Search

With the increasing application and study of lightweight and high strength fiber reinforced polymer composites in ocean industry, the structural failure problem of composite pressure hulls has attracted great attention from many researchers in China and globally. Analysis of the structural failure mechanisms is foundational to the design of deep-sea composite pressure hulls, since nowadays the design rules of pressurized vessels is mostly formulated according to their failure modes. Hence, this paper aims to review the research on the structural failure of composite pressure hulls in deep sea settings. First of all, the applied research status on composite material in marine equipment is analyzed, including inspection modalities for composite pressure hulls. The review then focuses on the three main failure modes, namely overall buckling, material failure and snap buckling of the deep-sea composite pressure hulls. The study identifies further problems of composite pressure hulls to be solved through the application of the deep sea equipment research, aiming to provide a reference for the study of mechanical behavior, ultimate strength computation, and design of thick composite pressure hulls for deep sea equipment. [ABSTRACT FROM AUTHOR]

With the continuous in-depth development of oil and gas resources, unconventional oil and gas resources show great potential. In recent years, 70% of the proven oil and gas resources in China are unconventional oil and gas resources. Therefore, the development of unconventional oil and gas resources has a huge impact on the increase in the production capacity of the oil and gas resources in China. Staged fracturing of horizontal wells is the main technical measure to improve unconventional oil and gas production stimulation. In the fracturing process, the soluble bridge plug technology can not only effectively solve the problems of large investment in drilling and grinding of composite bridge plugs, high risks, self-locking of oil, casing deformation, etc., causing the bridge plug to be unable to be drilled and remove and affect subsequent production but also can successfully make up for the problems of subsequent production. The large-diameter bridge plug cannot achieve the full diameter of the wellbore and cannot carry out operations such as postproduction logging and refracturing, which can ensure the integrity of the wellbore to the greatest extent. In this paper, HNBR was used to modify MPU to prepare HNBR/MPU soluble rubber, and the effect of blending ratio on the vulcanization characteristics, mechanical properties, compatibility, thermal stability, and degradation behavior of composites was studied. The results showed that with the increase in the amount of HNBR, the degree of cross-linking of the composite material decreased gradually, and the mechanical properties decreased slightly. With the increase in the amount of HNBR, the thermal stability of the material decreases. The use of HNBR greatly reduced the degradation rate of MPU. When the amount of HNBR was changed from 20 phr to 40 phr, the tensile strength retention rate of the composite after soaking increased from 14.73% to 39.72%. [ABSTRACT FROM AUTHOR]

Bridge weathering steel produced in China has been used in bridges worldwide. Bridge weathering steel Q345qNH is the most common steel grade used in China and for export, the fatigue crack growth thresholds and fatigue crack growth rate (FCGR) parameters of Q345qDNH are investigated in this paper. The FCGR tests are conducted on 9 compact tension (CT) specimens of base metal and butt welds respectively, the fatigue crack growth threshold tests are conducted on 3 CT specimens of base metal. The fatigue crack growth threshold decreases as stress ratio increases, Q345qDNH has considerably higher fatigue crack growth thresholds than Q345qD and universal thresholds given in BS7910. Two data processing methods are employed to calculate characteristic FCGR parameters. The FCGR parameters determined by small sample method are more conservative than those obtained by group sample method. The FCGR of butt weld increases faster than that of base metal as stress intensity range increases, therefore butt weld is more brittle than base metal. Q345qDNH has slower FCGR than Q345qD and the universal parameters given in BS7910, hence Q345qDNH has better fatigue resistance. The fatigue crack growth was numerically simulated, the largest difference between predicted fatigue life and tested fatigue life is 11.3%, therefore fatigue life can be predicted reasonably accurately by numerical simulation on the basis of measured FCGR parameters, which illustrates the potential and effectiveness of numerical simulation for fatigue assessment of complex bridge structural details based on fracture mechanics and the significance to obtain accurate FCGR parameters. • Fatigue thresholds and FCGR parameters of Q345qDNH have been measured. • Bridge weathering steel Q345qDNH has proved to have excellent fatigue resistance. • This paper provides essential data for fatigue assessment of bridges using Q345qDNH. • Fatigue life of CT specimens are accurately predicted by numerical simulation. [ABSTRACT FROM AUTHOR]

Rock burst is a catastrophic dynamic disaster caused by sudden failure and instability of coal, loading paths play an important role in the failure of coal, the coal failure process is associated with charge exception information. Hence, violent coal failure mechanics and time-frequency domain distribution of charge signal such as rock burst under different loading paths should be studied in-depth. In this paper, grade and cyclic loading test were carried out for coal with impact tendency samples produced by blocks cored from 800 depth in Xiaoqing coal mine of the Tiefa coal group in northeast China. Theory discussion was carried out for the result of stress and strain, frequency-spectra analysis was conducted for the wavelet charge data, figures showing the evolution mechanism of mechanical properties and the relationship of time-frequency domain amplitude of charge signals in coal with different loading paths and stage were obtained. The failure process and characteristics of coal under different loading paths were summarized. It found that the loading path changed the manner of energy accelerate-release, there were more plastic strain generation in coal under cyclic loading than that under grade loading, the former was more likely to cause greater damage and failure, then the strength of coal under cyclic loading is generally lower than that under grade loading, an energy conversion mechanical model of stress, damage and deformation was developed and explained the effect of the loading path. Charge signal was primarily distributed in the strengthening and peak stages, where there was a high amplitude pulse at each stress drop. The charge pulse was a type of low frequency signal with a primary frequency distribution range of 1 -100 Hz. Discussion on the charge generating mechanism from the perspective of friction slip, it demonstrated that the charge obtained during the coal failure process directly to stress loaded on and damage, the result verified it better. We propose that the research results in this study could be efficiently applied to daily mining activities, to provide an early warning and effectively avoid rock burst disaster. [ABSTRACT FROM AUTHOR]

This paper predicts the possibility of water inrush from a confined aquifer under the action of mining activities and water pressure. The study uses numerical analyses to evaluate stress redistribution and crack growth which result from coal extraction operations. Two models are presented in this study. By simplifying the distribution of the disturbed vertical stress on the coal seam and floor around a working face, a model is established to analyze the additional stresses in the floor strata induced by mining activities. And some distribution features of all the additional stress components are described. By using the superposition principle in fracture mechanics, another model is developed to analyze the crack growth in the floor strata under the action of disturbed stresses and water pressure. And the stress intensity factors at the crack tip are presented and the process of crack growth is obtained in the advancement of a working face. Because of discretizing only loading areas and crack surfaces, the present methods can obtain the accurate numerical results. Finally, some suggestions are made for preventing the water inrush from a confined aquifer. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract  Whilst the Yingxiu-Beichuan fracture is considered to be the primary seismic fracture of the Wenchuan earthquake, surface ruptures, geohazards, abnormal hydrological phenomena and aftershocks occurred not only along this central fracture but also along the so-called “secondary seismic fractures” parallel to it. The paper reports some of the geohazards associated with these secondary seismic fractures and concludes that the contribution of these is important in the overall scale of the effect of the Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

The cracking damage of the high-speed railway slab track seriously affects its durability and service life. The cracking condition index (CCI) for the China railway track system (CRTS) III prefabricated slab track and the corresponding calculation methods were proposed to realize the scientific management and guide the maintenance more reasonably. In order to ensure the objectivity and rationality of each parameter's weight, first, the finite-element (FE) model of CRTS III prefabricated slab track with surface cracks was established to analyze the crack growth behavior, based on the fracture mechanics theory. The results show that the crack growth rate of transverse cracks at the same depth is greater than that of longitudinal cracks and significantly greater than that of through cracks. Then 437 valid questionnaires were obtained through expert consultation. Combining the subjective factors of expert evaluation with the objective factors of numerical simulation, the Delphi method and interval analytic hierarchy process (AHP) were used to establish the structural hierarchy, which could determine the weight of each parameter. After that, it was applied to an engineering example. The results show that the weights obtained by the comprehensive method combining the Delphi method and the interval AHP are more objective. Furthermore, the evaluation method was applied to the engineering example, obtaining the slab track CCI within [70.91, 72.83] points, which verified the rationality of the evaluation method. In short, the index evaluation method can quantitatively analyze the cracking severity of the track slab, which has important reference and practical value for line maintenance. [ABSTRACT FROM AUTHOR]

Goal rock is naturally comprised of a large number of randomly-distributed discontinuities such as joints/ cracks and weak interlayers. The geometry, size, distribution, contact and infillings of the discontinuities essentially govern the physical/mechanical properties and failure mechanism of coal rock. It is intractable to theoretically and accurately characterize the discontinuities and the governing influences on the deformation and failure of coal rock. This paper presented a 3D finite element model of fractured coal rock representing the irregular geometry, complex contact, intersection, and network features of interior joints and cracks by means of high-resolution GT techniques and 3D reconstruction methods. The birth-death clement algorithm was employed to characterize and visualize the stress field distribution, spatial distribution of failed areas, energy dissipation and release during the deformation and failure of fractured coal rock when subjected to the varied unloaded conditions. The analysis discloses the determinative influences of the unloading paths and the properties of joints and cracks on the macro mechanical behavior and the failure mechanism of fractured coal rock. The paper presents a promising way to identify and characterize the governing effect of unloading due to deep excavation on real fractured coal rock. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Civil Engineering is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As a kind of existing and potential geological disaster at the source of avalanche on cliffs or steep slopes, perilous rock has developed in the western area of China widely, and it has posed a serious threat to highways, railways, pipelines, cities, and mining for a long time. More than ten years of engineering experience have shown the necessity and importance to pay our attention to the avalanche sources in active collapse mitigation. The support–anchor combined technique is devoted to the active hazard mitigation measures of perilous rock. This paper introduces fracture mechanics to investigate the design procedure of the support–anchor combined technique. To obtain reasonable design parameters of the technique, both stability assessment criterion and three safety classes of protection engineering for perilous rock is proposed, further, stable analysis methods for various types of perilous rock are established by using fracture mechanics. Abiding by the idea that to improve stability coefficient to a higher level, the support force of structure and the anchorage force of anchorbolt from the support–anchor combined technique are introduced into stability analysis methods established above, which can estimate the section dimension of support subunit and the amount of anchorbolt of the technique. Engineering applications of the technique in thousands of protection engineering have identified the remarkable effectiveness [ABSTRACT FROM AUTHOR]

• The crack propagations and patterns are described. • Presence of reinforcement bars has a significant influence on cracking behavior. • Influences of primary design parameters on cracking behavior are discussed. • Equations for estimating the reinforcement stress are developed. • Analytical model for predicting crack width is proposed. This paper aims to clarify flexural cracking behaviors (e.g., crack width and crack spacing) of a lightweight deck system composed of steel orthotropic deck and thin ultra-high performance concrete (UHPC) layer. Forty steel-UHPC composite plates and eight steel-UHPC composite beams were tested to explore their cracking characteristics in transverse and longitudinal directions, respectively. The influences of reinforcement ratios, the thickness of the UHPC cover, the spacing of stud shear connectors, and the depth of the UHPC layers on cracking behaviors were examined. According to the bridges with composite deck systems recently constructed in China, the same spacing of reinforcing bars were employed in longitudinal and transverse directions, which ranged from 30 and 75 mm. The test results show that the occurrence and growth of cracks in UHPC layers could be effectively restricted by increasing reinforcement ratios, reducing the thickness of UHPC cover and the spacing of stud shear connectors, as well as thickening the UHPC layer. For the average crack spacing, the most significant factor was the reinforcement ratio. For the longitudinal behavior of the lightweight deck system, the steel-UHPC composite beams would fail due to the yield of steel U-ribs and the crack width in the UHPC layer would be small (e.g., 0.09–0.14 mm) when the reinforcement ratio and the thickness of UHPC cover are design appropriately. Based on the experimental observations, the formulas were developed to estimate the stress in reinforcements for the composite (steel + UHPC) lightweight deck system. The reinforcement stresses predicted by these formulas were in agreement with the experimental data. A new formulation for predicting the maximum crack width in composite (steel + UHPC) lightweight deck system was also proposed. Good agreements were achieved between the test results and the predicted values of the flexural crack width. [ABSTRACT FROM AUTHOR]

Abstract Casing failures have been systematically encountered during multi-stage hydraulic fracturing of horizontal wells in some unconventional reservoirs of China. Such failures usually manifest themselves as excessive localized radial deformation of the casing after some fracturing stages, which prevent the bridge plugs of subsequent stages from being installed to the design depths, or even result in abandonment of all remaining fracturing stages. In this paper, aiming at clarifying the casing failure mechanism during hydraulic fracturing, we performed an integrated investigation of tight-oil wells in northwest China, by analyzing the field-gained data and modeling possible mechanisms responsible for the casing failure. The investigation results show that, radial deformation of the casing due to alteration of in-situ stresses, even jointly with poor cement sheath, is far smaller than that observed in the field which could reach the order of magnitude of 1–3 cm. In contrast, we find that shear slips of pre-existing fractures/faults crossing the casing can deform the casing to the observed magnitude, justifying a conclusion that casing failure is due to the shear of pre-existing fractures/faults. This modeling-derived conclusion is supported by the records of micro-seismic events. In addition, lead impression blocks have also run for determining the deformed shape of the casing, and the results match the modeling results for the casing sheared by activated fractures/faults. Effect of fracturing pressure, in-situ stresses, length and orientation of pre-existing fractures/faults on the slippage of the fractures/faults and thus the deformation of the casing are demonstrated by a series of sensitivity studies. Additionally, some insights are derived for mitigating the casing failure in future fracturing jobs in similar unconventional reservoirs. Highlights • A statistical analysis of casing failure during multi-stage hydraulic fracturing was performed. • The reduction of casing diameter was adopted as the casing-failure criterion. • The reactivation of pre-existing fracture/faults may be the primary reason for casing failure during hydraulic fracturing. [ABSTRACT FROM AUTHOR]

In most thermal coalbed methane production practices, the average single well production is low and the economic benefit is low. In order to improve the production of thermal coalbed methane (CBM), this paper presents a dual diagnosis method for fracture morphology of thermal CBM reservoir to improve hydraulic fracturing effect. The study is carried out as follows: Firstly, improved log-log curve method to adapt to coal seam fracturing construction. Secondly, establish the inclined stress calculation model of coal seam to obtain the critical depth value. Finally, combine the improved log-log method and critical depth method to form a dual diagnosis approach. Take Baiyang River in Xinjiang as an example, obtain the traffic, rock mechanics and other parameters suitable for the Baiyang River block, the fracture morphology is verified by fracturing data. The experimental results show that the approach can diagnose fracture morphology accurately. [ABSTRACT FROM AUTHOR]

Abstract In Tarim oilfield of China, the true vertical depth (TVD) is >6000 m, non-API 140 casings are often used as the intermediate and production casing. No industry standard has been established for such high grade casing, although there are some obvious shortcomings, such as sensitivity to defects and service environment, so that the fracture failure accidents of non-API 140 casing in Tarim oilfield happens frequently. Hence, this paper presents a discussion on the technical specifications of non-API 140 casing. Firstly, the system test and analysis of physical and chemical performance of non-API 140 casing material from various manufacturers have been conducted, including chemical composition, tensile properties, impact toughness, hardness, metallographic structure. Secondly, the analysis method of required minimum transverse impact toughness (CVN min) to resist cracking for pipe body and coupling of non-API casing has been presented, by which the effects of effective thickness and non-destructive examination level (defect depth) on CVN min to resist cracking of pipe body and coupling under diffenrent internal pressure have been analyzed.Thirdly, the full-scale ductile burst test of non-API 140 casing has been performed based on API 5CT standard. Finally, based on experimental data and analysis, the technical specifications for non-API 140 casing are proposed. Cases Analysis of Tarim field shows no fracture failure occurs during the subsequent service after adopting those technical specifications. Highlights • Physical/chemical performance of non-API 140 casing from three manufacturers is obtained. • Analysis method of required impact toughness to resist cracking has been presented. • Full-scale ductile burst test of non-API 140 casing has been performed. • Technical specifications for application of non-API 140 casing are proposed. [ABSTRACT FROM AUTHOR]

The non-negligible fissures, cracks or pore structures, which are contained in rocks, have significant influences on their mechanical properties. In this paper, a novel method is developed to reconstruct a three-dimensional multiple phase rock model from the limited morphological information of Chongqing sandstone. Firstly, X-ray micro-CT experimental tests are applied to Chongqing sandstone to obtain the raw CT images and the mechanical parameters. Then, the two-point correlation function, lineal-path function and fractal function are combined to generate the reconstructed model. Next, the reconstructed model subjected to compressive loads is simulated by Finite Element-Smoothed Particle Hydrodynamics (FE-SPH) coupling method with a brittle model formulation. Finally, the initiation, propagation and coalescence of cracks are investigated during the numerical simulation. The topological and mechanical properties of the reconstructed model are carefully analyzed and compared with that of the reference model. The comparison of the numerical modeling results and the experimental data shows a good consistency. This indicates that the proposed method has the ability to predict macroscopic behaviors of rocks from limited experimental information. [ABSTRACT FROM AUTHOR]

Goaf-side entry with small coal pillars (GESCPs) has an intrinsic advantage of improving the coal recovery ratio by implementing drifts with a small pillar size next to previous goafs. This technology is increasingly gaining popularity in the longwall mining of underground coal mines in China. This study focuses on understanding the critical condition of the main roof failure above the solid coal side of the goaf-side entry and investigating the key parameters that affect the structural stability of the surrounding rocks for GESCP. Mechanical models of the main roof and multi-layer cracking structures of the side wall of GESCP were established and the limiting equilibrium equation for the structural stability of the surrounding rock was proposed. The characteristics affecting the main parameters of the structural stability of the surrounding rock were analyzed. The research findings suggest that the integrity of the coal side walls plays a major role in maintaining the structural stability of the surrounding rock for GESCP under the given cross-sectional dimensions. Other factors, including the uniform load of overburden, the width of the coal pillar, the length of the roof hanging along the goaf side, and the fracture length in the main roof of the entry side wall, are less important. The key to achieving structural stability of the surrounding rocks for GESCP is to enhance the strength of the supporting coal side walls and, especially, to ensure the integrity of the small coal pillars. These conclusions were verified by engineering practice at the 1252(1) haulage gateway in a Coal Mine in China. [ABSTRACT FROM AUTHOR]

In the present study, a total of 20 fatigue tests on notched plates of Q460C steel were carried out, where the effects of relative stress amplitude, Δσ/fy, and relative nominal maximum stress, σmax/fy, on the fatigue life of these notched plates were carefully examined. Theoretical analyses and numerical simulations were subsequently conducted, based on an ellipsoidal fracture model originally proposed by the second author, which has been validated for use as a fracture criterion of fatigue crack, to investigate the fatigue cracking in the Q460C steel notched plates. The theoretical model was further developed to estimate the fatigue life of the Q460C steel notched plates using a unified crack growth approach originally proposed by the second author. Based on the theoretical and simulated results, the accuracy of the unified crack growth approach, and the allowable stress fatigue life formula recommended in China's code GB50017-2017, were assessed. The comparisons indicate that the unified crack growth approach is able to provide a reliable fatigue life assessment for the Q460C steel notched plates. [ABSTRACT FROM AUTHOR]

Orthotropic steel decks (OSDs) are inevitably subjected to fatigue damage caused by cycled vehicle loads in long-span bridges. This study establishes a probabilistic analysis framework integrating the dynamic Bayesian network (DBN) and fracture mechanics to model the fatigue crack propagation considering mutual correlations among multiple fatigue details. Both the observations of fatigue crack length from field inspection and monitoring data of vehicle loads from the weight-in-motion (WIM) system are utilized. First, fracture mechanics-based uncertainty analysis is performed to determine the multiple uncertainty sources in the Paris crack propagation model, material property, and observation data of crack length. The uncertainty of monitoring data of vehicle loads is ignored because of its high accuracy; consequently, the vehicle-load-related uncertainty is spontaneously ignored, which is also demonstrated to be very small on the investigated actual bridges. Second, a hierarchical DBN model is introduced to construct mutual dependencies among various uncertainties and intra-correlations in the propagation process of multiple fatigue cracks at different components. Third, a stochastic traffic model is established based on the WIM monitoring data and multi-scale finite element analysis via substructure techniques to determine the probability distribution of vehicle-load-related parameters. After variable discretization, a refined exact inference algorithm in a forward–backward–forward manner is adopted to estimate the posterior distribution of equivalent initial crack length and update the established DBN model. Finally, the proposed method is demonstrated by a numerical case study and a typical application on an actual cable-stayed bridge with steel box girders using OSDs in China. The results show that the probability distribution of equivalent initial crack size can be spontaneously derived with a larger mean value than the results of conventional empirical analysis. Furthermore, the component-level fatigue reliability is tracked and predicted based on the established crack propagation model. [ABSTRACT FROM AUTHOR]

Using undrained triaxial shear tests, this study investigates the mechanical properties of fine-grained materials (silty clay and sand) which are extensively used for China's Qinghai–Tibet Railway (QTR) under different confining pressures (σ3) and freezing temperatures (T). The results show that a reduction in T causes an increase in the shear strength and elastic modulus of all the materials tested in the present study. In addition, the freezing of the silty clay has no significant effect on the type of soil behaviour (strain-hardening), whereas the freezing of the sand changes its strain-hardening behaviour to strain-softening. Supposing that the deviatoric stress–strain curves of the silty clay and sand can be divided into two segments due to a reverse bending point, it was assumed that the first segment follows a hyperbolic function. Meanwhile, the second segment is also a hyperbola, with the reverse bending point as the origin and the residual strength as the asymptote. Accordingly, a nonlinear relation constitutive model that considers σ3 and T is derived. All model parameters are identified. The reasonability of the new model was verified using the test results of the materials. A comparison of the predicted and test results shows that this model can well simulate the deviatoric stress–strain response in the failure process of the tested materials. In particular, it can reflect the residual deviatoric stress after the materials' failure. [ABSTRACT FROM AUTHOR]

With the development of infrastructure in China, the slope rock mass is affected by external forces such as roads. Tunnel excavation will cause a series of engineering problems, which are essentially caused by the instability caused by the propagation of internal cracks in the rock mass. At the same time, in practical engineering, the rock mass is formed through long-term geological structure, and its internal cracks have a variety of properties. Based on granite samples as the research object, two different angles of sample shapes, cylindrical and circular, are prefabricated. The uniaxial compression test is studied to analyze the law of inclined crack propagation in rock under uniaxial compression. Combined with the theory of fracture mechanics, the influence mechanism of crack inclination and sample shape on crack propagation in rock mass is discussed. The test results show that under uniaxial compression, the stress-strain curves of granite samples can be divided into four stages: compaction, elasticity, plasticity, and residual deformation. For the cylindrical sample, the crack initiation occurs at the tip. The compressive strength of the sample decreases with the increase in the prefabricated crack angle, and the crack initiation angle increases with the increase in the prefabricated crack angle. The fracture of the sample changes from shear slip failure to split shear failure of the penetrating rock mass with the increase in the crack angle. For the disk sample, the crack initiation location gradually increases from the tip to the middle with the increase in the crack angle, and the crack initiation angle increases with the increase in the crack angle. The peak load of the sample decreases first and then increases with the increase in the crack angle and is the minimum at 60°. The fracture mode of the sample is mainly tensile failure. When the crack dip angle is between 30° and 60°, the tensile and shear composite failure of the sample occurs, and the influence mechanism of crack dip angle and sample shape on the crack propagation path is analyzed. The research results have important theoretical significance for engineering design, construction, and stability maintenance. [ABSTRACT FROM AUTHOR]

The present work shows the application of a new methodology, to find out the seepage evolution law in the earth dam under its flow conditions, to a real field case: the Longyangxia Dam, in Qinghai, China. Under the influence of various loads and environmental factors, the creep property of dam foundation fault will be bound to be changes in the evolution of its seepage field. Therefore, the normal compression creep and the tangential shear creep of dam foundation faults are studied in this paper. Moreover, assuming the non-tension capacity in the normal direction, the normal compression creeping model and the tangential shear creeping model of the fault are established. On the basis, the time series evolution model on the permeability coefficient of the fault in dam foundation is built. These methods have already been applied into Longyangxia Dam and the result indicates that the seepage condition is normal when dam foundation seepage coefficient are getting smaller. [ABSTRACT FROM AUTHOR]

Abstract: Fracture data available for one of the rock masses (limestone) in the dam site of Yujian River Reservoir were used to build and validate a stochastic 3-D fracture network model, and to perform a REV and equivalent continuum study in 3-D. A number of relations are developed in the paper between the rock mass mechanical parameters and fracture tensor components in 3-D. Based on the mechanical parameter values obtained in every 45° direction in 3-D, the principal parameter values, principal directions and tensors are developed for rock mass mechanical parameters to represent the REV block size properties. An incrementally linear elastic, orthotropic constitutive model is suggested to represent the equivalent continuum pre-failure mechanical behavior of the jointed rock mass by incorporating the effect of joint geometry network by the fracture tensor components. [Copyright &y& Elsevier]

Abstract: To correctly simulate and predict the performance of fractured gas condensate reservoirs, based on the previous related studies, a new numerical dual porosity compositional simulation approach, suitable for fractured gas-condensate reservoirs, is proposed in this paper. Firstly, the proper pseudo-components should be correctly divided, and then single-matrix block models are built to establish the pseudo capillary pressure curve through comparing the single porosity model with the dual-porosity model. Based on the results, the dual-porosity compositional simulation method is studied. The study shows that dual-porosity compositional models can be used for fractured gas-condensate reservoirs without water drive, but not for reservoirs with strong water drive, whereas the model with pseudo capillary pressure curve can be correctly used for all kinds of fractured gas-condensate reservoirs. This new method was applied for a fractured gas-condensate reservoir of Tazhong No.1 gas field in the Tarim Basin, NW China, and the performance history of this field was well matched with the fitting error less than 5%. [ABSTRACT FROM AUTHOR]

Abstract: Pores, cavities and fractures are well-developed in deep volcanic reservoirs. The simultaneous break and extension of multi-fractures in treatment caused early stage sand-block, low fracturing success ratios, small treatment scales, and bad stimulation results. Based on the geologic characteristics of the deep volcanic rocks, this paper establishes a fracturing break and extension model of volcanic rocks, introducing with time-relevant leaking factor, peak factor and modulus factor. The model is used to better describe the process of initiation and extension of major fractures and micro-fractures. On this basis, the evaluating method for the “three highs” of pump off pressure gradient, near well friction resistance and leakoff coefficient is studied, and through field practices, integrated processing measures are formed: use rubber plugging to deal with near-well high friction resistances, use silts to seal natural fractures, adjust prepad fluid ratios, and inject multi-section silt slugs. The success ratio of volcanic rock gas reservoir fracturing is raised to 90.0% from the past 36.0%. [Copyright &y& Elsevier]

• The evolution of cracking in graphite is examined by using a displacement field obtained through ESPI. • The consumed energy during the fracturing process of nuclear graphite is estimated by using the cohesive crack model. • The influence of tension softening curves with different shapes on the fracture behavior of graphite is discussed. • Correlation of the dissipated energy during the fracturing process and fracture mechanism in FPZ is evaluated by using a tri-linear model. General consensus in the literature is that the fracturing behavior of nuclear graphite shows a certain degree of nonlinearity due to the presence of a fracture process zone (FPZ) ahead of the crack tip. Thus, it is necessary to take the influence of the FPZ into account when investigating the fracture properties of nuclear graphite. Besides, the energy-based approach has been widely used to characterize the fracturing process in the FPZ. In consideration of all these factors, this paper examines the dissipated energy during the entire fracturing process of local graphite (NG-CT-01) found in China based on the cohesive crack model. The calculation process for estimating the average dissipated energy of the crack growth is obtained through three-point bending tests, which are performed on center-notched graphite beams. Electronic speckle pattern interferometry (ESPI) is used to obtain the full-field deformation of the beams. The experimental and analytical results are then compared, and a satisfactory agreement between the two results are found, which validates the reliability and accuracy of the formulas. The tension softening curves of graphite are plotted and different cohesive laws (bilinear, tri-linear and exponential) are used to determine the average amount of energy consumed during crack extension. The tri-linear softening model is used to evaluate the relationship between the energy dissipation of the fracturing process and the fracture mechanism in the FPZ of the graphite. Furthermore, the energy dissipated during the entire fracturing process of the graphite beam is examined based on three stages: initiation of cracking, propagation of stable cracking and unstable fracturing. [ABSTRACT FROM AUTHOR]

The low temperature fracture toughness of SA738Gr.B steel is as important as its strength, but the low temperature fracture toughness tends to be more complicated due to some deterministic and random inhomogeneity, such as sampling locations. In this paper, the fracture toughness of the 60 mm thick SA738Gr.B sheet in DBT region from the surface to the center was characterized by MC method and BMC method. Considering the sampling position, a distinct trend is found that the toughness of the surface is significantly higher than that of the center and T 0 is lower from center to the surface due to the faster quenching rate. Special attention was paid on the discussion of the applicability of China-produced SA738Gr.B steel to the master curve (MC) method. It was found that the inhomogeneity caused the actual distribution of the experimental data to deviate from the three-parameter weibull distribution, and the weibull slope was lower than the theoretical value of 4. Characterizing the fracture toughness of materials containing heterogeneous components by the MC method can overestimate the fracture toughness and may pose a potential hazard. The BMC method can effectively solve the problem of material inhomogeneity. It is an evaluation method based on the actual toughness distribution of materials. The BMC method takes into account the influence of material inhomogeneity on the toughness estimation, and more accurate assessment of material scatter than the standard MC method yields a slightly conservative but more appropriate prediction. • The effect of heterogeneity on the standard master curve method is mainly to destroy the Weibull distribution. • BMC method is used to predict the fracture toughness of the transition zone of heterogeneous materials. • The MC method produces non-conservative estimates for heterogeneous materials, but the BMC method provide a suitable estimation. [ABSTRACT FROM AUTHOR]

Fracture fields in coal rocks are the main channels for gas seepage, migration, and extraction. The development, evolution, and spatial distribution of fractures in coal rocks directly affect the permeability of the coal rock as well as gas migration and flow. In this work, the Ji-15-14120 mining face at the No. 8 Coal Mine of Pingdingshan Tian'an Coal Mining Co. Ltd., Pingdingshan, China, was selected as the test site to develop a full-parameter fracture observation instrument and a dynamic fracture observation technique. The acquired video information of fractures in the walls of the boreholes was vectorized and converted to planarly expanded images on a computer-aided design platform. Based on the relative spatial distances between the openings of the boreholes, simultaneous planar images of isolated fractures in the walls of the boreholes along the mining direction were obtained from the boreholes located at various distances from the mining face. Using this information, a 3-D fracture network under mining conditions was established. The gas migration pattern was calculated using a COMSOL computation platform. The results showed that between 10 hours and 1 day the fracture network controlled the gas-flow, rather than the coal seam itself. After one day, the migration of gas was completely controlled by the fractures. The presence of fractures in the overlying rock enables the gas in coal seam to migrate more easily to the surrounding rocks or extraction tunnels situated relatively far away from the coal rock. These conclusions provide an important theoretical basis for gas extraction. [ABSTRACT FROM AUTHOR]

The impacts of temperature on the surface thermal damage of rock salt, gypsum and mudstone from the Yingcheng salt mine, China were investigated by the surface crack growth and propagation tests at different temperatures. We found that: (a) high temperature could strengthen the rock salt molecular thermal motion and weaken the cohesion among the rock salt grains, so that the grain boundaries were more prone to slip and thus develop into cracks; (b) high temperature could make the water molecules evaporate from rock specimens, which should change the physical properties of gypsum and mudstone; and (c) high temperature had a significant effect on the interface between rock salt and gypsum and mudstone, therefore it should be easy to produce cracks with white or light yellow cumulate powder here. The surface crack growth and propagation of the rock salt, gypsum and mudstone have a positive correlation with the temperature by stereo microscope and the method of binary images, which could observe the surface thermal damage properties. Finally, the fractal dimension of the rock salt surface cracks was calculated based on fractal theory, and the evolution of the surface thermal damage was found from 50 to 260 °C. [ABSTRACT FROM AUTHOR]

Water inrush of tunnel face is one of the most common geological disasters during tunnel construction in China. Aiming at the rock mass with multi fractures in water-resistant strata ahead of karst tunnel, the compressive-shear cracking property is analyzed by fracture mechanics theory and the change law of rock bridge shear strength with branch crack propagated length under karst water pressure and geo-stress is studied according to Mohr-Coulomb strength criterion. Moreover, the critical water pressure of water-resistant strata with multi fractures under tension-shear failure is deduced. The safe thickness of water-resistant strata with multi fractures ahead of karst tunnel is established based on two band theory and critical water pressure, and the influence of karst water pressure, initial crack length, crack spacing, array pitch of cracks, lateral pressure coefficient and the angle between the crack and the maximum principal stress on the minimum safe thickness of water-resistant strata are discussed. A 3 Dimension Distinct Element Code (3DEC) considering the fluid-solid coupling effect and structural characteristics of rock mass is adopted to study the catastrophe process and the influence of karst cavity scale on displacement and seepage field in water-resistant rock mass ahead of tunnel in the process of sequential excavation. The numerical simulation results show that: The transition from the single effect of unloading on the extrusion displacement of karst tunnel face to combined action of unloading and karst water pressure occurs with the tunnel face advance; The displacement at each measuring point in water-resistant strata continues to increase in the process of tunnel excavation; The extrusion displacement and water flow velocity in tunnel face suddenly increase when the water inrush pathway is about to form; With the increase of karst cavity size, the minimum thickness of water-resistant strata, the displacement of measuring point and pore pressure of crack increase. The study results provide a reference for early warning and prevention of water inrush in karst tunnel face. [ABSTRACT FROM AUTHOR]