifier first performs initial scanning of the anomaly type of interest in every region of the sampled frames. Then our latency mechanism is applied to refine the initial scanning results by flagging up a region as an “anomaly” indication only when “anomaly” is detected by CNN in the current frame and also in a sequence of previous consecutive frames of the same region. We present a case study of crack feature detection in superheater inspection videos to illustrate the performance of the proposed framework. The results show that the latency mechanism can effectively remove the original false-positive detections seen in the initial scanning. In order to provide a primary exploration of suggesting possible formats for addressing reasoning traceability, knowledge graphs of the reasoning process in the video-level detection framework are built to provide a better understanding of why a specific section of the video is flagged as anomaly contents by the video-level detection framework.

Failure Causes Analysis of Circumferential Cracking on Gathering Pipeline in M Oil and Gas Field

The gathering and transportation pipeline experienced corrosion cracking failure after 2 years of operation. This paper conducted an analysis on the reasons for the pipeline failure by integrating background information on its usage, as well as observations, analyses, and detection of the morphology of failure samples. The results indicated that the fracture originated from the inner wall of the pipeline and extended to the outer wall along the wall thickness until complete fracture occurred. Based on microstructure analysis of the fracture and original microcracks at the top of the pipeline, it was determined that the fracture was a multi-source brittle fracture, spreading in both inter-granular and trans-granular forms with obvious radial quasi-cleavage fractures accompanied by secondary cracks. EDS analysis revealed that the element S was present in all zones related to fracture initiation, spreading, and transient zones. XRD analysis showed that corrosion products on the fracture surface were mainly composed of FeS, indicating the presence of H2S in the service environment leading to sulfide stress corrosion cracking characteristics in line with pipeline failure. It is recommended to confirm the source of H2S in the service medium and test residual stress within the same pipeline for potential risk assessment regarding cracking in other areas.

Cracking characteristics and axial capacity degradation of precast segmental bridge piers connected with corroded tapered sleeve locking-type splicing

The tapered sleeve locking-type splicing (TSLTS) reinforcement joint is a new method of steel mechanical connection. When applying the TSLTS reinforcement joints to precast segmental bridge (PSB) piers in coastal environments, the durability need to be taken into account. After the TSLTS reinforcement joint is corroded, the connection performance of the TSLTS reinforcement joint is degraded. Therefore, the mechanical properties of PSB piers decrease due to the corrosion of the joints, which brings some potential hazards. In this paper, the durability of PSB piers with TSLTS reinforcement joints in coastal environments was studied. The corrosion-induced cracking characteristics, failure mode and axial capacity of PSB piers with TSLTS reinforcement joints were investigated by means of accelerated corrosion test and axial loading failure test. The results indicated that, with increasing corrosion time, the total length of the corrosion-induced cracks on the surface of the pier specimens increased, but the increasing rate of the total length decreased. The mass loss of TSLTS reinforcement joints increased linearly with the maximum width of longitudinal corrosion-induced cracks on the concrete cover of the self-compacting concrete (SCC) segment, and the critical mass loss of TSLTS reinforcement joints was approximately 4.0 % when longitudinal corrosion-induced cracks appeared. The probability distribution type of the width of longitudinal corrosion-induced cracks on the surface of pier specimens was mainly a lognormal distribution. When the mass loss of TSLTS reinforcement joints was less than 7.2 %, the damaged area of the pier specimens was mainly concentrated in the SCC segment, while when the mass loss of TSLTS reinforcement joints was higher than 8.8 %, the damaged area was located in the ordinary Portland concrete segment near the top of the pier specimens, and the corroded TSLTS reinforcement joints could still maintain their connection performance. With an increase in the corrosion degree of the steel bars, the axial capacity of the pier specimens decreased, and the equivalent stiffness also degraded. The degradation rate of the axial capacity of the pier specimens increased linearly with the mass loss of the TSLTS reinforcement joints, and the degradation of the axial capacity occurred only when the mass loss of the TSLTS reinforcement joints was greater than 4.6 %. The research results can provide a theoretical reference for the application of TSLTS reinforcement joints in PSB piers.

Study on crack resistance of basalt fiber reinforced asphalt mixture modified by titanate coupling agent based on digital image correlation

Basalt fiber is widely used in pavement engineering for its strong mechanical properties, while its smooth surface can cause it to slip within the asphalt mixture, weakening the overall structure. In addressing this issue, this study explores the application of a titanate coupling agent (TCA) for surface treatment of the fibers. The aim was to enhance the interfacial adhesion between the fiber and the asphalt, consequently improving the crack resistance of the fiber-asphalt mixture. This study delves into the modification mechanism and chemical grafting process of TCA, employing scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Furthermore, digital image correlation (DIC) was utilized to dynamically monitor the semicircular bending test (SCB) specimens under full loading conditions, facilitating a comprehensive evaluation of the damage evolution process. Through comparative analyses of load-displacement profiles, full-field strain, displacement, and crack development, alongside the characterization of the fracture process zone (FPZ), a multi-scale quantitative assessment was conducted on the cracking characteristics of the asphalt mixtures. Throughout the damage evolution process, the toughening and crack-blocking effects of the modified fibers were extensively evaluated. Concurrently, SEM was employed to observe the fiber distribution and microscopic morphology of the fracture surface, shedding light on the mechanism of action of TBF (Treated basalt fiber) from a microscopic perspective. The augmentation of TBFAM (Treated basalt fiber Asphalt Mixture) is systematically elucidated, demonstrating that, in comparison with the other two asphalt mixtures, TBFAM exhibits prolonged microcrack formation time and a slower rate of crack propagation. The strain at the crack initiation point increased by 1.45 %, with a concurrent 2.49 % increase in the high strain area, indicative of its superior crack resistance. TCA facilitates the generation of a biomimetic coating via chemical grafting, thereby enhancing the surface structure of basalt fibers and fortifying the fiber-asphalt interface bonding ability. The favorable surface structure of TBF fosters a three-dimensional network within the asphalt mixture system, offering functionalities such as bridging, adsorption, toughening, and crack prevention. This impedes crack development and further bolsters the stability of TBFAM.

Pixel-level concrete bridge crack detection using Convolutional Neural Networks, gabor filters, and attention mechanisms

Cracks are prominent defects that significantly impact concrete bridges' structural safety and serviceability assessments. The conventional manual bridge inspection is often limited by multiple challenges related to the heavy workload of data processing and the subjective judgment of inspectors. Several vision-based methods, from traditional image processing techniques to segmentation deep learning models, have been employed for pixel-level crack detection to detect cracks in various scenes and surfaces automatically. The present paper proposes an end-to-end framework for crack segmentation combining UNet, Gabor filters, and Convolutional Block Attention Modules leveraging the complementary strengths of both spatial and frequency domains to enhance crack feature extraction and mitigate the impact of image disturbances. The designed network is trained and evaluated on a multi-source annotated crack dataset established by the authors to expose the model to multiple instances of crack appearance and background complexity and enhance its generalization ability. The proposed model achieves an Intersection over Union (IoU) of 60.62 % and an F1-score of 74.49 %, outperforming benchmark segmentation networks. Experimental results demonstrate the effectiveness of cross-domain feature fusion and multi-source data utilization in segmenting cracks with diverse patterns and background interference scenarios.

Low energy impact damage characteristics of epoxy coating on the surface of sandwich composites

AbstractWhen composite honeycomb sandwich structures are subjected to low energy impacts, the upper panel or the honeycomb core often suffers large damages, which are invisible, but affects the structural safety. To achieve non‐destructive detection of these damages, an epoxy resin film (ERF) was designed as a functional coating to display the internal failure features on structure surface. The effect of low energy impacts on the damage modes of the ERF was first investigated, and then the correspondence between the ERF damage characteristics and the structural internal failure was established. Simulation and experimental results showed that with the increase of impact energy, cracks on the ERF expanded from the radial radiation with a small extension range to the circumferential pattern with an increased damage area. Meanwhile, localized delamination appeared inside the upper panel, while the degree of flexure and crushing damage to the core continued to increase. Based on the analysis of the crack extension pattern of the ERF and the damage state of the upper panel and honeycomb core, a specific link between the ERF crack characteristics and the internal damages to the sandwich structure was obtained, which might offer a novel non‐destructive testing method for composite impact damages.Highlights Low energy impact damage in honeycomb sandwich structures was a concern. Impact damage response of epoxy resin film on sandwich composites was studied. Crack extension of the film showed a particular change from radial to annular. Specific link between film crack and sandwich structure damage was established. The results would be helpful for damage detection in sandwich composites.

Multi-scale study of subsurface fatigue cracking behavior of laser-powder bed fused Inconel 718 at stress ratios and temperatures

Multi-scale subsurface fatigue cracking is a significant lifetime-limiting failure mode, not yet fully understood, of additively manufactured superalloys in mechanical applications. Here, combined with some technologies of electron-backscattered diffraction, focused Ion beam, and transmission electron microscope, a series of axial fatigue tests at different stress ratios and temperatures are conducted to investigate the fatigue failure behavior of as-built superalloy manufactured by Laser-Powder Bed Fusion. The main results show that especially in the assistance of defect (e.g., pore or inclusion) and elevated temperature, the faceted cracking related to the grain feature is a typical failure mode. It is mainly controlled by shear stress and is more popular under a positive stress ratio. Based on the analysis of dislocation structure with faceted cracking, the localized plastic deformation at 25 °C is controlled by anti-phase boundary (APB) shearing, whereas that at 650 °C is caused by a combination of APB shearing, precipitation bypassing, and stacking fault shearing mechanisms. Combined with the definition of threshold value at the crack tip, a crack nucleation life prediction approach associated with the faceted cracking characteristics is proposed, and the predicted results show a good agreement with the experimental results.

A Study on the Time-Frequency Characteristics of Electromagnetic Radiation Signals during Concrete Cracking

At the moment of rupture, brittle materials such as concrete, coal, and rocks experience a sudden stress change that generates oscillating positive and negative charges on the crack surfaces, resulting in the emission of electromagnetic radiation (EMR) signals. While extensive investigations have focused on the EMR signals produced during the fracturing of coal and rocks, limited attention has been given to the signals arising from concrete cracking. Previous studies have predominantly concentrated on amplitude or frequency domain analysis, lacking a comprehensive exploration of the time-frequency domain characteristics of electromagnetic signals. In this study, various magnetic field sensors capable of collecting frequencies ranging from 50Hz to 50kHz are developed. A laboratory testing platform for concrete cracking is established with the utilization of data acquisition equipment and a rock point load tester. The EMR signals are subjected to time-frequency domain analysis using Short-Time Fourier Transform (STFT) methods. The study explores the relationship between the time-frequency domain characteristics of EMR signals and concrete dimensions, as well as the instantaneous stress drop at the moment of fracture. Experimental findings reveal that the dominant frequency of EMR signals generated during concrete cracking is in the low-frequency range. As the instantaneous stress drop at the moment of fracture increases, the frequency of the EMR signals shifts towards higher frequencies and the signals around 10kHz has a longer duration. When the frequency of EMR signals is lower than 20kHz, larger concrete dimensions result in greater stress drops at the moment of fracture, leading to higher intensity EMR signals. Utilizing the EMR characteristics of concrete cracking holds promise for monitoring the safety and stability of concrete structures.

Macro‐meso damage cracking and energy dissipation of rock‐backfill composites: Effect of cyclic disturbance frequency

AbstractThis work aims to reveal the effect of cyclic disturbance frequency, i.e., 0.01, 0.1, 0.5, and 1.0 Hz, on damage cracking and energy dissipation characteristics of rock‐backfill composite structure specimens. Testing results show that fatigue volumetric strain and secant modulus increase with increasing disturbance frequency; low‐frequency disturbance leads to an earlier volume expansion. In addition, the strain energy density increases with increasing disturbance frequency. Most of the energy are dissipated for the specimen under high‐frequency disturbance. Finally, computed tomography scanning reveals the influence of disturbance frequency on failure modes; tensile‐splitting failure and shear failure are likely to occur at the surrounding rock subjected to high‐ and low‐ frequency disturbances. Low‐frequency‐disturbed stress seems easily to stimulate cracks at the rock‐backfill interfaces and the backfill; the backfill can absorb much more energy than high‐frequency circumstances. It is suggested that the role of backfill in energy absorption, contact support, and stress isolation is disturbance frequency dependent.

Influence of crack and pore structure characteristics on the thermal protective performance of thermal barrier coatings based on LBM

Influence of crack and pore structure characteristics on the thermal protective performance of thermal barrier coatings based on LBM

Reinforcement effects on the tensile properties of seawater sea-sand engineered cementitious composites reinforced with multi-scale hybrid fibers

The preparation of seawater sea-sand engineered cementitious composites (SS-ECC) holds great potential for development in coastal and marine infrastructure. To improve its compressive strength and tensile properties, this study investigated the crack characteristics, micro-mechanics, fibers reinforcement index, and micro-structure of SS-ECC using a multi-scale hybrid fibers system consisting of CaCO3 whiskers, PE fibers, and stainless steel fibers (SSF). The aim was to gain an in-depth understanding of the reinforcement effects of multi-scale hybrid fibers. The results showed that CaCO3 whiskers increased the number of cracks and the stability of crack evolution in SS-ECC during the ultimate tensile state by bridging the matrix micro-cracks, improving the matrix micro-structure, and filling the interfacial transition zones between the macroscopic fibers (PE fibers and SSF) and the matrix. Additionally, it reduced the crack width and spacing. These combined effects increased the compressive strength, tensile strength, and tensile strain capacity of SS-ECC by 12.8%, 15.7%, and 11.9%, respectively. Finally, the reinforcement effects of multi-scale hybrid fibers were analyzed based on the pseudo-strain hardening performance indices, fibers reinforcement index, and hybrid coefficient. This analysis provides reference and insight for the design of multi-scale hybrid fibers SS-ECC with a high comprehensive performance (σcσtε/w) index.

Cyclic loading and unloading strain equations and damage evolution of gypsum specimens considering damping effects

This study aims to establish a strain instanton equation and damage factor evolution law for gypsum specimens by considering damping. First, damping energy is calculated based on the single-degree-of-freedom vibration model, and the instantaneous strain equation is obtained based on the stress balance equation. Second, the dissipation energy is divided into damping and damage energies, and a damage-factor correction algorithm is obtained. Third, cyclic loading and unloading tests were performed at different loading rates and stress amplitudes to verify the accuracy of the strain equation. Finally, the specimens’ magnitude curves and crack characteristics were monitored using moment–tensor acoustic emission simulations. The factors influencing the damping energy and strain equations, energy and damage evolution laws of the specimens, and damage patterns of the specimens at different loading rates were analysed. The results show that the instantaneous strain equation and the modified damage factor considering the damping effect can effectively reflect the deformation law and damage state of the specimens. In contrast, the damage to the specimens in the lower limit of the variable stress experiment was lower than that in the lower limit of the constant stress experiment. As the loading rate increases, the damage energy density of the specimen decreases, and the damage factor within a single cycle gradually decreases. As the loading rate increases, the number of crack events in the model increases significantly, size becomes more uniform, and sequentially exhibits dense and sparse distribution patterns, percentage of shear cracks decreases significantly, number of mixed cracks increases significantly, brittle behaviour of the specimen becomes obvious, and a complete damage state is attained known as the ‘crushed’ state. This study provides a theoretical reference for damage assessments of viscoelastic–plastic materials subjected to perturbing loads.

Pavement Crack Detection Based on the Improved Swin-Unet Model

Accurate pavement surface crack detection is crucial for analyzing pavement survey data and the development of maintenance strategies. On the basis of Swin-Unet, this study develops the improved Swin-Unet (iSwin-Unet) model with the developed skip attention module and the residual Swin Transformer block. Based on the channel attention mechanism, the pavement crack region can be better captured while the crack feature channels can be assigned more weights. Taking advantage of the developed residual Swin Transformer block, the encoder architecture can globally model the pavement crack feature. Meanwhile, the crack feature information can be efficiently exchanged. To verify the pavement crack detection performance of the proposed model, we compare the training performance and visualization results with the other three models, which are Swin-Unet, Swin Transformer, and Unet, respectively. Three public benchmarks (CFD, Crack500, and CrackSC) have been adopted for the purpose of training, validation, and testing. Based on the test results, it can be found that the developed iSwin-Unet achieves a significant increase in mF1 score, mPrecision, and mRecall compared to the existing models, thereby establishing its efficacy in pavement crack detection and underlining its significant advancements over current methodologies.

Верификация структурно-тектонического строения массива карьера «Нежданинское»

The article analyses the studies of factors affecting bench stability in the Nezhdaninskoye open-pit mine. It is determined that the fundamental factors are the vector location and characteristics of faults and cracks in the rock massif. The following three main systems of extended block-forming fractures were defined: (1) subvertical fractures with strike azimuth A = 290°; (2) fractures with strike azimuth A = 50° and dip angle into the massif P = 70°; (3) fractures with strike azimuth A = 50° and dip angle into the pit P = 40°. Basically, the extended surfaces of the rock massif discontinuities lie at an angle of 40-50° towards the mined-out space of the open pit. Individual extended fractures have the angle of 60-65°. The most probable fracture systems involved in bench collapse in case of unfavourable combination of the slope direction and the fracture system's dip are traced in the south-eastern walls of the open pits. There are no unfavourable surfaces in the northern and north-western walls. At the same time, faults running parallel to the benches create potential collapse zones, which requires additional measures to ensure their stability. The structural and tectonic framework of the rock massif has a significant impact on the formation and stability of the open-pit benches. Taking these factors into account when planning and developing open-pit mines will help ensure occupational safety and high efficiency of the mining process.

Experimental Study on the Strength and Damage Characteristics of Cement–Fly Ash–Slag–Gangue Cemented Backfill

In order to achieve the goal of effectively utilizing solid waste resources and improving mining stability, it is necessary to incorporate various types of solid wastes in the production of cemented backfill. For investigating the compressive strength and damage characteristics of Cement–Fly Ash–Slag–Gangue (CFSG) cemented backfill under loading, real-time X-ray Computed Tomography (CT) scanning was employed to capture two-dimensional (2D) grayscale slices and three-dimensional (3D) fracture models during uniaxial compression testing. The study quantitatively assessed the evolution of cracks and microstructural damage in CFSG cemented backfill. The results indicate that the specimens underwent four stages of transformation, including compaction, linear elasticity, yielding, and residual deformation, during the uniaxial compression process. The specimens exhibited a measured compressive strength of 3.44 MPa and a failure strain of 0.95%. As the axial strain increased, there was an increase in 2D porosity observed in the CT images and a greater dispersion of crack distribution. A 3D model constructed from CT slices illustrated the feature of cracking expansion, with the fracture volume gradually increasing during the elastic deformation phase and experiencing rapid growth during the yielding and residual deformation phases. The damage variable, obtained from the volume of 3D cracks, exhibited a slow-growth pattern, characterized by a rapid increase followed by a more gradual rise with the increase in axial strain. This study serves as a significant reference for comprehending the micro-mechanisms involved in the damage process and cracking characteristics of cemented backfill mixed with solid wastes under external loading conditions.