Fracture characteristics and ductility of cracked concrete beam post-strengthened with CFRP Sheet

The present paper concerns the fracture characteristics and ductility of cracked concrete beam externally bonded with carbon fiber-reinforced polymer (CFRP) sheet as well as the integration behaviors between CFRP/concrete interfacial debonding and concrete cracking.Three-point bending tests were carried out on the CFRP-strengthened cracked concrete beams with varying specimen depth and initial crack length.A straingauge method was developed to monitor the crack initiation and propagation in concrete,and the CFRP/concrete interfacial bonding behaviors,respectively.Clip gauges were used to measure crack mouth opening displacement (CMOD) and the deflection at midspan.Experimental results revealed that CFRP-strengthened specimen shows a higher load capacity under the same deformation level and a better inelastic deformation capacity compared with the unstrengthened one.For there are two manifest peak values in the obtained load versus displacement curve,the ductility of CFRP-strengthened concrete beams were investigated using index expressed as area ratio on the load versus displacement curve.The calculated results indicated that the contribution from CFRP sheet to the ductility improvement of specimen is notable when the deflection at midspan exceeded 10.5 times the first-crack deflection.

THE FRACTURE CHARACTERISTICS OF δ-Al_2O_3/AlALLOY COMPOSITES

The initial location of the crack and the controlling factor of the fracture in the squeeze casting composites δ-Al2 O3/Al-5.5Zn, δ-Al2 O3/Al-5.5Mg and δ-Al2 O3/Al-cSi were studied by microscopic observation and finite element analysis(FEA). The in situ failure processes were obseroed bU the scanning electronic microscope mp. The distributions of stress components along the fiber length in matta, and the tnean axial stresses of fiber and matrix were calculated by three-dimensional elasto-plastic FEA.It is found that the failure modes of short fiber reiwtreed metal matrix composites change with the variations of the micro-structurnl characteristics of composites, such as fiber orientation, matrix strength and intedecial bonding, etc.

Bonding and Fracture Characteristics at α_2/γ Interface in TiAl Alloy with B Addition

The bonding characteristics of (0001)α2||(111)γ interface in two-phase TiAl alloy have beeninvestigated with the recursion method. The results of bond order integral and interaction energybetween atoms are presented. The effects of B on atoms bonding both in constituent phase andat the α2/γ interface have been studied. The correlation between the mechanical propertiesof the alloy and the bonding at the interface has been discussed. The results suggest that Bsegregation to the interface benefits the ductility. This is supported by the related experiment.

Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons

The fracture characteristics of austempered spheroidal graphite aluminum cast iron had been investigated. The chemical content of the alloy was C3.2, Al2.2, Ni0.8 and Mg0.05 （in mass percent, %）. Impact test samples were produced from keel blocks cast in CO2 molding process. The oversized impact samples were austenitized at 850 and 950 ℃ for 2h followed by austempering at 300 and 400 ℃ for 30, 60, 120 and 180min. The austempered samples were machined and tested at room temperature. The impact strength values for those samples austempered at 400 ℃ varied between 90 and 110J. Lower bainitie structures showed impact strength values of 22 to 50J. The fractures of the samples were examined using SEM. The results showed that the upper bainitic fracture revealed a honey Comb-like topography, which confirmed the ductile fracture behavior. The lower bainitic fractures of those samples austempered for short times revealed brittle fracture.

Implications for identification of principal stress directions from acoustic emission characteristics of granite under biaxial compression experiments

The rock fracture characteristics and principal stress directions are crucial for prevention of geological disasters.In this study,we carried out biaxial compression tests on cubic granite samples of 100 mm in side length with different intermediate principal stress gradients in combination with acoustic emission(AE)technique.Results show that the fracture characteristics of granite samples change from‘sudden and aggregated’to‘continuous and dispersed’with the increase of the intermediate principal stress.The effect of increasing intermediate principal stress on AE amplitude is not significant,but it increases the proportions of high-frequency AE signals and shear cracks,which in turn increases the possibility of unstable rock failure.The difference of stress in different directions causes the anisotropy of rock fracture and thus leads to the obvious anisotropic characteristics of wave velocity variations.The anisotropy of wave velocity variations with stress difference is probable to identify the principal stress directions.The AE characteristics and the anisotropy of wave velocity variations of granite under two-dimensional stress are not only beneficial complements for rock fracture characteristic and principal stress direction identification,but also can provide a new analysis method for stability monitoring in practical rock engineering.

Influence of gravel content and cement on conglomerate fracture

Tight reservoirs are typically developed by horizontal wells and multi-stage hydraulic fracturing.The conglomerate reservoir is one type of tight reservoirs,which is different from homogeneous rock,such as tight sandstone.This is because that the existence of gravels makes conglomerate have strong hetero-geneity.Thus,it is difficult to grasp the fracture mechanism and the law of fracture propagation of conglomerate,which limits the efficient development of the conglomerate reservoir.In this paper,the fracture characteristics and factors influencing the fracturing of Mahu conglomerate were studied by uniaxial compression,acoustic emission monitoring and X-ray computed tomography(CT)scanning experiments.The results show that the fracture characteristics of conglomerates are influenced by the gravel content and cement.The conglomerate in the study area is mainly divided into carbonate cemented conglomerate and clay cemented conglomerate.The fracture complexity of carbonate cemented conglomerate first increases and then decreases with increasing gravel content.However,for clay cemented conglomerates,the fracture complexity increases over the gravel content.The crack development stress is a significant parameter in the crack assessment of conglomerates.This study is useful to understand the influence of meso-fabric characteristics of conglomerates on their fracturing and crack evolution and guides the design of hydraulic fracturing.

Studies on Characteristics of Fracture Lithofaciesin Wenxipo W-Cs-Rb Polymetallic Deposit,Hainan Province,China

1 Introduction Wenxipo W-Cs-Rb polymetallic depositis located in Fengshou rubidium-cesium polymetallic prospecting area in Danzhou City,Hainan Province,China.The hornstone facies,a thermal metamorphic facies,

Development Phases and Mechanisms of Tectonic Fractures in the Longmaxi Formation Shale of the Dingshan Area in Southeast Sichuan Basin, China

Shale gas has currently attracted much attention during oil and gas exploration and development. Fractures in shale have an important influence on the enrichment and preservation of shale gas. This work studied the developmental period and formation mechanism of tectonic fractures in the Longmaxi Formation shale in the Dingshan area of southeastern Sichuan Basin based on extensive observations of outcrops and cores, rock acoustic emission(Kaiser) experiments, homogenization temperature of fracture fill inclusions, apatite fission track, thermal burial history. The research shows that the fracture types of the Longmaxi Formation include tectonic fractures, diagenetic fractures and horizontal slip fractures. The main types are tectonic high-angle shear and horizontal slip fractures, with small openings, large spacing, low densities, and high degrees of filling. Six dominant directions of the fractures after correction by plane included NWW, nearly SN, NNW, NEE, nearly EW and NW. The analysis of field fracture stage and fracture system of the borehole suggests that the fractures in the Longmaxi Formation could be paired with two sets of plane X-shaped conjugate shear fractures, i.e., profile X-shaped conjugate shear fractures and extension fractures. The combination of qualitative geological analysis and quantitative experimental testing techniques indicates that the tectonic fractures in the Longmaxi Formation have undergone three periods of tectonic movement, namely mid-late Yanshanian movement(82–71.1 Ma), late Yanshanian and middle Himalaya movements(71.1–22.3 Ma), and the late Himalayan movement(22.3–0 Ma). The middle-late period of the Yanshanian movement and end of the Yanshanian movement-middle period of the Himalayan movement were the main fractureforming periods. The fractures were mostly filled with minerals, such as calcite and siliceous. The homogenization temperature of fracture fill inclusions was high, and the paleo-stress value was large; the tectonic movement from the late to present period was mainly a slight transformation and superposition of existing fractures and tectonic systems. Based on the principle of tectonic analysis and theory of geomechanics, we clarified the mechanism of the fractures in the Longmaxi Formation, and established the genetic model of the Longmaxi Formation. The research on the qualitative and quantitative techniques of the fracture-phase study could be effectively used to analyze the causes of the marine shale gas fractures in the Sichuan Basin. The research findings and results provide important references and technical support for further exploration and development of marine shale gas in South China.

Fracture development and hydrocarbon accumulation in tight sandstone reservoirs of the Paleogene Huagang Formation in the central reversal tectonic belt of the Xihu Sag, East China Sea

By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement(12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement(9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging(9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day(0–3 Ma ago), the fractures are tectonic ones developing successively;matching with the second stage(0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas.

Numerical analysis of seepage flow characteristic of collapse column under the influence of mining

Based on the importance of fractured rock mass seepage research, in order to analyze seepage flow characteristics of collapse column under the influence of mining, a method by embedding fractured rock mass flow solid coupling relationship into FLAC3D internal flow models is presented according to fluid-solid coupling theory and strength criterion. A calculation model of numerical analysis was established, and the influences of mining pressure and plastic damage to pore water pressure and seepage vector change rule were studied. The results show that collapse column is the main channel of confined water seepage upward. The impact is not big when the workface is away from the collapse column. But when the workface is nearing a collapse column, there will be a seepage channel on a side near the workface, in which seepage vector and head are comparatively large. With workface pushing through collapse column, the seepage channel transfers to the other side of the column. In addition, when the plastic damage area within the collapse column breaks through, a "pipeline flow" will be formed within the column, and seepage field will change remarkably and the possibility of water bursting will be greater.

Effect of hydraulic fracturing induced stress field on weak surface activation during unconventional reservoir development

Unconventional reservoirs usually contain many weak surfaces such as faults,laminae and natural fractures,and effective activation and utilization of these weak surfaces in reservoirs can significantly improve the extraction effect.In hydraulic fracturing,when the artificial fracture approaches the natural fracture,the natural fracture would be influenced by both the original in-situ stress field and the hydraulic fracturing-induced stress field.In this paper,the hydraulic fracturing-induced stress field is calculated based on the relative position of hydraulic fracture and natural fracture,the original in-situ stress,the net pressure inside the hydraulic fracture and the pore pressure of the formation.Furthermore,the stability model of the natural fracture is established by combining the Mohr-Coulomb rupture criterion,and extensive parametric studies are conducted to explore the impact of each parameter on the stability of the natural fracture.The validity of the proposed model is verified by comparing with the reservoir characteristics and fracturing process of the X-well 150e155 formation in the Songliao Basin.It is found that the stress field induced by the hydraulic fracture inhibits the activation of the natural fracture after the artificial fracture crossed the natural fracture.Therefore,for similar reservoirs as X-well 150e155,it is suggested to connect natural fractures with hydraulic fractures first and then activate natural fractures which can effectively utilize the natural fractures and form a complex fracture network.

Microstructure and properties of an Al-Zn-Mg-Cu alloy pre-stretched plate under various ageing conditions

The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging （RRA） treatment led to a combination of high strength and stress corrosion cracking （SCC） resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very fine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse η MgZn2 precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.

Study on adhesively-bonded surface of tapered double cantilever specimen made of aluminum foam affected with shear force

Aluminum foam is widely used in diverse areas to minimize the weight and maximize the absorption of shock energy in lightweight structures and various bio-materials.It presents a number of advantages,such as low density,incombustibility,non-rigidity,excellent energy absorptivity,sound absorptivity and low heat conductivity.The aluminum foam with an air cell structure was placed under the TDCB Mode II tensile load by using Landmark equipment manufactured by MTS to examine the shear failure behavior.The angle of the tapered adhesively-bonded surfaces of specimens was designated as a variable,and three models were developed with the inclined angles differing from one another at 6°,8° and 10°.The specimens with the inclined angles of 6°,8° and 10° have the maximum reaction forces of 168 N,194 N when the forced displacements are 6,5 and 4.2 mm respectively.There are three specimens with the inclined angles of 10°,8° and 6° in the order of maximum reaction force.As the analysis result,the maximum equivalent stresses of 0.813 MPa and 0.895 MPa happened when the forced displacements of 6 mm and 5 mm proceeded at the models of 6° and 8°,respectively.A simulation was carried out on the basis of finite element method and the experimental design.The results of the experiment and the simulation analysis are shown not different from each other significantly.Thus,only a simulation could be confirmed to be performed in substitution of an experiment,which is costly and time-consuming in order to determine the shearing properties of materials made of aluminum foam with artificial data.

A review of fracture mechanic behaviors of rocks containing various defects

The existence of defects in a rock mass is a vital factor complicating the mechanical response of the rock mass under stress.The characteristics of defects control the rock mass’s mechanical behaviors.This report reviews the experimental studies on the influence of simulating the internal defects of rock mass’s mechanical properties.The effects of preexisting defects(shape,size,number,angle,and other factors)on the mechanical properties and failure modes of the rock are investigated from the perspectives of rock failure and fracture mechanics.The fracture propagation mechanisms of different defective rock masses are analyzed by studying the stress state of different defective rock masses under uniaxial compression.After the defective rock samples with varying inclination angles are destroyed,the relationship among the compressive strength,the fracture propagation,and the failure characteristics is explored.The primary findings are as follows.The increase in the size and number of defects rapidly damages the mechanical properties of the rock and intensifies defect penetration and expansion.The rock mechanics characteristics of fissured rocks with different inclination angles show two primary variations:one is a continuous increase,and the other is a decrease followed by an increase.This result is strongly related to the failure pattern of rocks with preexisting fissures.This investigation is crucial for further studying the failure modes of complex defective rock masses and the geotechnical engineering related to defective rocks.

High cycle rotating bending fatigue performance and fracture behavior in a pearlite-ferrite dual-phase steel

To clarify the effects of ferrite morphologies and contents on high-cycle fatigue property of pearlite-ferrite dual-phase(DP)steel used for fabrication of commercial vehicle crankshafts,two types of DP steels with different ferrite grain sizes(S10:13.1μm and S30:21.4μm)and ferrite contents(S10:~9.5 vol.%and S30:~30.4 vol.%)were prepared.Stress amplitude-logarithm of number of high cycles to failure fatigue of the two DP steels was evaluated.Experimental results showed a fatigue strength of 510 and 400 MPa for S10 and S30 steels,respectively,at 10^(7) cycles.Fatigue cracks in S10 steel extended preferentially along the grain boundary,but it was easy for crack propagation to extend within a pearlite colony to form a zigzag crack morphology.Crack roughness was enhanced and high stress was introduced to the crack surface due to this kind of crack propagation behavior,which has positive effects on slowing down crack propagation.However,the crack propagation in S30 steel mainly occurred inside the soft equiaxed coarse ferrite grain.Analysis revealed that little stress was introduced to the crack surface.These results show that it is possible to improve high cycle fatigue strength of pearlite-ferrite DP steel by appropriately manipulating the volume fraction and microstructure morphology of ferrite phase.

Augmented reality(AR)and fracture mapping model on middle-aged femoral neck fracture:A proof-of-concept towards interactive visualization

A thorough understanding of the fracture characteristics can assist the decision-making process for surgery.This study aimed to characterize the femoral neck fractures among middle-aged patients and illustrated a biomedical visualization method using a fracture mapping model and augmented reality.We collected plain radiography and computed tomography(CT)data from 156 adult patients with a femoral neck fracture.The descriptive study showed that Type I and Type II fractures accounted for 8(5%)and 64(41%)cases.In comparison,Type IV fractures accounted for 44(28%)and 40(25%)cases according to the Garden classification.Comminuted fractures and cortical defects were identified in 14.74%and 29.49%of the cases.A fracture mapping model was reconstructed based on the CT data and demonstrated the location and distribution of the major fracture lines surrounding the head-neck junction.We also illustrated the application of augmented reality technology to visualize and interact with the patient-specific fracture model and the fracture mapping model that facilitated education,training,and surgical planning.Future studies may consider mapping other biomechanical data,such as joint loading and stress distribution,and exploring artificial intelligence via deep reinforcement learning for computer-aided fracture reduction and procedure planning.

Multi-scale fractures formation and distribution in tight sandstones—a case study of Triassic Chang 8 Member in the southwestern Ordos Basin

Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member,southwestern Ordos Basin,China.A systematic characterization of the multi-scale natural fractures is a basis for the efficient tight oil production.Based on outcrops,seismic reflections,well cores,well logs(image and conventional logging),casting thin sections,and scanning electron microscope observation,the multi-scale fractures occurrences and their influences on Chang 8 tight sandstone reservoirs are revealed.The results show that three periods of strike-slip faults and four scales of natural fractures developed,namely mega-scale(length>7×10^(7) mm),macro-scale(3.5×10^(5)<length<7×10^(7) mm),meso-scale(10<length<3.5×10^(5) mm),and micro-scale(length<10 mm)fractures.The mega-and macro-scale fractures developed by strike-slip faults are characterized by strike-segmentation and lateral zonation,which connect the source and reservoir.These scale fractures also influence the distribution and effectiveness of traps and reservoirs,which directly influence the hydrocarbon charging and distribution.The meso fractures include the tectonic,diagenetic,as well as hydrocarbon generation-related overpressure types.The meso-and micro-scale fractures improve the sandstone physical properties and also the tight oil well production performance.This integrated study helps to understand the distribution of multi-scale fractures in tight sandstones and provides a referable case and workflow for multi-scale fracture evaluation.

Effect of Long-Term Thermal Exposures on Tensile Behaviors of K416B Nickel-Based Superalloy

The effect of long-term thermal exposure on the tensile behavior of a high W content nickel-based superalloy K416B was investigated.The microstructure and the deformation characteristics were observed by scanning electron microscopy and transmission electron microscopy,and the phase transformation of the alloy during long-term thermal exposure was analyzed by X-ray diffraction patterns and differential thermal analysis.Results showed that after thermal exposure at 1000℃,the MC carbides in the K416 B alloy decomposed into M_(6)C.During tensile deformation,dislocations slipping inγmatrix crossed over the M_(6)C by Orowan bowing mechanism.With the increase of thermal exposure time,the secondary M_(6)C reduced greatly the yield strength of the alloy at room temperature.Meanwhile,the continuous distribution of the secondary M_(6)C with great brittleness in the grain boundary could become the main source of crack,which might change the fracture characteristic of the alloy from trans-granular to intergranular.