Total Cracks Research Articles

Experimental and numerical investigations of bending mechanical properties and fracture characteristics of cemented tailings-waste rock backfill under three-point bending

The bending mechanical properties and fracture characteristics of cemented tailings-waste rock backfill (CT-WRB) have a significant impact on efficient mine production. In this study, the effects of waste rock content (WRC) and waste rock size (WRS) on the bending mechanical properties and macroscopic failure characteristics of CT-WRB were investigated through experiments. Concurrently, PFC3D numerical simulation software was used to establish a three-point bending numerical model of CT-WRB with actual waste rock shapes to explore the crack evolution law and microscopic damage characteristics. Results showed that adding appropriate waste rock improved the initial stiffness, flexural strength, and modulus of the backfill, with optimal WRC and WRS at 40 % and 4–6 mm, respectively. Simulations indicated that under load, the displacement field of CT-WRB formed a vortex, with tensile stress in the middle and lower parts and compressive stress in an upper arch. Cracks started at the bottom and extended upward, leading to failure. Compared to OCTB (without waste rock incorporation), CT-WRB had more total cracks and tension-shear composite failure of internal particles. Waste rock altered stress distribution and crack paths, resulting in curved, bifurcated, and discontinuous main cracks. These findings provide a reference for efficiently recycling tailings, waste rock and ensuring safe mine production.

Study of CH4-H2-CO-Air premixed combustion characteristics based on fractal dimension and flame crack method

The laminar combustion characteristics of CH4-H2-CO-Air are studied in a constant volume combustion chamber with different hydrogen content ratios and the initial conditions of 300 K and 0.2 MPa by schlieren technology. The influence laws of flame instability are analyzed through the influence laws of hydrogen content ratio and equivalence ratio (Φ) on the fractal dimensions of the flame contour and the ratio of the total flame face cracks' lengths to the flame radius (L/R), as well as the influence laws of hydrodynamic instability and thermal-diffusive instabilities on the flames. The results show that L/R is more sensitive to flame instabilities than flame front fractal dimensions due to a wider sampling range. After R exceeds a certain value, for lean and rich combustion conditions with the same distance from the stoichiometric ratio, the L/R is greater during lean combustion, and as the equivalence ratio gradually increases from 0.6 to 1.4, the effect of increasing hydrogen content (Hydrogen mole fraction ranges from 60% to 66.67%) on flame instability gradually shifts from promotion to suppression. This study has certain reference value for the combustion application of biomass synthesis gas.

Crack-filling effect of gel on time-dependent mechanical behavior of concrete damaged by alkali–silica reaction

This study aims to investigate the crack-filling effect of gel generated by the alkali–silica reaction (ASR) on the mechanical behavior of concrete and propose an analytical model to predict the compression behavior of ASR-damaged concrete. To identify the mechanical contribution of the gel-filling cracks, the distribution of the chemical compositions and elastic moduli of the crack-filling gels were observed using SEM–EDS analysis and nano-indentation tests. The experimental results showed that the elastic modulus of the gel with a high-calcium composition ranged from 30 to 40 GPa, which is comparable to the elastic modulus of concrete. In addition, through literature reviews of the time-dependent compression behaviors of ASR-affected concrete, it was hypothesized that crack-filling gels can contribute to the recovery of ASR damage, and that the mechanical contribution of gels depends on the ASR crack patterns and changes with time. Based on these assumptions, the resistance of the gels filling the cracks in the compression, shear, and recontact behaviors of ASR-damaged concrete was reflected in the proposed model, in which the gel resistance increased with time and became larger as the ratio of microcracks to total cracks increased. The proposed model was verified through a comprehensive comparison of analysis and test results of compression behavior of ASR-damaged specimens, and it appeared that the change in mechanical properties of concrete according to the degree of damage (ASR expansion) and time was well simulated by the proposed model.

Damage Generated and Propagated by the AAR Reactive Aggregate from Kingston, Ontario, Canada.

It remains unclear in the literature what the cause of the so-called alkali-carbonate reaction (ACR) damage to concrete is. However, expansion and cracks as distress features are often attributed to the alkali-silica reaction (ASR). Therefore, this work aims to assess the damage to concrete generated and propagated by the so-called ACR-susceptible reactive aggregate through mechanical testing (i.e., the direct shear test), microscopy (the damage rating index-DRI), and other techniques. Distinct induced expansion levels (i.e., 0%, 0.05%, 0.12%, and 0.20%) were selected to compare the distress caused by ACR to concrete affected by ASR. The results show that the behavior of ACR, namely, as captured through the DRI, is inconsistent with that of ASR, thus attesting to ACR being a distinct distress mechanism. The damage captured through mechanical testing does not distinguish ACR from ASR; however, microscopy reveals that cracks in the cement paste are the main damage mechanism. The proportions of cracks in the cement paste are 40-50% of the total number of cracks, whereas open cracks in the aggregates normally characterizing ASR represent only up to 20% of the total cracks.

Natural joint effect on mechanical characteristics and fracture evolution of In-Site rocks under uniaxial compression

Understanding the strength features and failure characteristics of in-site rocks with various natural joints is an important fundament for studying the stability of underground engineering. The rock specimens with different joint angles, joint lengths, and joint distribution types were prepared and tested in uniaxial compressive test UCT combined with the acoustic emission AE detection. The natural joint categories of rock specimens were divided into single-joint type, cross-joint type, and mixed-joint type according to the number of joints and the gradual deterioration degree of integrity. The research results showed that the lower integrity could degrade the mechanical properties. The AE parameters as well as three types of energies all exhibited a firstly increasing and then decreasing trend with the increase of dominant joint angle αA. Rock specimens with lower intactness generated AE events with higher intensity and energy, resulting in a higher proportion of energy loss and lower severity of damage. The integrity of the natural jointed rock was verified by fractal dimension. The activity of micro-cracks within natural jointed rocks was characterized by both fractal dimension and AE b-value, and found that their trends were synchronized. A classification criterion that combined clustering techniques, AE parameter features, and genetic algorithms for tensile and shear cracks generated during the rock failure process was proposed. It’s demonstrated that the ratio of shear cracks to the total cracks showed an initial increase followed by a decrease with the increase of αA. Hoek-Brown's empirical formula was improved and the rating scores related to the uniaxial compressive strength Rucs-scores were continuouszed for natural jointed rock.

Numerical study on the rock breaking mechanism of high-pressure water jet-assisted TBM digging technique based on 2D-DEM modelling

In order to improve the digging efficiency of tunnel boring machine (TBM) in high-strength and highly abrasive formations, high-pressure water jet-assisted tunnel boring machine rock breaking technology has been developed and applied in steps. In this study, rock breaking mechanism by the new technology is investigated based on two-dimensional particle flow code (PFC2D) modelling. The force chain field distribution law and crack extension evolution characteristics of three typical rock breaking models are studied, and the influence of precutting slits parameters on force chain field distribution, rock sample rupture pattern, and peak load are further analysed. The results show that: 1) The rock breaking processes of the three typical modelling types (i.e., complete cutting model, same trajectory cutting model, and different trajectory cutting model) are different. Among them, the different trajectory cutting model is more likely to produce tensile failure and effectively reduce the penetration depth required for rock breaking. 2) The percentages of tension cracks to the total cracks in the three typical modellings are 90.8%, 93.9%, and 89.8%, respectively, indicating that the above three models are dominated by tension damage in the mesoscopic view. 3) With the increase of the depth of the precutting slit, the depth of the stress concentration zone beneath the disc cutter increases, inducing the increase of the angle between the edge of the stress concentration zone and the upper surface of the rock sample. Meanwhile, the peak load decreases, hence the difficulty of the tunnel boring machine disc cutter penetration is gradually reduced.

Numerical investigation on the effect of intergranular contact bonding strength on the mechanical properties of granite using PFC3D‐GBM

AbstractIn rock materials, the intergranular structure is more affected by various external conditions than the transgranular structure, and then dominates the macroscopic mechanical properties of rocks. In this paper, an innovative three‐dimensional grain‐based model based on Particle Flow Code (PFC3D‐GBM) for reproducing granite crystalline structures was developed. Based on this model, the Brazilian splitting test was conducted to investigate the effects of intergranular contact (IC) bonding strength on the mechanical properties and micro‐cracking behavior of the granite. Additionally, in this paper, the force chains and the orientation distribution of the force chains are shown in the form of three‐dimensional rose diagrams. The results are compared to the numerical results obtained by an initial particle‐based model to demonstrate the superiority of the proposed PFC3D‐GBM. The results indicate that the stress–strain curve of the sample in the Brazilian splitting test can be classified into four distinct stages: initial compaction, linear elasticity, damage, and splitting failure. ICs fracture at a higher rate than transgranular contacts (TCs) under static loading, as evidenced by experimental results of laboratory tests; during the entire loading process, all force chains uniformly distribute in all directions of a sample. As the external load increases, the number of total force chains decreases and the number of cracks increases. The main orientation of the high‐strength force chains is consistent with the external loading direction and is vertical with respect to the main orientation of the cracks. Furthermore, the number of high‐strength force chains increases with the level of external load. The sample's failure mechanism in the Brazilian splitting test is always a tensile failure, and this characteristic is unaffected by the modeling method or the IC bonding strength. Both the tensile strength and peak strain increase as the IC bonding strength increases. In this process, the number of total cracks in PFC3D‐GBM decreases, the proportion of transgranular cracks (Tcs) increases, and the proportion of intergranular cracks (Ics) decreases. The micro‐cracking behavior described above based on the PFC3D‐GBM can be explained more rationally. As the bonding strength of the IC increases, the particles involved in sample deformation are subjected to a greater stress concentration prior to the sample experiencing macro‐fracture, leading to an increase in the number of high‐strength force chains, which is the fundamental reason for the increase of the sample's tensile strength.

Study on mechanical and fracture characteristics of inclined weak-filled rough joint rock-like specimens

Weak-filled rough joint (WFRJ) strongly influence the mechanical and fracture properties of the rock mass. A uniaxial compression test with acoustic emission (AE) and digital image correlation (DIC) system was first conducted on the WFRJ rock-like specimens with a fixed inclination and filling thickness. The test results showed that the increasing JRC of WFRJ caused an increase and then a decrease in peak stress (σp), a steady increase in failure strain (εf) and a steady decrease in elastic modulus (E). And the increasing roughness of WFRJ changes the fracture pattern from sliding fracture along the hard-weak interface to mixed tensile-shear fracture. The AE results demonstrate that the increasing roughness in WFRJ caused the quantity increase in total cracks and the percentage decrease in shear cracks. Then, mechanical models considering local roughness and global structure of the weak filling layer were established to analyze the force characteristic of WFRJ rock-like specimens. Finally, ANSYS/LS-DYNA was used to numerical study the roughness effect of WFRJ specimens. The stress evolution in numerical results is consistent with the X-direction strain field evolution in DIC results. And the quantity and percentage variation laws of cracks in each part were verified by the numerical study results.

The role of prior creep duration on the acoustic emission characteristics of rock salt under cyclic loading

This study aimed to investigate the acoustic emission (AE) behavior of rock salt under combined creep and fatigue. The loading path was in a stress-controlled mode and contained a constant stress load with different durations, followed by cyclic loading. The AE counts, peak frequency, amplitude, b-value, and RA (rise time/amplitude)-AF (average frequency) distribution were systematically analyzed. The results show that the AE counts gradually increased during loading and existed sporadically within the prior creep stage and frequently in the subsequent cyclic loading phase. The peak frequency exhibited a zonal distribution and was divided into low, medium, and high parts. A longer prior creep duration induced a wider frequency distribution range, a higher average amplitude, and a reduction in b-value. According to the RA-AF distribution, the tensile cracks constituted more than 90% of the total cracks, and the prior creep duration decreased the shear crack propagation compared with the cases without prior creep. The results indicate that prior creep increases the crack dimensions and damage severity and makes the salt sample prone to fracture in the subsequent cyclic loading.

Split Hopkinson Pressure Bar Test and Its Numerical Analysis Based on Transparent Rock Samples

Rock crack propagation is an important direction in the analysis of rock mechanical properties. However, due to the opacity of rock mass, crack evolution process cannot be directly observed. In this study, a heterogeneous anisotropic transparent rock made by mixture randomly distributed different sizes molten quartz sand and pure epoxy resin. This study aims to provide an intuitive and useful method of observing the characteristics of crack evolution, also crack evolution process is recorded by a high-speed camera based on Split-Hopkinson pressure bar (SHPB) tests. The results show that: change of internal color is monitored during loading, which confirms the visualization of internal crack evolution. Failure occurs along the loading direction, and many cracks are distributed in the center of the sample. When the main crack passes through the broken glass sand, it directly passes through the glass sand, and when it passes through the unbroken glass sand, it extends along the edge of the glass sand. Particle flow code(PFC) is used to simulate crack propagation process of transparent rock sample in the SHPB test, and the process is divided into four stages: no crack, crack initiation, crack coalescence, and crack stability. When the final failure of the sample occurs during loading course, tensile cracks account for 78.3% of the total cracks and shear cracks account for 21.7%. Furthermore, fracture of the sample is dominated by tensile action. Stress—strain curve and failure mode of numerical simulation are in good agreement with laboratory test. The visualization of rock crack propagation is helpful to the follow-up study of hydraulic fracturing of roadway and debonding of bolt.

Effect of Joint Roughness and Infill Thickness on Shear Characteristics of Rock Mass

The joint planes in fractured rock mass will reduce the tensile strength and shear strength of rock mass, which has a decisive influence on the deformation and failure mode of engineering rock mass. Furthermore, infill also exists between the joint surfaces, which will also affect the shear characteristics of the joint. A method of importing standard joint profile into PFC2D was proposed, and a series of numerical simulation tests were carried out to study the effect of joint roughness and infill thickness on the shear characteristics of joints. The numerical results revealed that rock bolts profoundly improved the shear strength of the infilled rock joints, enhanced the toughness of the joint surface, increased the number of micro-cracks, and made the dilatation more obvious. The shear stress and the normal displacement of unbolted or bolted infilled rock joints increased with increasing the joint roughness and decreasing the infill thickness. The maximum horizontal compression stress in the middle of the bolt gradually increased with the increase of joint roughness coefficient. Different roughness has different effects on the number of micro-cracks in the sample. The number of total cracks and tensile cracks of the bolted and unbolted specimens increased with the increase of joint roughness coefficient, while the shear cracks remained almost the same. Through the study of the coupling effect of joint roughness and infill thickness on peak shear stress, results can be obtained as follows. The unbolted samples are highly sensitive to JRC changes. The greater the infill thickness, the greater the sensitivity of unbolted samples to JRC changes. The reinforcement effect of the bolt will strengthen the meshing strength between the joint surface and the filling material; that is, the meshing strength is positively correlated with joint roughness and negatively correlated with filling thickness.

Effect of calcium stearate and aluminum powder on free and restrained drying shrinkage, crack characteristic and mechanical properties of concrete

Shrinkage is the volume change of concrete without influence of external forces. Drying shrinkage, which occurs after the concrete hardening, is one of the most important causes of the cracking in concrete members. In addition to adverse effects on the appearance of concrete, cracks may reduce the strength and durability by increasing the permeability of concrete and facilitate the entry of aggressive agents into it. Free and restrained drying shrinkage are the most important types of shrinkage. The present study investigated the effect of two admixtures—including damp proofing and expansive materials— on compressive strength, tensile strength, electrical resistivity, modulus of elasticity, unrestrained and restrained shrinkage and water absorption of concrete specimens. In addition, the effect of admixtures on crack properties was investigated. The cracks properties, which occurred in unrestrained shrinkage test, such as average and maximum crack width and total cracks area, were computed by image processing. The results showed that the use of damp proofing additive, (i.e. calcium stearate) reduced maximum free drying shrinkage by 42.4%, maximum restrained drying shrinkage by 22.8%, maximum crack width by 51% and final crack area by 21%. Moreover, the use of expansive additive (i.e. aluminum powder) almost eliminated the free shrinkage, but on average, it increased the restrained drying shrinkage by 18% and reduced the maximum crack width and the final crack area by 2.67 and 27%, respectively.

Fluid-driven micro-cracking behaviour of crystalline rock using a coupled hydro-grain-based discrete element method

The grain-scale heterogeneity of rock has been found to greatly affect the cracking behaviour in mechanical tests. However, the heterogeneity has been insufficiently considered in hydraulic fracturing. To fill the gap, this paper proposes a coupled hydro-grain-based discrete element model and explores the fluid-driven micro-cracking of crystalline rock. The micro-heterogeneity of the rock is represented by a grain-based model. Three laboratory-scale fracturing cases over granite under different in-situ stresses are simulated. First, typical micro behaviour and hydro/hydro-mechanical responses are presented and interpreted. Next, sensitivity analysis of in-situ stress conditions is carried out. It is found that hydraulic fracturing of crystalline rock involves many unique behaviours on grain-scale such as non-symmetrical propagation, “zipper mode” initiation time order, generations of isolated and branching cracks, and re-closure of the cracks earlier initiated. Cracks can be induced in mineral grains and along the grain interfaces. The cracking mode is dominated by tension failure, with a small proportion in shear (between 3.5% and 1.5%). The interplays among orientation, aperture and number of different types of crack show that: 1. The inclination of cracks to the direction of maximum confining stress may occupy the entire range from 0° to 180°; 2. The aperture of cracks decreases statistically with their orientations turning to the direction of the minimum confining stress (σ3); 3. Increases in σ3 suppress the generation of total cracks and tend to force cracks to propagate in mineral grains; 4. A small in-situ stress difference results in fewer branching and isolated cracks. In this paper, evolutions of injection pressure and rock deformation during hydraulic fracturing are also discussed.

Numerical Simulation Study on Crack Evolution Characteristics of Coal Specimen Subjected to Conventional Compression Loading

The paper represents a simulation investigation about the crack evolution and acoustic emission characteristics of coal specimen subjected to conventional compression loading with four confining pressures. From the simulation tests, the following conclusions can be drawn. (1) Under the confining pressure of 0.5, 1.0 and 1.5 MPa, at the stress peak point, the number of total cracks in coal specimen are 3.33, 4.28 and 4.50 times of that under uniaxial stress state, respectively. That is, with the increase of confining pressure, the number of total cracks at coal specimen failure gradually increases, but the increase rate gradually decreases. (2) Under the confining pressure of 0, 0.5, 1.0 and 1.5 MPa, at the stress peak point, the proportion of tensile cracks in the total cracks decreases from 100% to 95.95%, and the proportion of shear cracks increases from 0 to 4.05%. These show that with the increase of confining pressure, the coal specimen presents a transition trend from tensile failure to shear failure. (3) When the confining pressure is less than 1.0 MPa, the confining pressure has an obvious effect on the crack distribution of coal specimen subjected to conventional compression loading, but when the confining pressure exceeds 1.0 MPa, this effect gradually weakens. (4) The maximum value of acoustic emission impact counts slightly lags behind the time point corresponding to the peak strength, namely, the coal specimen is destroyed at some point in the post-peak phase.

Investigation of the approximate decomposition of alternating current field measurement signals from crack colonies

Cracks, such as those caused by stress corrosion, are often present as colonies. The pick-up signals from the cracks in colonies obtained through the alternating current field measurement technique are composed of the signals from the total cracks. Using a superposed signal directly causes large errors in depth prediction. Therefore, this study investigates the approximate decomposition of alternating current field measurement signals from crack colonies. The effect of the number and the relative position of the cracks on the measured signals is studied. A method based on the signal circle is proposed to decompose the superposed signals of crack colonies. Further, a depth quantification algorithm, which is based on a generalized regression neural network, is developed for crack colonies. The results of the simulation and experiment indicate that the proposed algorithm can preliminarily predict crack depth in a colony, especially for deep cracks, which are dangerous. The predicted results can also be used as initial values for accelerating phenomenological inversion methods.

Failure characteristics and mechanical mechanism of study on red sandstone with combined defects

In this study, the strength and failure mechanism of red sandstones with combined defects were investigated by uniaxial compression tests on red sandstones with different crack angles using two-dimensional particle flow code numerical software, and their mechanical parameters and failure process were studied and analyzed. The results showed that the mechanical characteristics such as peak strength, peak strain, and elastic modulus of the samples with prefabricated combined defects were significantly inferior than those of the intact samples. With increasing crack angle from 15&deg to 60&deg, the weakening area of cracks increased, elastic modulus, peak strength, and peak strain gradually reduced, the total number of cracks increased, and more strain energy was released. In addition, the samples underwent initial brittle failure to plastic failure stage, and the failure form was more significant, leading to peeling phenomenon. However, with increasing crack angle from 75&deg to 90&deg, the crack–hole combination shared the stress concentration at the tip of the crack–crack combination, resulted in a gradual increase in elastic modulus, peak strain and peak strength, but a decrease in the number of total cracks, the release of strain energy reduced, the plastic failure state weakened, and the spalling phenomenon slowed down. On this basis, the samples with 30&deg and 45&degcrack-crack combination were selected for further experimental investigation. Through comparative analysis between the experimental and simulation results, the failure strength and final failure mode with cracks propagation of samples were found to be relatively similar.

Determination of damage levels of RC columns with a smart system oriented method

In this study, a method that is fast, economical and satisfying in terms of accuracy rate has been developed in order to determine the post-earthquake damage level of reinforced concrete column elements dependent on the damage image on the column surface. In order to represent the Turkish building stock, reinforced concrete columns were produced complying with the 2007 and 2018 Turkish Earthquake Code (TEC-2007 and TBEC-2018) and, in order to represent the existing building stock made before 2000, reinforced concrete columns which are non-complying with the code have been produced. A total of 12 reinforced concrete columns produced in 1/1 scale with square cross sections were tested under earthquake resembling reversible cycling lateral load and axial force. For each cycle, a data set was created by matching the surface images taken from the determined regions of the columns with the damage levels specified in TEC-2007 and TBEC-2018 depending on the load–displacement values measured on the column during the experiment. As a result of the experimental study, a total of 390 damage images were obtained for each load and displacement level. Image processing application was performed by using MATLAB on the damage images and the cracks on the column surface were separated. Parameters such as total cracks area, total cracks length, maximum crack length and maximum crack width have been obtained to represent the amount of damage on the column through the feature extraction process of the cracks in the images. The characteristics of the cracks were classified by support vector machines, decision trees, K-nearest neighborhood, Discriminant Analysis, Ensemble algorithms, which are machine learning classifiers, and the damage states for the columns were estimated. The estimation success from the classifiers ranges from 64 to 80%. In this study, it has been seen that the proposed and developed intelligent system will be open to development and will be a good alternative to existing conventional systems for the determination of column damage.

Crack Propagation of Rapid Setting Latex-Modified Concrete Pavement Slabs under Vibration

Abstract In recent years, rapid setting latex-modified concrete (RS-LMC), which allows vehicular traffic to flow after only 3 h of being placed, has been widely used as a material for improving bridge decks. As the restrictions for vehicular traffic when conducting repair work on pavements are difficult, bridge deck improvements using RS-LMC are exposed to vibrations during the initial curing stages, and cracks are generated because of the passing vehicles. To identify the characteristics of cracks caused by vibrations during the initial curing stage of RS-LMC, twelve specimens of this material were created and studied. Vibration tests were performed using a pavement shaking table, which simulates the vibration conditions of actual bridges. In this research, the main test variables were the amplitude of vibration and the latex-cement ratio (L/C). Test results revealed that the number and total cracks decreased with an increasing L/C, until 15 % L/C. Further, cracks occurred at a much lower strain rate than that stated by conventional design standards.

Effects of initial defects within mortar cover on corrosion of steel and cracking of cover using X-ray computed tomography

In actual reinforced concrete structures, plenty of initial defects exist, which may greatly influence the behavior of steel corrosion and cracking within concrete cover. Thus, this paper presents an experimental investigation into the effects of initial defects within mortar cover on the corrosion of a steel bar in mortar and on the cracking of mortar cover using X-ray computed tomography. The influence of initial defects with mortar cover at three different positions i.e. interface, middle and surface was investigated for 5%, 30% and 55% corrosion levels of steel. From experimental investigations, it was found that the specimen with interface defect has significant accumulation of corrosion products. The position of cracking within mortar cover induced by steel corrosion was examined. At least one crack connected steel with the initial defects or reached close to the defect. These cracks formed the basis of the development of other cracks. The cracking level was defined in terms of the number of cracks and the volumes of total cracks and followed the sequence of S-SF > S-MD > S-IF, which is consistent with the sequence of volume expansion ratio of steel. The degree of release of rusts was found to depend on the position of the initial defects i.e. the closer the initial defect is to the steel, the better would be the releasing effect of rusts. Finally, comparison between the external and internal corrosion parameters reflecting corrosion level was performed.

Influence of moisture on crack propagation in coal and its failure modes

Understanding of effects of water on crack propagation and crack failure modes of coal is important to determine the required width of the coal pillars in underground reservoirs and calculate the area of fractured zones containing flowing water. In this paper, we apply acoustic emission (AE) techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), alongside uniaxial compressive tests to develop new insights into the mechanics of crack developments and failure modes of partially saturated coal samples with a range of moisture contents (0%, 6.00%, 9.75%, and 10.96%). We present new relationships between tensile/shear cracks and failure modes incorporating the effects of water intrusion on the micro-tensile/shear cracks and macro tensile/shear failure modes.The results show that the presence of hydrophilic illite (clay mineral) has strong influence on water absorption capacity of coal. The uniaxial compressive tests carried out in this study, show that the AE activity is mainly appeared after the crack damage threshold (point D) and it is independent to water content. The AE activity decreases with increasing moisture content as the water reduces the internal connections between the coal particles and increases the possibility of sliding failure. Studying the RA (rise time divided by peak amplitude) and AF values (counts divided by duration) demonstrated that the cracks generated in the coal samples consist of a large number of micro tensile and shear cracks, whilst tensile- or shear-only cracks were not found to be evident. For the samples studied, the higher moisture content is associated with the lower AF and higher RA values. Under the scope of the experimental investigations, we conclude that the higher moisture content corresponds to a lower number of total cracks and tensile cracks in coal. However, a greater proportion of shear cracks can be created by the increasing water content. Increasing the water level can reduce the number of tensile failure planes and the percentage of tensile failure planes against total failure planes. Therefore, the higher water content can increase shear failure planes and promote the possibility of shear failure in coal.