COMPOSITES SCIENCE AND ENGINEERING

EXPERIMENTAL STUDY ON SHEAR PERFORMANCE OF EMBEDDED THROUGH SECTION SHEAR STRENGTHENING OF CONCRETE BRIDGE SLABS

DONG Peng-cheng, XIA Li-peng, WANG Xu-gang, ZHENG Yu

2020, 0(10): 5-12.

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The increase in traffic and heavy trucks loads close to supports have caused shear failure of concrete bridge decks. However, the existing strengthening methods commonly focus on the improvement of bending capacity of concrete bridge decks while ignoring the situation of shear failure problems. This paper reveals a comprehensive study of the behaviour of concrete slabs shear strengthened with Embedded Through Section (ETS) technique with fibre reinforced polymer (FRP) bars. Several structural variables like material type, diameter, and drilling damage were varied to assess the effectiveness of this shear strengthening technique. By analyzing the bearing capacity, crack distribution and failure mode of the test specimen, the results show that the strengthening method can effectively improve the shear capacity of concrete bridge slabs, and the failure mode changes from brittle shear failure to ductile bending failure or bending-shear composite failure. The drilling holes and different type of strengthening bars have little effect on the capacity and deformation of the bridge decks. Additionally, the use of FRP material can improve the durability of the bridge decks. Finally, a two-way theoretical model was proposed to predict the loading-carrying capacity of concrete slabs strengthened with ETS FRP bars. The ultimate strength predicted by this theoretical method shows good agreement with the test results.

EXPERIMENTAL RESEARCH ON BOND BEHAVIOR BETWEEN GFRP BARS AND STIRRUPS-CONFINED CONCRETE

HU Cheng-chao, GAO Kui, TU Jian-wei, FU Jin-hai

2020, 0(10): 13-20.

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Through the results of 120 pullout tests, the influence of different factors (the surface treatment of GFRP bars, the diameter of GFRP bars, concrete strength grade and the confined effect of stirrups) on the bond behavior of GFRP bars is studied and compared with steel bars. Then, the influence of the confined effect of stirrups on the bond failure and bond strength of GFRP bars is analyzed. Finally, the Bertero-Popov-Eligehausen (BPE) and Cosenza-Manfredi-Realfonzo (CMR) models are used to analyze the rising section of bond stress-slip (τ-s) relationship of GFRP bars in stirrup confined concrete. The test results indicate that when the bond failure interface only occurs on the surface of a GFRP bar, the bond strength is not dependent upon the concrete strength. Moreover, the results indicate that in comparison to specimens without stirrups, their stirrup-containing counterparts are more prone to pullout failure with greater ductility and higher bond strength and corresponding slip. The BPE and CMR models are able to investigate the τ-s relationship between GFRP bars and the stirrups-confined concrete with accuracy. With the experimental data, the specific parameters in the models classified by surface characteristics have been suggested.

STUDY ON CURING KINETICS OF TOUGHENED MODIFIED EPOXY RESIN AND DRAWING OF TTT DIAGRAM

TAO Lei, MIN Wei, QI Liang-liang, SUN Ze-yu

2020, 0(10): 21-29.

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In this study, the gelation and vitrification behavior of toughened modified epoxy resin were measured by small amplitude oscillatory shear method and differential scanning calorimetry, respectively, and the time-temperature-transition (TTT) diagram of the epoxy system was established. A dynamic differential scanning calorimetry (DSC) experiment was carried out on the toughened modified epoxy resin system, and a curing kinetic model was established using a phenomenological model. Using isothermal DSC and dynamic DSC combined with DiBenedetto equation to study the relationship between resin glass transition temperature and degree of curing, and gives the mathematical relationship between the glass transition temperature T_g and time t and temperature T. The gel time at different temperatures of the resin system was tested using a flat plate rheometer, and the relationship between the gel time and temperature was obtained by linear regression analysis. The TTT diagram based on the toughened modified epoxy resin system is drawn to provide theoretical guidance for the molding process of epoxy resin and its prepreg, which is conducive to improving manufacturing efficiency.

EFFECT OF GLASS FRITS ON THE ABLATIVE PERFORMANCE OF MoSi₂ MODIFIED BORON PHENOLIC RESIN CERAMICABLE COMPOSITES

YANG Wei, HUANG Zhi-xiong, WANG Yan-bing

2020, 0(10): 30-38.

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Due to the high temperature required for ceramization of ceramicable composites, the ceramic layer formed in the early stage of ablation is not dense enough, which will affect the ablation performance of the composites. While the glass frits with low melting point can decline ceramization temperature of the composites. In this work, two low melting point glass frits G1 and G2 were introduced into the MoSi₂ modified boron phenolic resin (BPR). Two vitreous silica fabric reinforced composites were prepared by prepreg-molding process. The effects of the glass frits on the thermal properties of the pyrolysis residues of MoSi₂ modified BPR and the flexural properties of vitrified structure and ablative properties of its high-silica-fiber reinforced composites were studied. Test results show that the ceramifiable temperature of the composites declines by about 600 ℃,and room temperature bending strength of vitrified structure at 800 ℃~1200 ℃ of the composites with glass frit G2 increases by 6.5 MPa~7.6 MPa. In addition, with the introduction of glass frit, a dense melt structure is formed on the surface of the composites in the process of oxyacetylene ablation, which inhibits the diffusion of hot oxygen to the inner carbonized matrix. Furthermore, glass frit G2 with more SiO₂ content is beneficial to promote the interaction between MoSi₂ particles and BPR pyrolysis products to form high-melting ceramic phases of Mo_4.8Si₃C_0.6 and MoB, which reduce the mass ablation rate of the composites by 10.7%.

INFLUENCES OF AIR NOZZLE AND HEAT-CONDUCTING PLATE ON THE INDUCTION WELDING OF CF/PPS COMPOSITE

QIAN Ying, WANG Jia-feng,SONG Yu-nuo, ZHU Shu

2020, 0(10): 39-46.

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With the increasing use of thermoplastic composite materials in aircraft, induction welding has attracted more and more attention as the most important developing direction for the connection technologies of the future thermoplastic structure. At present, the main problem of induction welding technology is that the highest temperature surface is hard to be confined to the weld line, and it is often located near the surface closer to the coil. In this paper, the influences of air nozzle and ceramic heat-conducting plate on the induction welding process of carbon fiber reinforced polyphenylene sulfide (CF/PPS) composite was studied. The results show that the use of air nozzle and heat-conducting plate could effectively reduce the temperature of the upper surface of the laminated plate during the welding process, minimize the high temperature melting or degradation of the resin near the coil, and guarantee the shape integrity of CF/PPS welding joint. In addition, the feasibility of induction welding of CF/PPS laminates made of unidirectional prepreg was studied. The results show that the CF/PPS laminates with alternating layers of 0° and 90° can be induction welded at low frequencies and high power due to the existence of alternating current circuits inside.

STUDY ON PREPARATION AND PROPERTIES OF HIGH PERFORMANCE THERMOPLASTIC RESIN EMULSION SIZING AGENT

CHEN Bo, WANG Jin-yan, LI Nan, LIU Cheng

2020, 0(10): 47-54.

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In this paper, a thermoplastic emulsion sizing agent was prepared by using phase inversion emulsification method, with thermoplastic resin PPBES as the main resin. The effects of emulsifiers, emulsifier content, emulsification temperature, shear rate, and molecular weight of the resin on the properties of the emulsion were discussed, and the optimal concentration of sizing agent was determined. The results showed that when using STAC as the emulsifier, mass ratio of emulsifier was 18.75%, emulsification temperature was 25 ℃, the shear rate was 11000 r/min, the resin number-average molecular weight was 5000, the average particle size of the emulsion was 110.5 nm, the centrifugal precipitation mass fraction was 2.01%, and Zeta potential was 55.1 mV. The surface energy of the carbon fiber reached 49.02 mJ/m², the optimal concentration of the sizing agent was 1.0%, the flexural strength and the interlaminar shear strength reached 1824 MPa and 80.2 MPa, respectively, which increased by 13.0% and 9.4%.

EFFECT OF IN-SITU STAMPING PROCESS PARAMETER ON GFRTP VISCOSITY AND PARAMETER OPTIMIZATION

WANG Yu, YANG Rui, LIANG Yi-nan, ZHANG Yi-ming

2020, 0(10): 55-59.

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Because of the high viscosity properties of GFRTP and vertically stamping and debonding in the in-situ stamping process, the adheres and detach between the mold and prepreg becomes an very important factor for forming effect. In order to study the influence on viscosity under different paremeters and optimize the combination of the parameter, probe test was used to analyze the viscosity. To define the maximum debonding force to present viscosity, and analyze the influence of important process parameters on viscosity and obtain the optimal parameter combination using the control method and RSM, the results show that the viscosity increase first and then decrease with the increase of temperature, increase with the increase of pressure, increase first and then decrease or remains still with the increase of pressure time. And there is a strong coupling relationship between the process parameter. It provides a theoretical guidance for the subsequent in-situ stamping process.

STUDY ON C-BEAM PREFORM BASED ON ADVANCED PULTRUSION

GAO Shao-nan, QI Jun-wei,WANG Yue-quan, CHEN Yi-chuan

2020, 0(10): 60-66.

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The advanced pultrusion (Advanced Pultrusion, ADP)using prepreg as the raw material can realize the efficient production of truss composite members. In the advanced pultrusion process, the preforming stage is to bend the straight prepreg strip to form a specified section. In this paper, the mold is designed to complete the flat-loaded prepreg tape to form a C-shaped cross section. By establishing a deformation model, the optimal length of the mold is determined to be 1100 mm. A constraint curve is designed to effectively constrain the fiber deformation process, and the tension is proposed. The control scheme revised the model and calculated that the fiber path difference was the smallest when the height of the discharge end was 50.9375 mm, which reduced it from 2.42‰ to 1.22‰. Based on this, the C-shaped preform mold design was completed. The BA3202 curing process was studied, and the advanced pultrusion process parameters were determined to be preforming 100 ℃, hot pressing 180 ℃, and holding for 60 minutes. Based on the above, a pultrusion experiment was performed. During the experiment, there was no wrinkle accumulation on the surface of the prepreg. The bending effect of the part is good, the surface quality of the corner area is high, and the internal fiber of the part is not buckled by metallographic observation, which indicates that the process parameters are reasonable and the preforming scheme is feasible.

NUMERICAL MODEL OF PROCESS-INDUCED DEFORMATION IN COMPOSITE PARTS CONSIDERING MOULD AND MATERIAL VISCOELASTICITY

QIAO Wei, YAO Wei-xing, HUANG Jie

2020, 0(10): 67-74.

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Considering the mould influence and material viscoelasticity, a three-dimensional numerical model of process-induced deformation and residual stress in composite parts was established. The interfacial shear stress between the mould and part was introduced to simulate the mould-part interaction during heat-up. The influences of temperature and degree of cure on the material mechanical properties were analyzed, and the linear elastic and viscoelastic constitutive equations were used to describe the material stress-strain relationship before and after the gelation, respectively. The finite element model simulating the composite curing process was established in ABAQUS software by using thermal-stress sequential coupling analysis method, and the temperature field, cure degree field, residual stress and deformation during curing were analyzed. Finally, the rationality and accuracy of the model established are verified by comparing with the experimental results of process-induced deformation. The results show that the residual stress generated during curing in the unidirectional and unsymmetric laminates is asymmetrically distributed in the thickness direction, which is the primary reason for laminates deformation after demoulding.

STUDY ON PREPARATION AND PROPERTIES OF PHYSICALLY THICKENING BULK MOLDING COMPOUND

XU Xin-min, HU Ming-feng, ZHANG Song, JI Ling

2020, 0(10): 75-81.

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It takes about 7 days to mature in the ordinary bulk molding compound(BMC) before it can be used for production. This leads to large inventory, high production cost and low efficiency, and also limits the development of BMC production and applications. In this paper, physical thickener was used instead of calcium hydroxide thickeners to prepare BMC composite material and the performance of BMC was studied and analyzed. The viscosity of the BMC's resin paste was tested and characterized by the laminar viscometer, so the effect of physical thickener's amount on the properties of the BMC was studied. Plate Vulcanizing Press Machine, Universal Tensile Testing Machine, Breakdown Voltage Test, high resistance meter are used to measure and characterize the mechanical properties, electrical properties, and other related properties of BMC, to study the effects of physical thickener on the properties of BMC. The results show that the thickening effect of the physical thickener on BMC is prominent, which significantly reduces the time needed for achieving the molding viscosity. The thickening effect of BMC when the physical thickener accounts for 20% of the UP mass is perfect, its viscosity reaches the required viscosity for molding after 24 hours of kneading preparation. When the viscosity meet the need of the molding pressure, the fluidity D_min of bulk molding compound is increased by 6.4%, which is more conducive to molding. And the mechanical properties of BMC are only slightly reduced. So, physical thickener can improve the production efficiency of BMC and save the production cost.

INFLUENCE OF CONFINEMENT STRUCTURE ON PROPERTIES OF CONTINUOUS GLASS FIBER REINFORCED POLYURETHANE COMPOSITE LEAF SPRINGS

LI Bo, ZHANG Min-ai, YANG Jian, WANG Jun

2020, 0(10): 82-87.

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In order to improve the mechanical properties of continuous glass fiber reinforced polyurethane (GF/PUR) composites as elastomers, the composite leaf springs with circumferential winding glass fiber yarn, glass fiber cloth, glass fiber bushing and two layers of glass fiber bushing were prepared by vacuum resin introduction molding process. The bending modulus, bending strength, fatigue property and damping property were studied. The results show that the restrained structure can reduce the bending modulus and strength of the composite spring to some extent, but its fatigue and damping properties are improved obviously. Among them, when the fiber cloth belt is used as the confinement structure, the composite leaf spring has better comprehensive performance, the bending modulus is 26201.16 MPa, the bending strength is 443.12 MPa, the cycle times of 40% stress level is 2109, and the natural frequency and loss factor are 215 Hz and 0.071, respectively.

COMPLIANCE DEMONSTRATION ON LIGHTNING PROTECTION OF Y12F AIRCRAFT COMPOSITE RADOME

SUN Jin-bo

2020, 0(10): 88-95.

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The Y12F aircraft is a brand-new commuter aircraft of Part 23 manufactured by HAFEI Aviation Industry. It is developed on the basis of the original Y12 series aircraft, and is also the largest commuter aircraft in the world at present. The Y12F aircraft is the first civil aircraft type in China to be certificated and approved by the FAA and CAAC simultaneously. The radome is usually located in the nose of the aircraft. It is vulnerable to lightning strikes in bad weather and may cause catastrophic accidents. Radome is the key part of aircraft structure lightning protection design and lightning protection compliance demonstration. Taking the compliance demonstration of Y12F aircraft radome lightning protection as an example, this paper introduces the relevant airworthiness requirements, the design idea, demonstration method, as well as the specific test process and results of the radome lightning protection. The relevant experiences and lessons can be used for reference by the applicant and the authority in the civil aircraft radome lightning protection design, compliance demonstration, and the airworthiness certification. It has important reference significance.

STUDY ON PREPARATION AND PROPERTIES OF NOVEL RESIN-BASED THERMAL PROTECTION STRUCTURES

XIA Yu, WANG Dong, XU Kong-li, WU Xiao

2020, 0(10): 96-100.

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According to the requirements on low cost and light-weight thermal protection structure and materials for space vehicles, a novel integrated resin-based thermal protection structure was proposed. The sandwich structure was prepared successfully by the optimization of materials selection and manufacturing process. It consists of high temperature carbon fiber reinforced polyimide composites as outer thermal protection layer, silica aerogels blanket as heat insulation layer, and high performance carbon fiber reinforced epoxy resin composites as internal bearing layer, respectively. The mechanical property and thermal protective performance were tested, and the influence of insulation layer thickness on the thermal protective performance was investigated. Experiment results show that the resin-based thermal protection structures have excellent compression properties. The integral structures still remain intact after the quartz lamp heating test, and the thermal insulation property can be improved with the increase of insulation layer thickness. The back temperature of the sample with 13 mm thick insulation layer was only about 100 ℃ after quartz lamp heating at 500 ℃ for 1500 s, exhibiting superior thermal protection performance.

PREPARATON AND PROPERTIES OF QUARTZ FIBER REINFORED BORON NITRIDE CERAMIC MATRIX COMPOSITES

GAO Long-fei, CHAI Xiao-xiao, MA Jun-yi, LI Song

2020, 0(10): 101-104.

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The 2.5D quartz fiber reinforced boron nitride ceramic matrix (2.5D SiO_2f/BN) composites were prepared by using borazine as the boron nitride ceramic precursor and 2.5D quartz fiber preforms as the reinforcement.The structure characterization of borazine was performed. The densification effect, mechanical properties and dielectric properties of 2.5D SiO_2f/BN composites were investigated. Experimental study shows that 2.5D SiO_2f/BN composites have a dense outer layer and a loose, porous interior. They have excellent mechanical and dielectric properties which can be used for radome wave-transmitting materials.

APPLICATION OF INTEGRATED ONLINE RESIN INFUSION MOULDING TECHNOLOGY FOR LARGE WIND TURBINE BLADES

YAN Chen, BU Li-jing, ZHAO Kang-nan, ZHOU Rui-rui

2020, 0(10): 105-109.

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With the advent of the era of the same price of wind power and thermal power, the capacity and sweeping area of wind turbines have been increasing, and the light-weight & large-scale wind turbine blades are becoming more and more popular which poses a higher challenge to the resin infusion moulding (RIM) technology. There is a big gap in labor, quality, cost, energy conservation and emission reduction of conventional RIM technology. The larger the blade, the higher the quality risk and cost, therefore blade manufacturing plants urgently need a new RIM technology suitable for light-weight & large-scale blades to support quality improvement, efficiency increase and cost reduction. This article introduces the application of integrated online RIM technology in moulding process of large wind turbine blades, and compares its advantages with conventional RIM technology. Evidence proves the higher application value of this technology in combination with the conventional RIM technology. Production practice shows that the integrated on-line RIM technology with auto-mixing function reduces the staffing by 33%, reduces the single-blade infusion defect quantity by 80%, and reduces the single-blade resin consumption by 60 kg, which has higher engineering application value.

EXPERIMENTAL AND SIMULATION STATUS OF BEARING CAPACITY OF COMPOSITE PANELS WITH DAMAGE

NI Ying-ge, ZOU Peng, BI Xue

2020, 0(10): 110-121.

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Stiffened composite panels are widely used in modern aviation industry because of its thin-walled and high bearing efficiency. It is inevitable to cause damages by the low-speed impaction during manufacturing and servicing processes. Thus, how to predict the bearing capability of stiffened composite panels with damages has become an important problem to ensure the safety of composite structures during the servicing process. In this paper, the failure mechanism of composite stiffened panel is summarized from two aspects of experiment and simulation. In the experimental study, four typical experimental types including the compression after impact, compression after impact fatigue, shear after impact and shear after impact fatigue are illustrated and analyzed. In the simulation research, the modeling method of stiffened panel, damage modeling method, failure criteria and property degradation rules are summarized. Finally, the existing problems in the current simulation are proposed, which provides a useful reference for further research on the bearing ability of composite stiffened panel.

RESEARCH PROGRESS OF EQUIVALENT MODEL OF HONEYCOMB SANDWICH STRUCTURE

YANG Wen, ZHANG Sheng-lan, LI Ying

2020, 0(10): 122-128.

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Honeycomb sandwich structure composite with light weight and high specific strength and specific stiffness has been widely used in many fields in recent years. In this paper, the research results at home and abroad are combed and summarized from the aspects of equivalent parameters of honeycomb core and honeycomb equivalent model. The equivalent parameters of honeycomb core layer are classified and discussed from the aspects of theoretical formula derivation, homogenization method and finite element method, and the equivalent models are compared based on sandwich plate theory, honeycomb plate theory, equivalent plate theory and other equivalent methods. Finally, some suggestions on the research direction of the equivalent model of honeycomb sandwich structure are put forward.

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