Crack Damage Evolution Research Articles

A deep learning informed-mesoscale cohesive numerical model for investigating the mechanical behavior of shield tunnels with crack damage

The present deep learning (DL)-based detection of crack damage of shield tunnels through images can only provide part of geometric information of the crack damage, which prevents the comprehensive understanding of the mechanical behavior of the cracked tunnel linings. This study bridges the gap between the geometric information identified by DL models and the transformation law of mechanical behavior of tunnel linings revealed by numerical methods. A mask-region-based hybrid attention convolutional neural network (Mask R-HACNN) and mesoscale cohesive numerical model are developed to obtain the geometric information of crack damage and investigate the structural response of segments and joints with crack damage, respectively. The numerical simulation results demonstrate that the relationship between in-depth crack length and crack opening displacement and the relationship between the degradation stiffness and in-depth crack length are independent of the loading path. Based on these findings, prediction formulas are established to interpret the relationships between the stiffness degradation coefficients, flexural stiffness, in-depth crack length, joint openings, and crack opening displacement. Then, the geometric information of crack damage obtained by the Mask R-HACNN is provided to the mesoscale cohesive numerical model to estimate the in-depth crack length, flexural stiffness, and stiffness degradation coefficient of cracked segments. Results show that the deep learning informed-mesoscale cohesive numerical model is satisfying in understanding the mechanical behavior of segments and joints during crack damage evolution.

Bridge crack segmentation and measurement based on SOLOv2 segmentation model

With the continuous increase of vehicular traffic, the safety caused by bridge crack damage is becoming increasingly prominent. Bridge crack analysis and measurement are of great significance for promoting road traffic safety. However, existing bridge crack image segmentation methods have shortcomings in processing image detail features, resulting in the inability to better measure the actual size of bridge cracks. Therefore, to further optimize the calculation method, a bridge crack image segmentation method based on improved SOLOv2 is designed to achieve more accurate bridge image segmentation. Based on the image segmentation results and combined with the skeleton data extraction method, a bridge crack calculation method is designed. From the results, the segmentation accuracy for crack images was 92.05 % and 93.57 %, respectively. The average mIoU of AM-SOLOv2 method was 0.75, significantly lower than commonly used crack image segmentation methods. In addition, the mIoU value variation amplitude of the AM-SOLOv2 method was relatively smaller. The crack length and width errors were within 0.05 mm and 0.06 mm, significantly lower than the comparison method. It indicates that this method can achieve more accurate crack image segmentation and calculation. This is beneficial for a deeper understanding of the performance degradation and crack damage evolution of bridge structures, thereby improving bridge design and construction technology.

Multiscale fracture, physical and mechanical properties of stromboli volcano (Italy) edifice

The physical, mechanical and fracture properties at Stromboli volcano have been integrated at multiple scales to understand whether the interplay between a presumed NE/SW rift zone and the Sciara del Fuoco (SDF) depression has resulted in a zone of weakness able to promote fracturing prone to flank instability. Multiscale fracture quantification by imaging via FracPaQ toolbox both fractures and sample scale fractures has been integrated with rock physics and rock mechanics experiments on cm-scale samples belonging to the Paleostromboli, Vancori, Neostromboli, Pizzo and Present Deposit volcanic cycles that have been taken from within and outside the rift zone. The structural changes to the edifice have been quantitively assessed by mapping at different scale fracture properties such density and orientation within and outside the rift zone allowing to identify the potential damaged zones that could reduce the edifice strength.Results indicate that basalt textures, microfracture density, porosity, chemical zoning and preferential alignments, despite lithologically dependent, can be related to the NE/SW zone of weakness at the regional scale and to collapsed volumes that have been subject to continuous intrusive activity. Numerical inversion models have been performed to cross correlate fracture density in the basalts at multiple scales.A link between microfracture density and seismic velocities has been also established via numerical modelling, allowing to interpret in terms of degree of fracturing the results of seismic tomographies at the field scale, providing a novel method to image crack damage evolution within the inner structure of the volcano edifice.

Bending-damaged fracture criterion and numerical implementation of limestone beam under three-point bending test

To study the characteristics of tensile cracking by rock-bending damage, the bending stress–strain curves and the cumulative ringing counts were obtained by a three-point bending test and acoustic emission (AE) monitoring of limestone beams. Based on the Lemaitre strain equivalent principle and the continuous damage theory, the bending damage variable D was defined by the AE cumulative ringing counts, and the bending crack damage evolution equation was established according to the Weibull distribution of the rock element strength. To realize the numerical test of the bending deformation, the damage variable D was used as the intermediate variable, and the specific process of damage bending stress transformation and the realization of the tensile crack criterion were secondary developed by FISH language in FLAC3D. According to the results, the central part of rock beam deforms downward under the action of bending stress, and both ends tilt up. Therefore, the bending failure begins with the tensile crack at the bottom of the rock beam and gradually extends to the compression zone. The maximum damage value is about 0.402 before the peak stress. The compressive stress in the x direction increases from the neutral layer to the top of the rock beam, and the tensile stress in the x direction increases from the neutral layer to the bottom of the rock beam. The maximum tensile stress is distributed in the center of the bottom of the specimen, where the bending effect is obvious. The stress–time curve was divided into the (Ⅰ) compaction stage, (Ⅱ) expansion stage, and (Ⅲ) penetration stage, accordingly, and the evolution of damage equation was divided into three stages: initial damage stage, slow damage stage, and accelerated damage stage. The curves of the experimental result, theoretical model prediction result, and numerical simulation result were in good agreement with each other, which indicates that the numerical simulation based on the criterion of rock damaged fracture can better reflect the bending process of rock beams under three-point bending stress.

On the fracture and energy characteristics of granite containing circular cavity under variable frequency-amplitude fatigue loads

The cavities left after engineering construction obviously affect the stability of underground rock mass. Extensive studies have been devoted to investigate the mechanical behaviors of cavity-contained rock under static or impact stress paths, however, the damage process and energy dissipation characteristics were not well understood. The presented studies aim to reveal the volumetric deformation and energy dissipation characteristics of granite samples with different cavity orientations under unconventional cyclic loading conditions. The applied stress path is characterized with variable frequency-amplitude sinusoidal loads. Testing results show that the intersection between the maximum principal stress and cavity orientation affects the strength, deformation, energy, and failure modes. Crack damage evolution and hole collapse were revealed using strain rate and energy rate. It is found that the dissipated energy rate gives an early instability warning than the volumetric strain rate.A series of CT images of the failed rock give new insights into the mesoscopic mechanism of hole spalling and crack coalescence. A transition from tensile failure to shear failure was found as the cavity orientation increases with respect to the principal stress.

Experimental investigation of rock–shotcrete interface damage under dynamic splitting using digital image correlation and acoustic emission

AbstractInterface failure is the main type of rock–concrete structure failure, which affects the stability and safety. To study the interface crack propagation and damage characteristics, this article carried out the rock–shotcrete cube interface splitting test under different loading rates. The rock–shotcrete specimens are composed of four material types. Based on the nondestructive monitoring technology, digital image correlation and acoustic emission (AE) were used to monitor the whole process of splitting failure. The results indicate that, both peak load and critical displacement exhibit significant strain rate effects. The main stage of interfacial microcrack development is between 0.8 Pmax and Pmax. And the damage evolution of cracks is more sufficient and stable under low loading rate. The splitting damage at the rock–shotcrete interface shows phased characteristics. When the load is near the peak, the surge of the AE signal indicates that the damage is intensified.

Research on the Evolution of Shield Segment Cracks Based on Acoustic Emission and CMOD.

In order to explore the cracking law and failure characteristics of segments, a model test of shield segment cracking was conducted. The microscopic and macroscopic crack evolution process of the segment is studied by using acoustic emission detection technology and crack opening displacement (CMOD). According to the acoustic emission signal and CMOD, characteristics generated in the process of segment cracking, in the form of numerical value, the evolution characteristics of each stage of segment cracking are directly reflected. Based on acoustic emission energy and CMOD, the segment cracking damage model was established to determine the segment fracture damage degree. The result shows that segment cracking can be divided into three stages, and the acoustic emission detection results and CMOD have different degrees of change in each cracking stage. This proves that both the acoustic emission acquisition results and CMOD can be used as evaluation indicators of damage degree. Acoustic emission can accurately identify the crack evolution process, and the yield strengthening is an important stage of crack damage evolution. The damage data points in this stage account for 76.83% of all the damage data points, the occurrence rate of damage data points is 0.225 s, and the density of data points in the damaged area is 3.219 × 10−4 mm3, which is larger than the other two stages. The segment cracking damage model can effectively reflect the segment cracking degree and provide a reference for the actual segment cracking assessment.

Crack damage evolution in concrete coarse aggregates under microwave-induced thermal stress

Crack damage evolution in concrete coarse aggregates under microwave-induced thermal stress

A new fatigue crack growth mechanism of high-strength steels

In this study, the fatigue crack damage evolution procedure of AISI 4340 high-strength steel was systematically studied via three-dimensional X-ray technology and scanning electron microscopy. Based on the experimental results and theoretical analysis, a new FCG mechanism is proposed, i.e. the blunting and re-sharpening to micro-scale void coalescence (BRS-MVC) mixed mechanism. Few micro-scale voids initiated near the crack tip are surrounded by the blunting crack tip at low ΔK zone, which is controlled by the BRS mechanism. More micro-scale void clusters would occur around the crack tip at high ΔK zone, which may be attributed to the MVC mechanism. The new FCG mechanism provides a unique perspective for the FCG behavior of high-strength steels, which could promote the fatigue life prediction of metallic materials.

A New Fatigue Crack Growth Mechanism of High-Strength Steels

In this study, the fatigue crack damage evolution procedure of AISI 4340 high-strength steel was systematically studied in quasi-in-situ via three-dimensional X-ray technology and scanning electron microscopy. It is found that the surface roughness and uniformity of crack growth in high-toughness steel are higher than those in low-toughness steel. The crack bifurcation and crack closure effect would both prolong the effective crack length; accordingly, the effective fatigue crack growth (FCG) rate decreases due to above crack growth characteristics in high-toughness steel. Based on the experimental results and theoretical analysis, a new FCG mechanism is proposed, i.e. the blunting and re-sharpening to micro-scale void coalescence (BRS-MVC) mixed mechanism. Few micro-scale voids initiated near the crack tip would be surrounded by the blunting crack tip at low Δ K zone, which is controlled by the BRS mechanism. More micro-scale void clusters would occur around the crack tip at high Δ K zone, which may be attributed to the MVC mechanism. The new FCG mechanism provides a unique perspective for the FCG behavior of high-strength steels, which could promote the fatigue life prediction of metallic materials.

Characterization of Dynamic Mechanical Parameters of Rock Fracture Evolution Based on Dry Coupling Point Contact Ultrasonic Technology

The nonlinear characteristics of deep rock masses are very prominent. Accurately obtaining the dynamic mechanical parameters of rock masses and crack damage evolution can provide the necessary data basis for studying the nonlinear behavioral changes of the stress release core during the pressure relief of rock masses. An ultrasonic acquisition system based on a compact dry-coupled acoustic wave probe can adapt to the surface of the rock being monitored with various curvatures. It can monitor the ultrasonic wave at multiple locations during the entire process of uniaxial failure from the field sandstone standard sample. It can also comprehensively collect the ultrasonic wave changes in different areas of the rock during the entire process of failure. Furthermore, the evolution of cracks characterized by dynamic mechanical parameters of sandstone samples is analyzed. The results show that the dynamic Poisson ratio is relatively stable at the beginning and then gradually increases during crack development. While the dynamic elastic wave begins to accelerate down during crack develop steady gradually accelerates to decrease, and falls rapidly when the rock damage. This can serve as a precursor to damage, providing certain feasibility for the characterization of onsite rock damage.

In situ X-ray computed tomography (CT) investigation of crack damage evolution for cemented paste backfill with marble waste block admixture under uniaxial deformation

Cemented waste rock backfill (CWRB), which is a mixture of tailings, waste rock, cement, and water, is subjected to the combination actions in underground mining operations. While the strength requirement of CWRB has been widely studied, the mesoscopic damage evolution mechanisms of waste block proportion (WBP) on its geomechanical characteristics are still not well understood. In the present paper, the crack damage evolution of CWRB samples were characterized using real-time X-ray CT under uniaxial compression. The effects of waste block on the stress strain responses, stress dilatancy behaviors, meso-structure changes, and the cracking behaviors were investigated. The results show that strength of CWBR is influenced by waste block proportion, and strength increases are due to geomechanical effects related to propagation of tortuous failure surfaces. Besides, for sample with high WBP, interlocking among the waste blocks also improves the overall stiffness and strength of CWRB compared with tailing paste sample. Volumetric dilatancy caused by the damage and cracking behavior has a close link with the meso-structural changes, which are controlled by the interactions between the waste rock and cemented tailing paste. The crack damage behavior is quite inhomogeneous and strongly influenced by the waste block size, shape, and distribution. Our study demonstrates that it is feasible to reveal the physical mesoscopic mechanism of crack damage evolution for CWRB by using X-ray CT, and the meso-damage cracking behaviors should be deeply studied to ensure stability of pillar during mining activities.

In Situ X‐Ray Computed Tomography (CT) Investigation on Localized Deformation and Crack Damage Evolution in a Bimrock by Tracking Rock Blocks

Instability of rock mass with block‐in‐matrix‐rocks (bimrocks) often poses a threat to the geological and ecological environment; thus, investigation of the localized deformation and crack damage evolution is critical to predict the bimrock hazards. In this work, triaxial compression testing on block‐in‐matrix‐soils (bimsoils) with a rock block percentage of 40% (mass ratio) was performed under tomographic monitoring using an original experimental setup specially designed to match the 450 kV industrial x‐ray Computerized Tomography (CT) apparatus. A series of 2D CT images were obtained by carrying out CT scanning at key points throughout the test and from different positions in the sample. The physical strain localization phenomenon was well investigated using the proposed Block Tracking Movement (BTM) method to track the trajectory of rock blocks during deformation. The distribution and morphology of cracks are strongly influenced by the interactions between the rock block and the soil matrix including the repeating contact and separation between them that finally results in the macroscopic pattern of cracking. The displacement vector analysis revealed the spatial kinematics of rock blocks during sample deformation and the associated localized band evolution, which was consistent with the macroscopic crack pattern observation. The cracks corresponding to the low‐density regions in the bimrock sample further indicate the inhomogeneous pattern of localized deformation. The meso‐structural changes and strain localization of the bimrock under triaxial deformation are discussed first by analyzing the rock block movement using x‐ray CT data.

Investigation on the effect of confining pressure on the geomechanical and ultrasonic properties of black shale using ultrasonic transmission and post-test CT visualization

Knowledge of the influence of confining pressure on the geomechanical and ultrasonic characteristics is essential to exploration and development of gas shale reservoirs. In contrast to the extensive studies on the physical and mechanical properties of shale, the velocities of P-wave and S-wave during continuous deformation are not thoroughly understood. In this study, a suite of triaxial compression tests were done on the shale samples collected from the Sichuan Basin and were cored with horizontal bedding structure under various confining pressure. In-situ ultrasonic transmission technique combined with post-test CT imaging analysis was used to reveal the velocity-pressure characteristics and crack patterns. The experimental results indicate that strength, elastic parameter, P- and S-wave velocities, and failure pattern are influenced by the applied confining pressure. At the pre-peak stage on the stress-strain curve, the P-wave velocity increases with elevated stress; however, the S-wave velocity first increases and then decreases, which is related to the crack damage evolution. The meso-structural changes during deformation are considered to be the main factor controlling the pressure sensitivity of the velocities. Under compression, the relatively soft pores, micro-cracks and minerals were compacted gradually, and it is the primary reason leading to the increment of velocities. Based on the CT scanning technique, the crack pattern was obtained after triaxial testing, the crack pattern analysis suggests that the fractal dimension of fracture distribution almost increases with confining pressure. What is more, the stimulated fracture density increases with increasing confining pressure. It suggests that, for the Longmaxi black shale, a high confining pressure condition is beneficial to form a complex fracture network.

Using X-ray computed tomography to investigate the effect of confining pressure on meso-structural changes and crack damage evolution in soil and rock mixture during triaxial deformation

This article aims to quantitatively investigate the influence of confining pressure on the mesoscopic structural changes using X-ray computerised tomography (CT) for soil and rock mixture (SRM). While the effects of confining pressure on the macroscopic mechanical characteristics have been widely studied, the physical meso-mechanisms governing the SRM deformation are still poorly understood. A triaxial compression experiment was conducted in SRM samples with a rock block proportion of 30%, under confining pressures of 60, 120 and 200 kPa, respectively. The meso-structural changes were quantitatively studied using a macroscopic stress–strain description and mesoscopic CT image identification. Under triaxial deformation, the stress–strain curves present strain hardening behaviour, and the strength of the sample increases with the increase of confining pressure. In addition, the crack distribution is extremely non-uniform and strongly affected by the existing rock blocks. The crack scale presents a reduction trend with increasing confining pressure. What is more, the most striking observation from the CT images is that cracks may disappear during deformation owing to the interactions between the soil matrix and rock blocks. Through a series of meso-structural evolution analysis, the meso-mechanisms of the effect of confining pressure on SRM under triaxial deformation have been presented first.

In Vivo X-ray Computed Tomography Investigations of Crack Damage Evolution of Cemented Waste Rock Backfills (CWRB) under Uniaxial Deformation

Cemented waste rock backfill (CWRB), which is a mixture of tailings, waste rock, cement, and water, is subjected to combination actions in underground mining operations and has been widely used in deep resource mining. While the strength requirement and macroscopic deformation behaviors of CWRB have been well studied, the mesoscopic damage evolution mechanisms are still not well understood. In this work, a CWRB sample with a waste rock proportion of 30% was studied with a uniaxial compression test under tomographic monitoring, using a 450 kV industrial X-ray computed tomography (CT). Clear CT images, CT value analysis, crack identification, and extraction reveal that CWRB damage evolution is extremely inhomogeneous and affected by the waste rock size, shape, and distribution. Furthermore, the crack initiation, propagation, and coalescence behaviors are limited to the existing waste rocks. When deformation grows to a certain extent, the cracks demonstrate an interlocking phenomenon and their propagation paths are affected by the waste rocks, which may improve the ability to resist compressive deformation. Volumetric dilatancy caused by the damage and cracking behavior has closed a link with the meso-structural changes, which are controlled by the interactions between the waste rocks and the cemented tailing paste.

In situ X-ray micro-CT for investigation of damage evolution in black shale under uniaxial compression

Crack damage evolution of shale is crucial to the hydraulic fracturing treatment and engineering stability. Although many effects have been done on the macroscopic characteristics of shale, yet the microscopic failure mechanism is not well understood. A uniaxial compressive test on black shale was conducted under topographic monitoring using in situ X-ray micro-tomography (µCT). A series of high-resolution reconstruction images were obtained by carrying out CT scans at six key points throughout the test to obtain the internal structure of shale sample. In addition, the CT values for the purpose of crack damage evolution in shale were identified. Clear 2D/3D CT images, CT value analysis and image segmentation analysis reveal that the sample experiences compression, damage, cracking, crack propagation, and collapse stages. Crack geometry and distribution in the shale sample is visualized by rendered CT images, and a combined tension and shear failure mode is observed from the fracture rose diagram. This work suggests that formation and propagation of fractures are influenced by the stratified structure and weak cementation medium between layers.

X-ray computed tomography (CT) observations of crack damage evolution in soil-rock mixture during uniaxial deformation

In most soil and rock mixture (SRM) experiments, the cracks develop during deformation are only observed after failure, and detailed mapping of crack evolution remains speculative. X-ray computerized tomography (X-ray CT) offers a feasible way to observe “in vivo” some of the characteristics of a tested specimen during its deformation. Therefore, we designed a special loading device combined with a 450-kV industrial X-ray CT machine to conduct deformation experiments for SRM. During testing, an axial load is applied during deformation of the SRM specimen with a rock block percentage of 30% (mass ratio), and six loading stages performed CT scanning in real-time. The crack initiation, propagation, and coalescence phenomenon can be vividly observed during deformation. The cracks have been identified and extracted using a series of digital image processing. In addition, the geometry characteristics of cracks (e.g., length, area, width, and fractal dimension) at each scanning stage have been statistically studied. What is more, the relationships between crack geometries and axial strain are discussed. The interaction between the soil matrix and rock blocks strongly influences the crack geometry morphology and probability density distribution during deformation.

Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading

AbstractWe present a comparative study of crack damage evolution in dry sandstone under both conventional (σ1 > σ2 = σ3), and true triaxial (σ1 > σ2 > σ3) stress conditions using results from measurements made on cubic samples deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, we measured the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output contemporaneously with stress and strain. We use acoustic wave velocities to model comparative crack densities and orientations. In essence, we create two end‐member crack distributions; one displaying cylindrical transverse isotropy (conventional triaxial) and the other planar transverse isotropy (true triaxial). Under the stress conditions in our experiments we observed an approximately fivefold decrease in the number of AE events between the conventional and true triaxial cases. When taken together, the AE data, the velocities, and the crack density data indicate that the intermediate principal stress suppresses the total number of cracks and restricts their growth to orientations subnormal to the minimum principal stress. However, the size of individual cracks remains essentially constant, controlled by the material grain size. Crack damage is only generated when the differential stress exceeds some threshold value. Cyclic loading experiments show that further damage commences only when that previous maximum differential stress is exceeded, regardless of the mean stress, whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.

A micromechanics-based damage constitutive model of porous rocks

A three-phase damage micromechanical model for rocks is developed through a combined consideration of micromechanical and thermodynamic theories. The rock is considered as a three-phase composite system, namely the rock skeleton without porosity, the initial pores, and the new cracks. The interaction among the three phases is considered. The kinetic equation of crack damage evolution is established in terms of the thermodynamic driving force derived from the reduction of Gibbs free energy of rocks. An equation that balances the thermodynamic driving force with the corresponding resistive force is obtained to calculate the damage volume fraction of crack under given stress. The stress-strain response of rocks is investigated under confining pressure or with different pore volume fraction. The theoretical results are found to be in good agreement with experimental data.