COMPOSITES SCIENCE AND ENGINEERING

Reliability study of shear strengths of FRP reinforced concrete beams using experimental test database

CHEN Tai-gu, HU Shou-wang, LUO Yuan-bin, ZHENG Yu, LI Ming

2022, 0(2): 5-10. DOI: 10.19936/j.cnki.2096-8000.20220228.001

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FRP bars have the advantages of corrosion resistance, high tensile strength and fatigue resistance, and are expected to replace steel bars to solve the problem of corrosion of steel bars in civil engineering structures. However, the shear performance of FRP reinforced concrete structures is different from that of steel reinforced concrete structures, due to the low elastic modulus and poor bonding behavior of FRP bars. The present theoretical models of shear capacity of FRP reinforced concrete structure are established based on limited test results, and their applicability and accuracy cannot be well guaranteed. Therefore, this paper first collects the shear test data of FRP reinforced concrete beams at home and abroad, and then establishes a test database containing 478 groups of shear capacity. The global sensitivity analysis of the above test beam parameters based on Monte Carlo method was carried out, and the applicability of four existing shear capacity calculation models based on the established database was analyzed. Considering the uncertainty of parameters, Monte Carlo method and reliability theory are used to analyze the reliability of the model. The results show that the calculation model of shear capacity by using CSA code is in good agreement with the test results. The reliability index is positively correlated with the living-constant ratio and the fractional coefficient of FRP material.

Modeling investigation of tension stiffening effect in FRP reinforced concrete beams

CHEN Guo-xiong, AN Ru, FU Shi-hua

2022, 0(2): 11-18. DOI: 10.19936/j.cnki.2096-8000.20220228.002

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Fiber reinforced polymer (FRP) reinforced concrete beams exhibit greater deflection and wider cracks than traditional reinforced concrete beams. The tension stiffening effect caused by the tensile force of the concrete between cracks has a significant impact on the deformation behavior of FRP reinforced concrete beams after cracking. This paper proposes a new constitutive relationship by introducing a weakening factor on the basis of the tensile stress-strain relationship of plain concrete, which reflects the effect of tension stiffening. The deformation behaviors of finite element simulation are consistent with the experimental results. The influence of the reinforcement ratio, concrete strength, elastic modulus of FRP bars, and concrete mixed with fibers on the tension stiffening effect was evaluated by the weakening coefficient. With the increase of reinforcement ratio and elastic modulus of FRP bars, the weakening coefficient increases and the tension stiffening effect decreases. With the increase of concrete strength or the incorporation of fiber into concrete, the weakening coefficient decreases and the tension stiffening effect increases. It indicated that the model can effectively predicate the deformation behavior of FRP reinforced concrete beams.

Effect of opening angle on free vibration of composite sandwich cylindrical shells

ZHAI Yan-chun, YUE Xiu-jie, WANG Peng-hao, ZHANG Ping, YU Xiao

2022, 0(2): 19-22. DOI: 10.19936/j.cnki.2096-8000.20220228.003

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Based on the first-order piecewise displacement model and Hamilton′s principle, the structural dynamics equation of composite sandwich cylindrical shell is derived. The influence of the opening angle on the free vibration of the structure is explored. The results show that with the increase of the opening angle, the first-order mode loss factor gradually decreases, and when the opening angle is less than a certain value, the reduction rate is faster. The first mode frequency of the short shell increases gradually. The first order modal frequency of the long shell shows a tendency of decreasing first and then increasing.

Buckling analysis of anisotropic laminated plates by finite difference method

FU Wei-gang, XIONG Huan-jie, LIAO Zhe, MA Jun-chi

2022, 0(2): 23-30. DOI: 10.19936/j.cnki.2096-8000.20220228.004

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To obtain the buckling capability of anisotropic laminated plates accurately and efficiently. Firstly, the buckling governing differential equations and the boundary conditions were discretized by the finite difference method (FDM). The idea of modularization was utilized to classify and establish the related coefficient submatrices, which were then combined to a global coefficient matrix. One set of FDM equations was established with the deflection values in the mesh points as the unknown coefficients, and corresponding numerical procedures can be used for different numbers of mesh points and boundary conditions. The simply supported anisotropic laminated plates subjected to uniaxial loading condition was considered as the case study. Comparisons among the proposed FDM results, FEM results and analytical results show that the proposed FDM can get the buckling capability accurately and easily. And the coupled effect of the ply number, the aspect ratio, the ply angle and the stacking sequence on the buckling coefficient were also investigated. The study shows that the buckling coefficient increases first quickly and then slowly with respect to the ply number. The anisotropic laminated plates with aspect ratio less than one show better buckling capability, whereas the aspect ratio increases to some extent, the buckling coefficient is not sensitive to the aspect ratio. Reasonable laying angles can greatly improve the buckling behavior of the laminated plates, the optimal angle of stacking sequence for the symmetrical angle-ply laminated plates is around 60°, and antisymmetric angle-ply laminated plates have better buckling capacity than symmetrical angle-ply laminated plates.

Performance of moment redistribution in RC continuous beams strengthened with externally bonded CFRP

ZHOU Man-qing, ZHANG Zhi-mei, CHENG Wen

2022, 0(2): 31-37. DOI: 10.19936/j.cnki.2096-8000.20220228.005

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The finite element analysis model of RC continuous beams strengthened with externally bonded CFRP is established by using ABAQUS. The effect of amount of CFRP, bonded length and position of CFRP on the flexural performance and moment redistribution characteristics of RC continuous beam is discussed, and the calculation formulas of the moment modulation coefficient at mid-span and central support which varies with the amount and bonded length of CFRP are established respectively for the beams strengthened in positive moment zone and in negative moment zone. Then, the correctness of the formulas are verified by the experimental data. The results show that both the yield load and the ultimate load increase nonlinearly with the increase of amount of CFRP and bonded length for beams strengthened in either positive or negative moment zone, and the effect of amount of CFRP on the flexural capacity of beam is more significant than that of CFRP bonded length, and the moment values adjusted by the formulas of moment modulation coefficient are more consistent with the experimental moment values, which shows that the formulas established in this paper are effective and feasible.

Regression analysis of shear strength of FRP beams without web reinforcement

CUI Jun-qiang, XUE Xin

2022, 0(2): 38-44. DOI: 10.19936/j.cnki.2096-8000.20220228.006

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The difference in the properties between fiber reinforced polymer (FRP) bars and ordinary steel bars increases the complexity of the shear performance of concrete beams. For the evaluation approaches of the shear strength of FRP reinforced concrete beams without web reinforcements, the current different countries′ design codes differ in the influencing variables concerned and the influence degree of each variable. Thus, the evaluation results based on each design code differ a lot, and in addition, are generally conservative compared with that of the test results. To this end, based on a quantitative regression analysis on the test data of 184 FRP beams from published literature, this paper proposes a new evaluation formula for the shear strength of FRP beams without web reinforcement. This new formula systematically evaluate the effects of the influencing variables, and the evaluation results are more reasonable and accurate than that of the current code design formulas.

Design and testing of root junctional structure of high temperature composite rudders

ZHUANG Yang-peng, WEN Zi-hao, JIANG Sheng-da, WEI Zhong-wei, ZHAO Xi-shuai, LUO Chu-yang

2022, 0(2): 45-51. DOI: 10.19936/j.cnki.2096-8000.20220228.007

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According to the structure characteristics of the rudder and molding process of composite materials, four composite rudder schemes were designed to improve the joint strength between the metal frame and composite skin. Then the 3D finite element (FE) model was established to analyze the static strength of each rudder scheme. Finally, the optimal scheme by adding the gasket and rivets at the junction between the composite skin and metal skeleton was determined. Based on this scheme, the composite rudder was fabricated by resin transfer mould (RTM) process. The room temperature static strength test and thermo-mechanical test under bending load were completed. The analysis results show that the stress of the resin junctional area and skeleton is reduced greatly when the skeleton and skin root are strengthened by adding gaskets and adhesive bolted joint, which improves the bearing capacity of the composite rudder significantly. The experimental results show that the composite rudder has no residual deformation under 150% of the service load at room temperature static strength test. Meanwhile, the rudder maintains the structural integrity and meets the structural design requirements under combined high temperature and load, which verifies the feasibility of the design scheme.

Analysis of calculation model for ultimate stress and strain ofFRP-confined seawater sea-sand concrete square columns

LI Ben-ben, LI Peng-ju, ZHAN Chang

2022, 0(2): 52-55. DOI: 10.19936/j.cnki.2096-8000.20220228.008

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The applicability and prediction accuracy of three calculation models (Lam′s model, Wei′s model and Lim′s model) for ultimate stress and strain was evaluated based on test database of FRP-confined seawater sea-sand concrete (SSC) square columns. Higher prediction accuracy of models for ultimate stress compared with that of models for ultimate strain can be found through the analysis results. Among the three models for predicting ultimate stress, Lam′s model exhibits relatively higher accuracy with the smallest average absolute error AAE equal to 12.4%. AAE of Lim′s model and Wei′s model is 16.8% and 18.2%, respectively. Among the three models for predicting ultimate strain, Lim′s model has the highest accuracy (AAE=14.3%) while Lam′s model and Wei′s model largely underestimate the ultimate strain of FRP-confined SSC square columns, with the average ratio of prediction value to experimental value Mean equal to 68.1% and 56.1%, respectively. Therefore, among the three models, Lam′s model shows the highest accuracy for ultimate stress prediction, while Lim′s model is the best for ultimate strain prediction.

Study on the effect of different tools on hole delamination of ultrasonic-assisted drilling CFRP

JIANG Zhen-qi, HUANG Wen-jian, CAO Shi-yu, WU Chao-qun

2022, 0(2): 56-61. DOI: 10.19936/j.cnki.2096-8000.20220228.009

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In the ultrasonic-assisted drilling of carbon fiber composite material (CFRP), the selection of the tool has a significant influence on the hole quality. To analyze the influence of different kinds of drills on ultrasonic-assisted drilling of CFRP, core drills, twist drills, improved twist drills, and dagger drills were selected to conduct comparison experiments under the conditions of different spindle speeds and feed rates. The results show that the thrust force and the exit delamination area during core drilling are significantly affected by the machining parameters. The high spindle speed and the low feed rate should be selected with the utilization of core drilling. There was no evident change in the delamination area caused by the other three tools with the increase of the spindle speed. According to the characteristics of the delamination morphology of hole entrances and exits, twist drills and improved twist drills are appropriate for the drilling process with low hole quality and moderate drilling efficiency. The dagger drill is suitable for drilling thin CFRP plates with the requirements of high hole quality and moderate drilling efficiency.

Effect of carbon nanotubes on the thermal expansion coefficient ofpolyimide homogeneous composite membrane

WANG Ke-jie, LU Cheng, SHAO Hui-qi, JIANG Jin-hua, CHEN Nan-liang

2022, 0(2): 62-67. DOI: 10.19936/j.cnki.2096-8000.20220228.010

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Polyimide (PI) composite materials have excellent properties and a wide range of uses. In order to meet their different usage scenarios and thermal stability requirements, this article uses the thermal imidization process to prepare PI homogeneous composite membranes. Different content of carbon nanotubes (CNTs) were added to the PAA resin matrix, and its mechanical properties, thermal stability and coefficient of thermal expansion (CTE) were tested. The results show that when the content of CNTs is 0.1%, the tensile strength and elastic modulus of the composite film material increase by 5.6% and 8.3%, respectively. When the content of CNTs increases. The tensile strength and elastic modulus begin to decrease. The addition of CNTs does not change the molecular segments of the composite membrane material and have little effect on the thermal stability of the composite membrane. The resin matrix after CNTs are added is unevenly distributed among the fibers, and evenly accumulates on the surface of the composite membrane. CNTs have a significant effect on reducing the CTE of the composite membrane, and when the CNTs content is 0.1%, the CTE of the composite membrane is the lowest.

Study on interlaminar shear properties of PAN/BF core-spun yarn reinforced composites

QU Yan-ping, TIAN Hui-xia

2022, 0(2): 68-74. DOI: 10.19936/j.cnki.2096-8000.20220228.011

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Basalt fiber (BF) reinforced composites have been widely used in automotive, aviation, industrial buildings and other fields. However, the interlamellar properties of the composites are weak, which lead to the poor overall mechanical properties of BF reinforced composites, and the composites are easy to be layered in the process of application, which shorts their service life. In order to improve the interlaminar properties of BF reinforced composites, polyacrylonitrile (PAN)/BF core-spun yarns were prepared by conjugate electrospinning method with PAN nanofibers coated on the surface of BF. Then, PAN/BF core-spun yarn was woven into fabric and cured with resin to get composites. The interlaminar shear properties of the composites were tested and analyzed by short beam shear test. The results show that compared with BF reinforced composites, the interlaminar shear strength of unidirectional composites and plain composites reinforced with nanofiber core-spun yarns is increased by 11.5% and 8.72%, respectively. In the process of test loading, the load-displacement curve of plain weave fabric reinforced composite is more complicated than that of unidirectional strip fabric reinforced composite, and the load-displacement curve decreases in a stepped way.

Study of a new energy consumption model of composite panels under low-velocity impact

WANG Tao, YANG Yong-xin

2022, 0(2): 75-81. DOI: 10.19936/j.cnki.2096-8000.20220228.012

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Most of the current research on the impact resistance of composite materials has been carried out from the perspective of material energy dissipation, which is mainly in the form of deformation and fracture of the material. From the idea of material and structure integration, this paper proposes a new composite panel structure, which is based on the general impact resistant composite panel (called impact panel in this structure), and adds a sliding body placed inside the composite panel. Under the effect of low velocity impact, this structure form can increase the sliding body motion energy consumption in addition to the deformation energy consumption of the material, and improve the impact resistance of the composite plate. Based on the existing research, according to the characteristics of the structure type, the influence of the impact energy, the sliding angle of the sliding body, the mass of the sliding body and the thickness of the impact panel on the energy dissipation effect of the structure is numerically analyzed. The numerical simulation modeling and calculation were carried out by ABAQUS software, and the correlation analysis of the simulation results showed that the new energy dissipation mode of energy dissipation by sliding body motion is feasible. The best correlation factors with the sliding body energy dissipation effect are sliding angle, sliding body mass, impact energy, and thickness of the impact panel in order. Among them, the energy consumed by the sliding body is positively correlated with the impact energy, positively correlated with the sliding angle, negatively correlated with the sliding body mass, and negatively correlated with the thickness of the impact panel. Finally, the slider energy dissipation formula based on multiple linear regression method is established.

Effect of amino-functionalized carbon nanotube interleaves on theinterlaminar properties of carbon fibrous composites

FENG Xu, LIU Ling

2022, 0(2): 82-88. DOI: 10.19936/j.cnki.2096-8000.20220228.013

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Carbon fiber reinforced plastics are widely used, and their weak interlaminar performance can easily cause delamination damage. In order to improve the interlaminar shear resistance of the laminates, un-functionalized (CNT) and amino-functionalized carbon nanotube (NH₂-CNT) films were prepared in this paper, and their effects on the mode Ⅱ critical strain energy release rate (G_ⅡC) and interlaminar shear strength (ILSS) of laminates were investigated. The results show that when the thickness of the interleaves were 31.1 μm, compared with the blank laminate, the CNT interleaf would reduce the G_ⅡC and ILSS by 19.2% and 6.59%, respectively, while the NH₂-CNT interleaf could improve the G_ⅡC and ILSS by 51.7% and 15.4%, respectively. It is found that the good interfacial bonding between NH₂-CNT and epoxy resin contributes the improvement of interlaminar shear properties of the laminates.

Research on equivalent load in fatigue test of wind turbine generator blade

HU Jie-hua, DENG Hang, LIANG Peng-cheng, XIAO Qiong, FENG Xue-bin, FAN Sheng

2022, 0(2): 89-93. DOI: 10.19936/j.cnki.2096-8000.20220228.014

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During the operation of wind turbine generator blade, the time-domain load is constantly changing. It is the key to evaluate the fatigue life of the blade to convert the time-domain load into the equal life test load which can be loaded under the test conditions. Taking a certain type of wind turbine blade as the research object, the time-domain load of the blade in the life cycle was calculated by Bladed, and then it was converted into Markov matrix load with different amplitudes under different average values by rain flow counting method. Then the load of each section was converted into equivalent load without considering the average load. However, this method, which do not consider the influence of mean load on the fatigue life of composite materials, will lead to great error. In view of this, an equivalent conversion method considering the mean load was proposed to convert the load of each section into the equivalent load. Finally, the fatigue test scheme of the blade, which is more in line with the actual situation of the wind field and can better assess the fatigue performance of the blade structure, was designed.

Influence of moisture gain on electrical properties of quartz fiber/SiO₂ composite radome

PAN Rui, ZHOU Yong-xin, NA Wei, HAN Zi-jian, ZHANG Yan, SHI Bao-li

2022, 0(2): 94-98. DOI: 10.19936/j.cnki.2096-8000.20220228.015

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In order to understand the influence of moisture gain on the electrical properties of quartz fiber composite radome, both simulation and experiments are performed on the radome and test samples. In this paper, electrical properties of quartz fiber composite before and after moisture gain are measured using both test samples and a radome. The wave transmissivity of the radome as a function of moisture gain are presented. And, the moisture absorption model of the radome is also proposed. The results show that quartz fiber composites usually have relatively high porosity and specific surface area, and tend to absorb moisture in the air. The moisture gain influences the electrical properties of quartz composite radome. The dielectric constant is slightly affected by the moisture gain, but the loss angle tangent increases by c.a. 3 times. The unmiostureproofed radome reaches absorption equilibrium within 2 weeks at the temperature of 20 ℃~30 ℃ and humidity of 65%~75%, the equilibrium and maximum absorption is 0.083 kg/m² and 6.3‰, respectively. The wave transmissivity of the radome drops by 7%~11% during the absorption, and keeps constant thereafter. The moisture absorption process basically agrees with the Langergren pseudo-second-order kinetic model. In summary, moisture gain has innegligible influence on the electrical properties and wave transmissivity of the radome. And, proper moistureproof measures must be taken to protect the quartz fiber composite radome from gaining moisture in the air.

Experimental research on the tensile properties of glass fiber textile reinforced mortar

ZHAO Hong-fei, ZHANG Yi-wen, WANG Wei-dong, DENG Zong-cai, ZHAO Yang, LIU Qing, LIU Hao

2022, 0(2): 99-106. DOI: 10.19936/j.cnki.2096-8000.20220228.016

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In order to study the uniaxial tensile mechanical properties of glass fiber textile mortar slabs, two groups of glass fiber textile mortar slabs of tensile were tested. The ordinary mortar matrix and the epoxy mortar matrix were used, respectively, which contained different glass fiber textile layers. Through tensile test of glass fiber textile reinforced mortar slabs, the failure process and failure mode were observed, and the influence of the number of layers of fiber textile, types of mortar and age on the tensile mechanical properties of the slabs was analyzed. The test results showed that the glass fiber textile can effectively improve the bearing capacity and deformation capacity of the specimens. The peak loads of ordinary mortar specimens with 1, 2, 3 and 4 layers were 16%, 107%, 211% and 357% higher than that without glass fiber textile. The peak loads of epoxy mortar specimens with 1, 2, 3 and 4 layers were 41%, 50%, 130% and 249% higher than that without glass fiber textile, respectively. The peak loads of slabs increased obviously with the increase of glass fiber textile layers. When the glass fiber textile layers were the same, the peak loads of epoxy mortar slabs and its corresponding deformation and ultimate deformation at failure were higher than that of ordinary mortar slabs. Compared with the specimens whose curing age was 28 days, the peak loads of ordinary mortar specimens reinforced with 1, 2, 3 and 4 fiber layers curing age 365 days increased by 34%, 37%, 29% and 7%, respectively. And compared with the specimens whose age was 28 days, the peak loads of epoxy mortar specimens with 1, 2, 3 and 4 fiber layers aged 365 days increased by 26%, 36%, 40% and 21%, respectively.

Die form study of composite T-stringer

AN Shen-shen, YAN Chao, SU Jia-zhi, CHEN Ping

2022, 0(2): 107-111. DOI: 10.19936/j.cnki.2096-8000.20220228.017

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In this study, we investigate the die form of the T-stringer based on the carbon fiber reinforced epoxy resin composite material. Two kinds of T-stringers are fabricated by employing the positive and inverted T-shaped die combination form, respectively. The thickness and shape quality of the different T-stringers are investigated through the direct measurements on the parts and careful data analyses. It is revealed that the T-stringers prepared using the positive T-shaped die combination form generally show more uniform thicknesses along both the cross-sectional and the lengthwise directions. The better thickness uniformity is attributed to the intimate contact between the part and the positive T-shaped die combination form. On the other hand, the inverted T-shaped die combination form provides T-shaped truss with lower degrees of springback and torsion deformation, mainly derived from the uniform temperature distribution across the parts during fabrication. The current study emphasizes the importance of the die combination form selection for the preparation of high quality T-stringers.

Study on the composite simulator of the aero-engine bypass duct with resistance above 350 ℃

LIANG Heng-liang, CHEN Yu-long, ZHOU Hong-fei

2022, 0(2): 112-118. DOI: 10.19936/j.cnki.2096-8000.20220228.018

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The composite aero-engine bypass duct manufactured by autoclave molding technology is a large-size tubular structure with flanged edge. Due to its complex structure and the adverse factors such as high temperature, high pressure and long curing time, it is difficult to guarantee the structure quality for the selected polyimide BMP370 resin matrix composite material. In this study, the molding qualities of the simulation parts of the case were better controlled by packaging technology, molding design and manufacturing technology and process techniques. The results of non-destructive testing, mechanical properties of furnace pieces, T_g test and 370 ℃ static strength test show that the case is qualified in non-destructive testing with porosity under 2% and mechanical properties at high temperature above 50% of that at room temperature. The T_g of 430.2 ℃ and 370 ℃ damage intensity also meets the design requirements.

Research progress on creep property of glass fiber reinforced polymer anchors

JING De-sheng, BAI Xiao-yu, WANG Hai-gang, ZHANG Ming-yi, LI Cui-cui, JIAO Yu-jin, YAN Jun, WANG Zhong-sheng

2022, 0(2): 119-128. DOI: 10.19936/j.cnki.2096-8000.20220228.019

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Glass fiber reinforced polymer (GFRP) anchors have excellent mechanical properties compared with traditional reinforced anchors, and have acid-base corrosion resistance that reinforced anchors do not have. Compared with other composite anchors, GFRP anchors have become a potential substitute for reinforced anchors because of their low price and high engineering cost performance ratio. This paper mainly introduces the research results of GFRP anchor in creep test, theoretical analysis and numerical simulation. The creep mechanical model and creep characteristics of GFRP anchor under continuous stress or cyclic load are summarized. The main influencing factors of creep performance of GFRP anchor are summarized. The long-term performance of GFRP anchor and the numerical simulation analysis results of durability based on experimental data are discussed. Finally, the deficiencies in the existing creep performance research of GFRP anchors are analyzed, and reasonable suggestions are put forward for the future development of GFRP anchors in terms of durability and geotechnical anchorage.

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