Crack Formation Process Research Articles

Analysis of Crack Width Development in Reinforced Concrete Beams.

The reliability and durability of reinforced concrete structures depend on the amount of concrete cracking. The risk associated with cracks generates a need for diagnostic methods for the evaluation of reinforced concrete structures. This paper presents the results of a study of 10 single-span reinforced concrete beams to follow the process of crack formation and changes in their width. The beams were loaded to failure with two forces in a monotonic manner with unloading and in a cyclic manner. Continuous observation of the crack formation process was provided by the digital image correlation system. The simplified method for estimating the maximum crack width is proposed. The presented results confirmed the stochastic character of the process of crack formation and development. The maximum crack widths calculated on the basis of the proposed formula were on the safe side in relation to those calculated according to Eurocode 2. It was also confirmed that the distances between cracks do not depend on the loading manner. Hence the density function describing the distribution of distances between cracks can be used to assess the condition of reinforced concrete elements. The research has also shown the suitability of the DIC system (ARAMIS) for testing concrete elements.

MATHEMATICAL MODELING OF THE PROCESS OF POSSIBLE DAMAGE TO NPP PIPELINES DUE TO THEIER LOCAL EROSION AND CORROSION SINKING

The article is devoted to computer modeling of the process of possible damage to the pipelines of nuclear power plants that received local thinning of the walls.The geometric features of pipelines are essential not only for assessing the intensity of erosion and corrosion of a metal, but also for the final strength assessment, in particular, when calculating local thinning of the walls of pipelines. The accuracy of forecasts of the life of pipeline structures directly depends on the ability to evaluate the strength of pipeline elements that have received local thinning, especially if these elements themselves have a complex shape. Such elements include elbows, nozzles, adapters (transitions), shells. All of these elements are thin-walled structures of complex shape. Computer modeling of the process of crack formation and growth requires solving several problems. A review and critical analysis of the available experimental information on the growth rates of cracks in specific structural materials and in the environment that is characteristic of given operating conditions is required. An assessment of the crack growth rate is impossible without determining the stresses and strains in the shell structure of complex spatial geometry. The development of routine criteria for crack growth and their attachment to a stress-strain state calculation program is the next task. To solve these problems, a complex of three-dimensional finite element methodologies and programs based on the theory of thick shells has been created. The article presents some results of calculating the stress-strain state of adapters and the growth of cracks in them.

In-situ observation and finite element analysis of fretting fatigue crack propagation behavior in 1045 steel

In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element analysis. in-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation process. The fretting fatigue tests under different load conditions are carried out, then the lifetime and fracture surface are obtained. The crack propagation rates under different loading conditions are measured by in-situ observations. With in-situ observation, crack initiation location and direction are analyzed. Finite element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate, and establishing crack growth rate model. From fitted S-N curve, it turns out that smaller load ratio leads to higher lifetime. Crack initiates slightly below the point equivalent to line contact of the contact surface in different test conditions, and crack direction shows no obvious relationship with load parameters. The established crack growth rate model well agrees with the test results.

Cracking of Ti2AlNb-based alloy after laser beam welding

Crack formation process in the Ti-23Al-23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy based on orthorhombic titanium aluminide (Ti2AlNb), especially in the weld zone (WZ), heat-affected zone (HAZ), and base metal (BM), during laser beam welding (LBW) at room temperature was considered. It was determined that the crack spread in the WZ throughout the crystallites of β-phase. In the HAZ, cracks spread throughout the globular β-phase crystals body and along the edges of globular α2- and O-phases. The cracking stopped in BM due to a mechanism similar to shear ligament toughening. Cracking can be effectively suppressed by increasing the LBW temperature.

ПРОЧНОСТЬ ИЗГИБАЕМЫХ ЖЕЛЕЗОБЕТОННЫХ ЭЛЕМЕНТОВ С ЗОННЫМ АРМИРОВАНИЕМ ИЗ СТАЛЬНОЙ ФИБРЫ

On the basis of theoretical and experimental studies, the assumptions and calculation method for flexible and compressed-curved reinforced concrete structures with zone reinforcement made of steel fiber operating under static and short-term dynamic loads are formulated. In the developed method for calculating the strength of normal and inclined sections, a nonlinear deformation model is implemented, which is based on the actual deformation diagrams of materials. To confirm the main results of this method, numerical and full-scale experimental studies of reinforced concrete beam structures reinforced with conventional reinforcement and a zone steel-fiber layer were planned and carried out. Experimental studies were carried out under static and short-term dynamic loads. As a result of the conducted experiments, data were obtained that characterize the process of destruction, deformation and crack formation of steel-reinforced concrete elements under such types of loading. The efficiency of fiber reinforcement of normal and inclined sections for increasing the strength and deformation characteristics of reinforced concrete structures is revealed.

Study on Formation Mechanism of Zonal Disintegration of Surrounding Rock in Deep Tunnel

With the construction of underground rock engineering, the surrounding rock in deep tunnels appears zonal disintegration of fracture and intact zones alternate distribution, which is a special engineering geological phenomenon. This study establishes a partitioned fracture model under the coupling of high in-situ stress and osmotic pressure, and identify the key influencing factors of the fracture model. Furthermore, a stress intensity factor (SIF) of initial cracks on surrounding rock elastoplastic boundary is derived using the transformation of complex functions. Considering the high seepage pressure of the surrounding rocks, a zonal fracture initiation criterion is established combined with the analysis of redistributed stress fields. Finally, the obtained criterion is embedded into an extended finite element method (XFEM) platform for numerical simulation. Taking the maximum circumferential tensile stress as a cracking criterion, the propagation trajectory of rock cracks is traced by contour methods. Calculation results have realized the modelling of a whole process of crack initiation, propagation, and formation. And the established criterion has be verified.

Controlling coal rock massif hydro-treating process by instrumental and geophysical methods

Coal rock massif hydro-treating methods accompanied by crack initiation and propagation under influence of injected fluid pressure and applied for preventing gas-dynamic phenomena are studied in the article. The perspectives of applying acoustic methods for controlling the process of crack formation and effective preventive measures are also studied.

Impact of cracks on vibration parameters and lifetime of hydraulic units

One of the key factors determining the hydraulic unit reliability and lifetime is the current technical state of the runner and, first and foremost, its lack of dangerous cracks. Dynamic load throughout the mode range affects the crack occurrence and development. Blade system crack formation process depends heavily on actual equipment operating modes. Yet, modern hydraulic unit diagnostics systems tend not to record crack formation on its early stages. As a result, routine maintenance often reveals long cracks which require significant financial and time resources to repair. Vibration-based diagnostics systems reveal cracks during operation ineffectively: calculations of the Francis (hydraulic) turbine’s runner prove that. High structural rigidity and specific eigenforms don’t permit correlating the blade dynamics to measured vibration parameters. Analytic approach based on calculating fatigue strength and fracture mechanics methods could solve the problem mentioned above.

On the issue of scientific substantiation of the method for forecasting the hazardous properties of coal seams

The study of filtration permeability of the barrier pillar between the isolated fire section and the worked longwall ventilation drift was completed using the mathematical model developed by the authors for coupled processes of rock deformation and gas filtration. The problem is solved involving a finite element method implemented in the author's programme. When solving, the parameters of width of the barrier pillar and thickness of the mined coal seam were varied. The article presents the results of calculating the values of geomechanical parameters, permeability coefficients and gas pressure in the filtration area. It is shown that with an increase in thickness of the coal seam, intensity of the crack formation process increases in the barrier pillar. With small pillar width, the coal seam along its entire length is in a disturbed state, as well as the rocks of its roof and soil. The filtration areas around the worked longwall ventilation drift and the isolated conveyor drift are connected both along the coal seam and along the host rocks. This means that at a certain value of the pressure drop, filtration of fire gases into the mine workings of the worked excavation section is possible. With a greater width of the pillar, between disturbed rocks around the mine workings of the worked section and isolated fire one, there is a barrier made of undisturbed rocks. That impermeable barrier prevents gas filtration from one mine workings to another. Its width decreases with an increase in the thickness of the coal seam. Based on the results of calculating gas pressure, it is shown that methane from the coal seam moves into atmosphere of the mine workings, where gas pressure has lower values. At the same time, for the accepted conditions, with a pillar width of 20 m and a seam thickness of 0.4 m, gas is filtered from the mine workings of isolated fire section into the adjacent excavation section along the disturbed rocks of the seam roof. In the rest of the cases, the areas of low pressure around the mine workings are separated by zones of higher pressure, exceeding pressure of fire gases in the isolated section. Filtration of fire gases into atmosphere of the worked excavation section is impossible under such conditions.

The influence of working seam thickness on permeability of barrier pillars near isolated fire sections

The study of filtration permeability of the barrier pillar between the isolated fire section and the worked longwall ventilation drift was completed using the mathematical model developed by the authors for coupled processes of rock deformation and gas filtration. The problem is solved involving a finite element method implemented in the author's programme. When solving, the parameters of width of the barrier pillar and thickness of the mined coal seam were varied. The article presents the results of calculating the values of geomechanical parameters, permeability coefficients and gas pressure in the filtration area. It is shown that with an increase in thickness of the coal seam, intensity of the crack formation process increases in the barrier pillar. With small pillar width, the coal seam along its entire length is in a disturbed state, as well as the rocks of its roof and soil. The filtration areas around the worked longwall ventilation drift and the isolated conveyor drift are connected both along the coal seam and along the host rocks. This means that at a certain value of the pressure drop, filtration of fire gases into the mine workings of the worked excavation section is possible. With a greater width of the pillar, between disturbed rocks around the mine workings of the worked section and isolated fire one, there is a barrier made of undisturbed rocks. That impermeable barrier prevents gas filtration from one mine workings to another. Its width decreases with an increase in the thickness of the coal seam. Based on the results of calculating gas pressure, it is shown that methane from the coal seam moves into atmosphere of the mine workings, where gas pressure has lower values. At the same time, for the accepted conditions, with a pillar width of 20 m and a seam thickness of 0.4 m, gas is filtered from the mine workings of isolated fire section into the adjacent excavation section along the disturbed rocks of the seam roof. In the rest of the cases, the areas of low pressure around the mine workings are separated by zones of higher pressure, exceeding pressure of fire gases in the isolated section. Filtration of fire gases into atmosphere of the worked excavation section is impossible under such conditions.

Numerical simulation of fiber-matrix debonding in single fiber pull-out tests

The present work deals with the numerical crack simulation of fiber-matrix debonding in single fiber pull-out tests. For this purpose, two models are used: a finite element model (FE model) with the cohesive zone approach and a peridynamic model. For calibration a reference experiment is applied. In addition analytical equations are used for reference values. The influence of the model parameters and the material parameters of the cohesive zone model on the force-displacement curve is investigated. Besides the free fiber length, the critical interface strength, the critical energy release rate as well as the initial interface stiffness have a great influence on the force-displacement curve of the pull-out test. From the crack simulation it can be seen that Mode I has an influence on the crack initiation, but further crack growth after initiation is dominated by Mode II. The FE model can be calibrated in a way that the crack initiation point and the maximum force correspond to the reference experiment. The peridynamic model depicts a comparable crack formation process.

Effects of High-Field Velocity Saturation on the Performance of V-Doped 6H Silicon-Carbide Photoconductive Switches

Velocity saturation characteristics of a V-doped 6H silicon-carbide (SiC) photoconductive switch under high electric field are presented. A vertical-geometry switch device based on vanadium-compensated SiC is triggered by a 532-nm laser pulse with light peak power of several hundred kW and operational voltages from 2 to 20 kV. The maximum electrical peak power achieved by the device is up to 1 MW (~50-A, 20-kV, 1.1-ns pulsewidth). When the working voltage is increased to increase output power, the carrier-velocity saturation under high electric field unavoidably impedes the current growth and directly limits the peak power. The parameters of parallel high-field-dependent mobility are obtained with different fields and laser energies: low-field mobility μ = 227 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V s and fitting parameter β = 1.7. With working voltage increasing steadily, a crack-forming process results in device degradation as a result of large current and high electric field. The I - V characteristic curves of the simulations and tests show current saturation and possible damage under high electric field.

An Integrated Architecture for Corrosion Monitoring and Testing, Data mining, Modeling and Diagnostics/Prognostics

It has been established that corrosion is one of the most important factors causing deterioration and decreased performance and reliability in critical aerospace and industrial systems. Corrosion monitoring, detection, and quantification are recognized as key enabling technologies to reduce the impact of corrosion on the integrity of aircraft and industrial assets. Accurate and reliable detection of corrosion initiation and propagation, with specified false alarm rates, requires novel tools and methods, including verifiable simulation and modeling methods. This paper reports an experimental investigation of the detection and quantification of pitting corrosion on aluminum alloy panels using 3D surface metrology methods and image processing techniques. Panel surfaces were evaluated by laser microscopy and stylus-based profilometry to characterize global and local surface features. Promising imaging and texture features were extracted and compared between coated and uncoated aluminum panels at different exposure times under accelerated corrosion conditions. Image processing, information processing, and data mining techniques were utilized to evaluate the presence and extent of pitting corrosion. A new modeling framework for corrosion stages is introduced that emphasizes the representation of pitting corrosion and ultimately the crack formation process. Detection and prediction of the evolution of corrosion stages relies on data, a particle filtering method, and the corrosion propagation model. Results from these experimental studies demonstrate the efficacy of this proposed methodology.

Вивчення процесів формування підповерхневих тріщин у ромбічній сортовій заготовці при виробництві в умовах металургійного мікро-заводу

Вивчення процесів формування підповерхневих тріщин у ромбічній сортовій заготовці при виробництві в умовах металургійного мікро-заводу

Mechanics of oscillations and waves in the ice of the Аrctic ocean during compression and ridging

One of the main scientific and practical problems in the Arctic is the study of the dynamic state of the sea ice cover. The main parameters in the general model of drifting ice are the drift velocity vector, friction stress at the air-ice and ice-water interfaces, and the forces of dynamic interaction of ice fields. Establishing the connection between the large-scale processes in the atmosphere-ice-ocean system is necessary for developing methods of forecasting ice compression and ridging and the formation of local and extended fractures and leads, which help improve the existing climate models. The main aim is to obtain results of full-scale instrumental measurements of parameters of ice large-scale mechanics and dynamics, which provide a physical basis for explaining the nature of observed large-scale ice processes and allow one to perform physical parametrization. To accomplish this aim and evaluate the physicomechanical condition of the drifting ice cover of the Arctic Ocean, the “Transarktika-2019” expedition performed a real-time ice monitoring in April 2019. The investigation was conducted using seismometers and tiltmeters installed on the ice such that they formed a triangle with the sides measuring up to two kilometers. Data has been obtained on the wave and oscillation processes of crack formation, compression and ridging of ice. The possibilities of deciphering the initial data on the physics of wave and oscillatory processes in the icewater system considerably increase when using the known methods of processing seismic signals. With use of spectral Fourier analysis wavelet-transformation of oscillations significanlty extending possibilities of the seismic method at revelation of prognostic signs of crack formation and compression was applied. It is shown that the dynamics of ice processes can be connected with oceanic swell and tidal events. A possibility is created for obtaining new results in the investigation of large-scale mechanics of sea ice.

Preparation and characterization of self-healing microcapsules of asphalt

Cracking is one of the main forms of damage to asphalt pavement. The self-healing process of asphalt pavement is the inverse process of crack formation and development. Strengthening the self-healing ability of asphalt materials could restrain cracking and prolong the service life of asphalt pavement. Microcapsule self-healing technology has achieved good results in the field of microcrack repair and has promising research prospects. In this study, two kinds of microcapsules, CA-mic and E-mic, were prepared by in situ polymerization with a self-developed rejuvenator CA and modified epoxy resin as core materials. The micromorphology, thermal stability and chemical structure of the microcapsules were characterized by fluorescence microscopy (FM), scanning electron microscopy (SEM), laser particle size analyzer (LPSA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). The freeze-heating cycle test was adopted to evaluate the durability of the microcapsules. The results showed that the two prepared microcapsules were spherical in shape and uniform in particle size distribution, with compact encapsulation and good durability. Additionally, they could survive under high temperature during asphalt construction. The rheological test of bending beams showed that the low temperature crack resistance of asphalt was improved after adding two kinds of microcapsules. The “Fracture - Healing- Fracture” tests showed that the two microcapsules could enhance the self-healing ability of the asphalt, and the maximum self-healing rate of the CA-mic could reach 52% and that of the E-mic could reach 47%.

A STOCHASTIC MODEL FOR CRACK INITIATION LIFE PREDICTION OF AN AUSTENITIC STAINLESS STEEL UNDER CONSTANT AMPLITUDE LOADING

Predicting crack initiation life (CIL) of a mechanical component or a structure in service remains difficult since the crack formation process is of stochastic nature. To ensure a high level of safety and reliability, it is essential to have an appropriate probability distribution law of the CIL to ensure that cracks can be detected before reaching a critical length. In the present study, a stochastic model is used to predict the number of cycles corresponding to the formation of a crack 500 μm long resulted from the nucleation, growth, and coalescence of multiple microcracks. The model is applied in the case of a 316L austenitic stainless steel for different plastic strain ranges.

Crack formation process in ceramic Nd:YAG

We estimated the crack formation process by analyzing the crack type and thermal stress of 2 at.% and 4 at.% ceramic Nd:YAGs damaged by pump power in an end-pumped ceramic laser. Lateral cracks, radial cracks, and median cracks occurred sequentially in the ceramic Nd:YAG when the tensile stress caused by the pump power increased over 160 MPa and decreased.

Experimental study of the main controlling factors of desiccation crack formation from mud to shale

Numerous studies have been done on the formation mechanism and distribution law of desiccation cracks. However, the formation mechanism of micro-cracks in shale is still unclear. In this study, the most common clay minerals of shale (montmorillonite, illite, kaolinite) were experimentally studied to identify the main controlling factors of desiccation crack formation, and show the influence of these factors on crack geometry. To imitate the process of desiccation crack formation, a diagenetic crack imitator machine and a laboratory air drying were used. The results show that clay content, temperature, and thickness of the clay layer are the main factors that control the crack formation. Our samples suggest that crack width increases with increased montmorillonite content, and decreases with increased kaolinite and illite content. Temperatures of 100 °C and 230 °C induced further desiccation cracks in pure mineral solutions, while the mixed samples did not show any additional crack propagation. Crack width was shown to increase with layer thickness. In addition, 92% of all samples exhibited the three-armed pattern, where the angle distribution was between 90-100 and 170–180°. We further propose that the water content in the dehydration process plays an important role in the formation of cracks, and the crack formation can be divided into four stages. The majority number of cracks occur at the initial and development stages, where the main mono-crack forms and starts to divide. The cracks intersect each other at the complex third stage. Ultimately, at the weak stage the last few random thin cracks might form. The formation of the first crack also depends on clay type and varies in time within an order from early to late: montmorillonite, kaolinite, illite. Based on the visual similarity between diagenetic micro-cracks in shale and mud desiccation cracks, as well as their mechanism of cracks formation, this study may provide a new perspective on the formation of the diagenetic micro-cracks in shale reservoirs and complements previous studies through virality of pure homogeneous and mixed clay samples.

Multiscale analysis and experimental validation of crack initiation in quasi-isotropic laminates

A multiscale approach comprising laminate-scale finite-element (FE) analysis and fiber-diameter-scale periodic unit cell (PUC) analysis was developed to predict matrix microcracking in quasi-isotropic laminates; further, this method was validated through comparison of our predictions with experimental results. In the mesoscopic FE analysis, the nonlinear deformation in the unidirectional laminae was incorporated to reproduce the deformation behavior of the laminate and obtain deformation histories at the locations of expected crack initiation in the laminate. In the microscopic analysis, the nonlinear behavior and crack initiation in the matrix resin were simply modeled by an elasto-viscoplastic law and a stress-based failure criterion, respectively. To predict crack initiation considering both the macroscopic deformation fields and the microscopic heterogeneity of the material, the mesoscopic FE analysis was conducted first. Subsequently, the microscopic PUC analysis was undertaken based on the strain histories obtained from the mesoscopic analysis. Our multiscale approach was applied to quasi-isotropic laminates with several laminate configurations to predict the matrix cracks in the 90∘ ply of the laminates. In addition to referring to experimental data cited in literature, initial and transverse cracks were observed when conducting tensile tests of quasi-isotropic laminates using the in situ replication technique and ex situ X-ray computed tomography. Through comparison of the predicted values with experimental results quoted in literature and obtained in this work, we validated the prediction capability of our multiscale analysis and evaluated the process of crack formation from the mesoscopic and microscopic points of view. Moreover, we examined the sensitivity of the predicted results to fiber arrangement and the influence of constitutive and failure modeling of the two-scale analysis on the predicted cracking strains. The reported method can predict initial and transverse cracks on quasi-isotropic laminates; further, it depicts the damage progress wherein microcrack nucleation and coalescence occurring before the full-width transverse cracking in laminated composites are observed under tensile loading conditions.