Crack Intensity Research Articles

Fracture features of brittle coal under uniaxial and cyclic compression loads

Under the effects of complex geological and stress environments, burst hazards continue to be a major challenge for underground space utilization and deep resources exploration as its occurrence can lead to personnel causalities, equipment damage and structural collapse. Considering the stress path experienced by in-situ coal body, cyclic loading appears in quite various forms for instance shearer cutting, overlying strata breakage, hydro-fracturing and blasting, during tunnel, mining and underground space utilizing process. The stability of the underground coal body subject to periodic loading/unloading stress is extremely important for maintain the function of designed engineering structure for waste storage, safe mining, roadway development, gas recovery, carbon sequestration and so on. The mechanical properties of hard rock subject to cyclic fatigue loads has been intensively investigated by many researchers as the rock burst induced by supercritical loads has long been a safety risk and engineering problems for civil and tunneling engineering under deep overburden. More recently, the mechanical properties of coal samples under cyclic fatigue loads is investigated from the aspect of hysteresis, energy dissipation and irreversible damage as the burst hazards of brittle coal is rising in many countries. However, the crack propagation and fracture pattern of brittle coal need more research to understand the micro mechanism of burst incubation subject to cyclic fatigue loads as brittle coal can store more elastic strain energy and rapidly release the energy when its ultimate strength once reached. This research studied the internal crack status corresponding to different cyclic fatigue loading stage of brittle coal samples. The AE monitoring was applied during the uniaxial and cyclic loading process of brittle coal samples to record the crack intensity of samples at different loading stages. The damage evolution curve corresponding to loading status was then determined. The fracture pattern of coal samples determined by micro-CT scan was observed and discussed. It has been found by this paper that brittle coal of uniaxial compression tests demonstrated sudden failure caused by major splitting fracture while that of cyclic fatigue tests experienced progressive failure with mixture fracture network.

Desiccation cracks prediction using a 3D finite elements model

ABSTRACT This paper focuses on the cracking of fine soil grains by desiccation using a 3D hydro-mechanical model. It further seeks to study the cracks growth across the depth during the desiccation time. Numerical results provide data for assessing the depth propagation of the cracks, field suction in 3D, the tensile stresses caused by the cracks. Cross sections have been considered to determine the cracks’ depth. Furthermore, water content and suction versus time have been obtained. It is still challenging to experimentally obtain a sample’s suction, water content, and distribution of stresses. Although there is a gap between numerical and experimental results concerning the appearance time of the first crack, the interpretation given in the text demonstrates that it is a wrong gap. Consequently, it can be stated that the numerical results agree with the experimental results, in terms of cracks intensity factor (CIF) and the evaporation rate as functions of desiccation time.

Morphological characteristics of cracks in soil with and without vegetation cover

An assertive understanding of cracks in expansive soils makes their typification a challenge, especially in field conditions. Seeking to overcome such difficulty, digital images have been used to estimate the fractal geometry of surface cracks in soils with and without vegetation in a semi-arid environment. This study was carried out in a cross-section along the main course of a first-order watershed in the Centro Sul region, Ceará State (Brazil). Two treatments comprised soil with and without vegetation cover. Experimental plots were installed to represent the upper, middle, and lower parts of the watershed studied (vegetated and non-vegetated), with an area of 1 m2 and spaced 2 m apart. The following morphometric characteristics of cracks were monitored and evaluated during 38 days, namely: area, weighted average between crack area and its perimeter, weighted average of the fractal dimension area of the cracks, connectivity between cracks and soil moisture. In the soil with and without vegetation, intensity of cracks in the soil had a significant correlation (p ≤ 0.01) with the perimeter, fractal area and crack connection. The intensity of cracks between covered and uncovered plots showed a significant difference (p ≤ 0.01), only in the lower part of the watershed. The middle stretch had higher sand content and differed statistically from the lower stretch for intensity of cracks in the soil. Vegetation influenced the correlation between cracks intensity and their connection, influencing their morphology in the sandy stretch.

Experimental Investigation of Pre-Flawed Rocks under Dynamic Loading: Insights from Fracturing Characteristics and Energy Evolution.

Different fractures exist widely in rock mass and play a significant role in their deformation and strength properties. Crack rocks are often subjected to dynamic disturbances, which exist in many fields of geotechnical engineering practices. In this study, dynamic compression tests were carried out on rock specimens with parallel cracks using a split hopkinson pressure bar apparatus. Tests determined the effects of strain rate and crack intensity on dynamic responses, including progressive failure behavior, rock fragmentation characteristics, and energy dissipation. Based on the crack classification method, tensile-shear mixed cracking dominates the failure of rock specimens under the action of impact loading. Increasing the flaw inclination angle from 0°-90° changes the predominant cracking mechanism from tensile cracking to mixed tensile-shear cracking. The larger the loading rate, the more obvious the cracking mechanism, which indicates that the loading rate can promote the cracking failure of rock specimens. The fragmentation analysis shows that rock samples are significantly broken at higher loading rates, and higher loading rates lead to smaller average fragment sizes; therefore, the larger the fractal dimension is, the more uniform the broken fragments of smaller sizes are. Energy utilization efficiency decreases while energy dissipation density increases with increasing strain rate. For a given loading rate, the energy absorption density and energy utilization efficiency first decrease and then increase with increasing flaw inclination, while the rockburst tendency of rock decreases initially and then increases. We also find that the elastic-plastic strain energy density increases linearly with the total input energy density, confirming that the linear energy property of granite has not been altered by the loading rate. According to this inherent property, the peak elastic strain energy of the crack specimen can be calculated accurately. On this basis, the rockburst proneness of granite can be determined quantitatively using the residual elastic energy index, and the result is consistent with the intensity of actual rockburst for the specimens.

An dynamical evaluation of size-dependent weakened nano-beam based on the nonlocal strain gradient theory

This paper examines the free lateral vibration of a cracked nano-beam based on Euler-Bernoulli beam theory and nonlocal strain gradient theory (NSGT). Due to the importance and application of nanostructures, their mechanical and mechanical properties are essential. The governing equations and boundary conditions related to using the Hamilton principle have been extracted. The beam separation with the nano-beams division into two parts attached to the Torsion spring is modeled. The model calls the excess strain energy due to crack and increases the discontinuity in the deflection slope. This study investigated the effects of crack propagation, crack intensity, material length scale parameter, and various nonlocal parameters. A comparison of previous studies has been published, where a good agreement is observed. The results show that the parameters mentioned above play an important role in dynamical behavior.

Pressure Container Surface Crack Analysis

High-pressure containers are widely used in modern industries, including chemical industry, oil, nuclear power, aviation and many other fields. As pressure vessels are widely used in various fields, it will face a more complex and changeable working environment. the effects of media environment and fatigue stress cause defects, which will greatly affect the normal work of the container. Therefore, this article conducts static analysis of the surface with cracks on the surface through the finite element numerical simulation method, analyzing the rules of crack expansion and the impact on the life of pressure vessels. Research results: The container was stretched vertically under the load of 10MPa, and the maximum deformation occurred at the top of the ellipse sealed head, with a maximum deformation of 1.63mm; The maximum stress value does not occur at the top of the container, but occurs in the crack, and the maximum stress value is 538.18MPa; The minimum value of the safety factor of the safety coefficient cloud diagram is 0.50, which is located at the crack. The intensity of cracks has a great impact on the intensity of high -pressure containers. It needs to be checked regularly on high -pressure containers to ensure the safe operation of the equipment.

Engineering high-performance cementitious matrices for improved projectile impact resistance with silane, micro fibrillated cellulose and fine calcined bauxite aggregate

This experimental study evaluated the effectiveness of passive and active strategies in improving the projectile impact resistance of cementitious matrices. The principle of the passive approach is to reinforce the matrix by substituting weak Van de Waal's forces between hydration products with covalent bonds which have the orders of magnitude higher dissociation energy. A silane coupling agent (SCA) and micro fibrillated cellulose (MFC) were selected as the vehicles to engineering the bond scheme at the molecular scale by forming a covalently linked organic-inorganic hybrid. The mechanism of the active strategy is to aggravate damage to the projectile and thus leave less energy available for the penetration process. This is achieved by incorporating exceptionally hard calcined bauxite aggregates (CBA). Impact tests were conducted utilizing 8-mm-diameter conical-nosed steel projectiles at a designed velocity of 650 m/s. Damage was quantified by the depth of penetration (DOP) and equivalent crater diameter (ECD). Results indicated that the SCA and MFC modified matrices resulted in a 30% and 20% reduction in DOP, respectively. The crack intensity was decreased tremendously or even absent for the SCA and MFC modified matrices compared with the prominent cracks of the control radiated from the cratering region. Matrices incorporating fine CBA or siliceous aggregates led to a 55% and 38% smaller DOP, respectively. Crushing of aggregates and appreciable deformation, severe scratches, and significant mass loss of retrieved projectiles were observed. Matrices including fine CBA or siliceous aggregate maintained excellent integrity and drastically less severe cracking on the impact surface as well as the interior region compared with the control.

Influence of Geotextile Materials on the Fractal Characteristics of Desiccation Cracking of Soil

In recent years, the irregular cracks formed during the damage evolution of civil engineering materials have been able to be quantitatively described by using fractals. In this study, the fractal characteristics of the desiccation cracking of soil were investigated under different substrate contact and permeability conditions through a natural drying test in the laboratory. Three kinds of base contact conditions of soil, namely, grease, geomembrane, and geotextile, were designed, and two samples for each contact condition, including one parallel sample, were used. The continuous drying experiment was carried out at a constant ambient temperature. The crack morphology under different spacings was analyzed quantitatively using digital image processing technology. The fractal dimensions of three soil substrate contact conditions (grease, geomembranes, and geotextiles) were between 1.238 and 1.93. When the crack network on the soil surface stops developing, the fractal dimensions under the three experimental conditions are 1.88, 1.93 and 1.79, respectively. In the final state of crack development, the crack intensity factor of the sample with grease at the bottom is 2.99% and 4.02% higher than that of the sample with geomembranes and geotextiles at the bottom, respectively. The residual water contents of the samples with bottom contact conditions of grease, geomembrane, and geotextile increase successively, which are 3.12%, 5.76% and 9.71%, respectively. The effects of interface friction and permeability on soil cracking behavior are analyzed, and the evolution characteristics and formation mechanisms of cracks in soil are revealed.

Dynamic analysis of effect of progressive crack and variation in center distance on vibration of spur gears

Gears are very important components of every mechanical system. The loading pattern in case of spur gears in mesh is highly dynamic in nature and has impact on vibrations. Excessive vibration hampers the performance of the machine. The presence of cracks on gears and change in center distance directly influences the mesh stiffness. This paper endeavors to investigate the effect of progressive cracks which are developed with passage of time. This is done by developing an improved dynamic model for different intensity of cracks considering misalignment of root circle and base circle. The improved model also takes into consideration the effect of change in center distance along with severity of crack. A MATLAB code is developed to solve the analytical equations to determine time varying mesh stiffness (TVMS) graphically. Through this study, it is established that TVMS is a good indicator for investigating the progression of crack on the gears and also change in center distance.

The effects of late Cenozoic climate change on the global distribution of frost cracking

Abstract. Frost cracking is a dominant mechanical weathering phenomenon facilitating the breakdown of bedrock in periglacial regions. Despite recent advances in understanding frost cracking processes, few studies have addressed how global climate change over the late Cenozoic may have impacted spatial variations in frost cracking intensity. In this study, we estimate global changes in frost cracking intensity (FCI) by segregation ice growth. Existing process-based models of FCI are applied in combination with soil thickness data from the Harmonized World Soil Database. Temporal and spatial variations in FCI are predicted using surface temperature changes obtained from ECHAM5 general circulation model simulations conducted for four different paleoclimate time slices. Time slices considered include pre-industrial (∼ 1850 CE, PI), mid-Holocene (∼ 6 ka, MH), Last Glacial Maximum (∼ 21 ka, LGM), and Pliocene (∼ 3 Ma, PLIO) times. Results indicate for all paleoclimate time slices that frost cracking was most prevalent (relative to PI times) in the middle- to high-latitude regions, as well as high-elevation lower-latitude areas such the Himalayas, Tibet, the European Alps, the Japanese Alps, the US Rocky Mountains, and the Andes Mountains. The smallest deviations in frost cracking (relative to PI conditions) were observed in the MH simulation, which yielded slightly higher FCI values in most of the areas. In contrast, larger deviations were observed in the simulations of the colder climate (LGM) and warmer climate (PLIO). Our results indicate that the impact of climate change on frost cracking was most severe during the PI–LGM period due to higher differences in temperatures and glaciation at higher latitudes. The PLIO results indicate low FCI in the Andes and higher values of FCI in Greenland and Canada due to the diminished extent of glaciation in the warmer PLIO climate.

Quantifying the Impact of Charge Rate and Number of Cycles on Structural Degeneration of Li-Ion Battery Electrodes

A quantitative link between crack evolution in lithium-ion positive electrodes and the degrading performance on cells is not yet well established nor is any single technique capable of doing so widely available. Here, we demonstrate a widely accessible high-throughput approach to quantifying crack evolution within electrodes. The approach applies super-resolution scanning electron microscopy (SEM) imaging of cross-sectioned NMC532 electrodes, followed by segmentation and quantification of crack features. Crack properties such as crack intensity, crack width and length are quantified as a function of charge rate (1C, 6C, and 9C) and cycle number (25, 225, and 600 cycles). Hundreds of particles are characterized for statistical confidence in the quantitative crack measurements. The data on crack evolution is compared to electrochemical data from full cells and half cells with the NMC532 positive electrodes. We show that while crack evolution strongly correlates with capacity fade in the first 25 cycles, it does not correlate well for the following hundreds of cycles indicating that cracking may not be the dominant cause of capacity fade throughout the cycle-life of cells.

Detection of Road Cracks Using Convolutional Neural Networks and Threshold Segmentation

Automatic road crack detection is an important transportation maintenance responsibility for ensuring driving comfort and safety. Manual inspection is considered to be a risky method because it is time consuming, costly, and dangerous for the inspectors. Automated road crack detecting techniques have been extensively researched and developed in order to overcome this issue. Despite the difficulties, most of the proposed methodologies and solutions involve machine vision and machine learning, which have lately acquired traction largely due to the increasingly more affordable processing power. Nonetheless, it remains a difficult task due to the inhomogeneity of crack intensity and the intricacy of the background. In this paper, a convolutional neural network-based method for crack detection is proposed. The method is inspired from recent advancements in applying machine learning to computer vision. The primary goal of this work is to employ convolutional neural networks to detect the road crack. Data in the form of images has been used as input, preprocessing and threshold segmentation is applied to the input data. The processed output is feed to CNN for feature extraction and classification. The training accuracy was found to be 96.20 %, the validation accuracy to be 96.50 %, and the testing accuracy to be 94.5 %.

Experimental study on failure and fracturing attributes of granite after thermal treatments with different cooling conditions

Thermal damage is one of the fundamental mechanisms affecting the stability of rocks encountered in many deep geo-engineering ventures such as nuclear waste isolation, geothermal extraction etc. In the present study, the findings on influence of heating and subsequent cooling conditions on the fracturing and mechanical behaviour of an Indian granite are discussed. The granite samples from Jalore region of India were heated from room temperature to 600 °C followed by slow cooling and water quenching treatments. It was observed that rapid cooling through quenching had a greater impact on strength, elastic properties than slow cooling rate. Moreover, a drastic shift in terms of fracturing thresholds was witnessed, as a function of both temperature and sudden thermal shock experienced by rock at higher cooling rate. The simultaneous AE monitoring results suggested an early crack damage at higher thermal regimes. With help of microscopic observations, it was observed that inter-granular crack boundaries widened with increasing thermal damage experienced by minerals under both cooling conditions. However, the fracturing process was intense under rapid cooling treatment due to the invasion of water due to quenching which accelerated severe intra-crack growths, especially at higher thermal stress because of the rapid cooling rate than the slow cooling. The increased intensity of thermal cracks with temperatures and with variation in cooling rate was inferred as the primary reason for decay of rock characteristics. This was very well supplemented by decay in strength properties and changes in fracturing attributes of the tested granite as inferred from acoustic monitoring.

Effect of laser pulse power on solidification cracking susceptibility in surface processing of a high carbon high chromium tool steel

High carbon, high chromium tool steels are deep hardening steels widely used for cold forming dies. A 400 W Nd:YAG laser was employed to melt the surface of the AISI D2 tool steel with 5 and 6 ms single pulses of laser at 4–10 J energy levels. The microstructural study showed that complete dissolution of large primary carbides occurred during melting, but the cooling rate was too fast to form any primary carbide. Increasing the laser energy (peak power) led to a coarser dendritic microstructure and a reduction in solidification cracking intensity. The analyses indicate that the coarser dendritic structure has promoted the backfilling effect. However, despite having a coarser dendritic structure, the material laser processed with a preheat of 300 °C showed the most severe crack intensity due to the provision of sufficient time in cooling for the formation of a network of brittle carbides in the inter-dendritic regions. The HAZ was found to be sound although reaching a hardness of 750 HV. • A fine columnar dendritic microstructure was formed by laser remelting, which became coarser with increased laser peak power. • Facilitation of the backfilling effect in higher laser peak powers reduced the intensity of solidification cracks.. • The formation of the inter-dendritic carbide phase in the preheated sample increased the solidification cracking intensity.

Free transverse vibrations analysis of size-dependent cracked piezoelectric nano-beam based on the strain gradient theory under mechanic-electro forces

This paper investigates the free lateral vibration of a cracked nano-beam to piezoelectric effects on Euler-Bernoulli beam theory and nonlocal strain gradient theory (NSGT). So far, the propagation and growth of cracks in nano-beams have not been done by modeling the crack area using torsion springs using NSGT. This article investigates the free vibrations of cracked nano-beams using piezoelectric effects. The governing equations, related boundary conditions, and piezoelectric effects were derived using the Hamilton principle. Nano-beam separation is modeled by splitting nanoparticles into two parts connected to a rotating spring. The model calls the extra strain energy due to cracking and increases the discontinuity in the deflection slope. This study investigates the effects of crack propagation, crack intensity, piezoelectric effects, longitudinal scale parameter, and nonlocal parameter. A comparison of previous studies has been published in which a good agreement is observed. The results show that the mentioned parameters play an important role in dynamic behavior.

Frost cracking dictated landslide distribution in response to temperature change since Last Glacial Maximum across the Eastern Qilian Mountains

AbstractTemperature‐dependent frost cracking is a major weathering process that dominates sediment supply and hillslope erosion by reducing shear strength and conditioning slopes to be more failure‐prone in cold mountain regions. Landslides are a common erosional process in such landscapes. The linkage between these two processes has been proposed but many of the mechanisms remain to be investigated. In this study we compile a landslide inventory in the Eastern Qilian Mountains including 2482 rockfalls and 5373 elongate transport zone footprints. We use a widely accepted frost cracking model to explore this weathering process as a mechanism of paleoclimate control on the modern landslide distribution. The results show that landslides concentrate in a narrow altitudinal range and that this elevation dependence varies with slope aspect. Based on the paleoclimate record, we calculate the time‐averaged frost cracking intensity (Fci) since Last Glacial Maximum (LGM) and adopt different parameters for two slope aspects. We find that time averaged Fci is a good predictor of the landslide elevation distribution for both aspects. We then introduce two slope‐based landslide initiation and runout models (i.e., SHALRUN and FCIRUN) to examine the relative roles of slope versus temperature in determining the landslide probability in space. The FCIRUN model, which incorporates Fci as a measure of sediment supply, consistently outperforms the SHALRUN model. This result suggests that frost cracking determines the pattern of landslide initiation, although topography still controls their runout path. Our study highlights the potential of introducing Fci into slope‐based landslide prediction models to improve model skill in identifying hazardous areas (e.g., rockfall) across weathering‐limited landscapes like the Qilian Mountains.

Microscopy of maize grains subjected to continuous and intermittent drying

Drying is an important step in the post-harvest processes as a way of product conservation and quality preservation. In this context, this study aimed to evaluate the effect of continuous and intermittent drying of maize grains with different rest periods on the integrity of their micro- and macroscopic structures. Maize grains were harvested with a moisture content of 0.3399 ± 0.001 dry basis (db) and subjected to continuous and intermittent drying with 4, 8, 12, and 16 hours of rest period. An experimental fixed-bed dryer, with controlled drying air conditions at a temperature of 100 °C and air flow of 1.5 m3 min.−1 m−2 (12 m3 min.−1 m−3), was used. Continuous drying was completed with a moisture content of 0.1628 ± 0.0003 db, whereas intermittent drying was interrupted with 0.2195 ± 0.0002 db and resumed after rest. The drying rate, integrity through grain images, the conformation of particles through scanning electron microscopy, and cell membrane integrity were evaluated. The drying rate increased with an increase in the rest period, the increase in rest period reduced the intensity of cracks, and the reduction in rest period led to higher dispersion and reduction in the size of starch granules and lower integrity of cell membranes.

Three‐dimensional DEM simulation of the nonlinear crack closure behaviour of rocks

AbstractCrack‐closure behaviour of rock materials is well‐known, while the three‐dimensional discrete element method (DEM) simulation of crack‐closure behaviour is still a challenge. Hence, a numerical method was proposed through the combination of notional surface method and flat‐joint model. The pre‐existing microcracks with finite width can be inserted into the three‐dimensional numerical sample through the present method. Since the two physical mechanisms, that is, closure of pre‐existing microcracks and fracture of flat‐joint bonds, are included in the simulations, various evolution trends of crack‐closure stage can be captured through the present method. Due to the high computational efficiency of flat‐joint model, the three‐dimensional DEM simulations of crack‐closure behaviour can be achieved. The results of parametric study show that the crack intensity determines the crack‐closure behaviour and the crack width significantly influences the evolution trend of crack‐closure stage. The simulated unconfined compressive strength (UCS) and tensile strength (TS) significantly decrease with the increase of crack intensity. The crack width significantly influences the deformation parameters, such as elastic modulus, Poisson's ratio and crack closure strain. A calibration procedure was proposed to ease the calibration of mechanical parameters and crack‐closure behaviour. Through the proposed numerical method and calibration procedure, the basic mechanical parameters and crack‐closure behaviour of sandstone were well matched.

New binary paper/wood industry waste blend for solidification/stabilisation of problematic soil subgrade: macro-micro study

This article presents the potential application of paper/wood industry waste by-products, i.e. lignosulphonate (LS) in conjunction with lime (LM) [i.e. LS-based cementing admixture (LSCA)] to mitigate desiccation induced cracking and mechanical vulnerabilities of expansive soil subgrade. The results showed that the LSCA and lime (LM) significantly ameliorated the soil consistency, subgrade strength characteristics (i.e. California bearing ratio and resilient modulus), and completely mitigated the swelling potential with enhanced resistance against Sr . Moreover, the LSCA and LM notably reduced the 3D-volumetric shrinkage and crack intensity factor (CIF) by 90.1 and 87.4%, respectively. The mineralogical and microstructural analyses manifested that the LSCA induced new cementitious compounds (i.e. hydrates of calcium silicate and calcium aluminate), and substantially enhanced the interlocking among the soil particles compared to the untreated and LS-treated soils. Besides, pavement thickness analyses showed that the LSCA treated subgrade, omits the provision of sand cushion and sub-base course promoting road material conservation.

Novel size-dependent finite element formulation for modal analysis of cracked nanorods

It is a remarkable engineering problem how possible structural damages in nano/micro scaled mechanical elements affect the working performance and mechanical behavior of nano/micro-electro-mechanical system (NEMS/MEMS) organizations. There are various nano/micro-scale mechanical elements that can be modelled as one-dimensional and continuous systems in NEMS/MEMS organizations. With this motivation, the free vibration analysis of nano scaled axial rods with a single crack is examined by employing the nonlocal finite element formulation for the first time in the literature in this study. Firstly, the differential equation of nonlocal vibration of nanorods is presented for a single crack. Next, a nonlocal finite element formulation developed for the case of crack is proposed. The local and nonlocal nondimensional frequency parameters of cracked nanorods are compared with some studies in the literature. Additionally, nondimensional frequencies of nanorods are presented with several tables and graphics according to different parameters such as nonlocal parameter, mode number, crack intensity and crack location. Detailed discussions of the numerical results are given, and the conclusions deduced from these discussions are summarized.