COMPOSITES SCIENCE AND ENGINEERING

Structural analysis of fiber winding composite case base on plate and shell theory

ZU Lei, MOU Xing, ZHANG Qian, FAN Wen-jun, WU Shi-jun, WU Qiao-guo, ZHANG Gui-ming, GENG Hong-bo

2022, 0(12): 5-16. DOI: 10.19936/j.cnki.2096-8000.20211228.031

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Generally, inconsistent radial displacement at the equatorial circle is caused by the calculation of stress field based on the cylindrical shell and Laplace film theory for the cylinder body and the head respectively, and this inconsistency arises the phenomenon of theoretical structural separation which is incompatible with the actual deformation during the overall loading process. Due to the variation of angle, thickness and curvature of the fiber-wound composite shell head, the calculation of stress field is very complicated. Taking the inconsistency of radial displacement and deformation at the equator as starting point, the method of theoretical analysis of complex force field of fiber-wound composite head not only considers the sudden change of the structure curvature at the equatorial circle, but also considers that of the stiffness of material on the structure at the same time. And by making the metal connector end face pressure load to be the equivalent load of contact part of the composite, the radial film displacement under the equivalent load of pole hole is used as the boundary condition of the control equation. Based on the plate and shell theory, this paper discussed the calculation method of the moment stress field of the fiber-wound composite case head, and established a theoretical model of composite rocket case with bending moment, explored the reason why pole hole always swell damage. A standard pressure test of a ø150 dry winding composite case was carried out, the comparison of analytical and numerical solutions show that the method has good approximation accuracy to describe the stress field distribution of the fiber winding composite material layers. According to the theoretical calculation results, reveals internal mechanism from the aspects of structural mutation and material stiffness mutation, as well as the law of shell stress under different winding schemes. This calculation method has the potential to lay a theoretical foundation for a reasonable structural design, which is of great significance to quickly understand the overall strain state of the case at the beginning of design.

Reliability and sensitivity analysis of edgewise compressive strength for plain woven faceplate honeycomb sandwich structure

TIAN Jing, ZOU Run-wen, WANG Xuan, ZHANG Feng

2022, 0(12): 17-23. DOI: 10.19936/j.cnki.2096-8000.20221228.002

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For plain woven faceplate honeycomb sandwich structure, the progressive damage failure analysis model is established to study the influence of the randomness of parameters on the lateral compressive strength. Firstly, the output responses corresponding to random input samples are obtained by the use of finite element analysis, and the surrogate model is established by Kriging method. The reliability analysis method based on Monte Carlo is applied for structural reliability analysis, and the sensitivity analysis methods based on variance and failure probability is respectively used for structural sensitivity analysis. The structural reliability index, the sensitivity index of the mechanical properties and thickness are computed, which provides a basis for the structural reliability optimization design.

Damage characteristics and energy dissipation of carbon fiber reinforced high strength concrete under multiple impact loads

MA Xue-si, XU Li, ZHANG Qiang

2022, 0(12): 24-30. DOI: 10.19936/j.cnki.2096-8000.20221228.003

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In order to study the safety of carbon fiber reinforced high strength concrete under multiple impact loads. The longitudinal wave velocity, damage factor, peak stress and energy dissipation of specimens with different carbon fiber contents (0%, 0.15%, 0.3%, 0.45% and 0.6%) under different impact times (1, 2, 3 and 4) were studied by using a separated Hopkinson pressure bar device with a diameter of 74 mm and a non-metallic ultrasonic testing device. Combined with scanning electron microscope scanning (SEM), the internal microstructure of the specimen under different impact times was observed, and the relationship between the mechanical properties of the specimen and the content of carbon fiber and impact times was analyzed. The results show that the addition of carbon fiber can increase the integrity of the specimen, reduce the internal cracks and increase the longitudinal wave velocity. The first impact will "compact" the internal cracks of the specimen, and the wave velocity of the specimen will increase. With the increase of impact times, the damage degree of the specimen increases and the wave velocity decreases. The addition of carbon fiber can effectively increase the peak stress of the specimen, and the effect is the best when the content is 0.45%. The impact load will cause compaction and damage to the specimen, and the peak stress of the specimen first increases slightly and then decreases greatly. The energy time history change of the specimen under impact load is mainly divided into three stages. The energy dissipation rate of the specimen is the highest when the fiber content is 0.3%. Multiple impact will cause damage accumulation to the specimen, and its energy consumption density will be significantly reduced. SEM test results show that the existence of carbon fiber can effectively reduce the internal crack propagation and the number of cracks under impact load, and the changes of microstructure and macro mechanical properties are consistent.

Monitoring the micro-damage repair process inside composite using line-field spectral-domain optical coherence tomography

YOU Ru-feng, HUANG Pin-bo, NI Zi-hao, XIE Sheng-li, BAI Yu-lei, DONG Bo

2022, 0(12): 31-36. DOI: 10.19936/j.cnki.2096-8000.20220528.032

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Detecting early micro-damage and monitoring repair process are of great significance to the safety of composite materials in service, a method based on line-field spectral-domain optical coherence tomography (LF-SD-OCT) is proposed to study the problem above in this paper, which can provide a micron scale section-image for composite material to inspect damage, and monitor damage repair process combined with image processing techniques. For verification, an inverted type LF-SD-OCT system is designed and established, which features the depth resolution of 5.20 μm, the depth range of 3.36 mm, the lateral resolution of 6.02 μm, and the viewfield width of 5.80 mm. Then, the internal micro-damage tomography of translucent silica gel, light-cured resin, and transparent ethylene-vinyl acetate copolymer (EVA) is carried out. Finally, based on the established system, the whole electron-thermal repair process of internal damage inside EVA is monitored in time. The results show that the method owns the excellent qualities of micron resolution and dynamic measurement, which is very suitable for damage inspection and repair sensing of composite materials.

Analysis of transfer length of pre-tensioned prestressed CFRP bars and sea water sea-sand concrete slab

HUANG Yu-xuan, FAN Ling-yun, GAO Jing

2022, 0(12): 37-45. DOI: 10.19936/j.cnki.2096-8000.20221228.005

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In order to study the prestress transfer length of pretensioned CFRP tendons, the bond performance of CFRP bars and sea water sea-sand concrete is tested. According to the theoretical analysis and finite element numerical simulation, several factors affecting the bond performance of CFRP bars and sea water sea-sand concrete are analyzed. The finite element model of pre-tensioned prestressed CFRP bars and sea water sea-sand concrete slab is established by ABAQUS software, and the transfer length of pre-tension prestressed CFRP bars and sea water sea-sand concrete slab is analyzed. The results show that the higher the concrete strength is, the shorter the transfer length of the prestressed member is, because the increase of the bond strength makes the transfer distance of the prestressed shorter. The transfer length can be significantly increased with the increase of CFRP reinforcement and tension control stress. This paper also puts forward the calculation formula of the transfer length CFRP bars sea water sea-sand concrete prestressed members.

Bonding influence on static strength of single bolted joint of carbon laminates and aluminum alloy plate

LIU Feng, WANG Zhuo-yu, DOU Guang-zheng

2022, 0(12): 46-53. DOI: 10.19936/j.cnki.2096-8000.20221228.006

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The static strength tensile tests of single-bolted joint and bonded-bolted joint of carbon fiber laminate and aluminum alloy plate are carried out. A finite element model of the bonded-bolted joint is established. The three-dimensional Hashin failure criterion is embedded in the analysis model by subroutine programming. The debonding mechanism of the adhesive layer between the laminate and aluminum plate is studied, and the failure evolution process of the carbon fiber laminates is analyzed. The numerical model is revised based on test data. After the correction, the peak load error between the finite element model and the test value is 2.9%, and the error of the second peak is 14.8%. It is showed that high-strength adhesive improves the joint strength and expands safety margin of the single-bolted and bonded joint. The higher the adhesive strength, the higher the peak load. When the debonding occurs between the laminate and aluminum plate, the secondary peak load is lower than the initial one. The secondary peak load is consistent with the peak load of the non-adhesive single-bolted joint. The joint stiffness and stability of the bonded-bolted joint with adhesive in the bolt hole is better than that of the joint without adhesive in the bolt hole. The debonding mechanism of the adhesive layer between the laminate and aluminum plate under displacement loading and the failure evolution process of carbon fiber laminates are simulated well by the finite element model established in this paper.

Analysis and optimization of mechanical properties of RTSF-PVA hybrid fiber concrete based on grey correlation analysis

CHEN Dong-lin, WANG Xue-zhi, LIU Hua-xin

2022, 0(12): 54-61. DOI: 10.19936/j.cnki.2096-8000.20221228.007

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The effects of recycled tire steel fiber (RTSF), polyvinyl alcohol fiber (PVA) and slag on cubic compressive strength, splitting tensile strength and flexural strength of RTSF-PVA/ slag concrete were studied by orthogonal design with three levels and four factors. The results show that, compared with C50 reference concrete, the maximum cubic compressive strength, splitting tensile strength and flexural strength of RTF-PVA/slag concrete are increased by 23.71%, 43.72% and 37.96% respectively. Through variance analysis, it is found that RTSF fiber is a significant factor affecting the three performance indexes, slag is a significant factor affecting the cube compressive strength and flexural strength, and PVA fiber is a significant factor affecting the splitting tensile strength. The performance indexes of RTSF-PVA/ slag concrete were optimized by using grey correlation analysis method and principal component analysis method. By comparing the grey correlation degree of each factor, the optimal scheme was obtained as followed. The volume content of RTSF was 0.6 kg/m³, the volume content of PVA was 0.075 kg/m³, and the replacement rate of GBFS was 15%. Compared with the original scheme, the cube compressive strength after optimization is slightly reduced, but the splitting tensile strength, bending strength and slump are increased by 1.24%, 5.11% and 30.91%, respectively. The results verify the effectiveness of the optimization method, and provide a reference for the mixing range of RTSF fiber, PVA fiber and slag.

Experimental study on the critical force of local buckling of GFRP equilateral angular members under axial compression

CHEN Jian, SUN Ze-yang, ZHAN Yang, LUO You-jian, ZENG Yi-hua

2022, 0(12): 62-68. DOI: 10.19936/j.cnki.2096-8000.20221228.008

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The stiffness of fiber-reinforced composite materials (FRP) is relatively low relative to its strength, so FRP axial compression members are generally controlled by the buckling bearing capacity rather than the compressive strength of the materials. In order to study the local stability performance of equilateral angular GFRP profiles under axial compression, 15 specimens were selected for testing, the test results were compared with the finite element model, and the calculation accuracy of several existing local buckling theoretical methods was evaluated. The results show that for GFRP angular axial compression members, the local instability failure position is mostly near the end, and the fiber at the end of the test piece is broken and delamination occurs. The finite element calculation results are in good agreement with the experimental results, indicating that the established finite element model can better simulate the local stability of equilateral angular GFRP profiles, and the end constraint conditions have a significant impact on the local buckling bearing capacity of the member. The buckling stability of the column is mainly controlled by the slenderness ratio of the member, while the buckling stability of the short column is mainly controlled by the width-to-thickness ratio. Comparing the existing calculation methods of local buckling theory, it is found that the equation proposed in Technical Specification for Composite Pultrusion Section Structures can better predict the local buckling critical force of equilateral angular cross-section FRP members.

Acoustic emission monitoring of tensile deformation of 3D printed composites

SUN Heng, MA Lian-hua, ZHOU Wei, LIU Jia, JI Xiao-long

2022, 0(12): 69-74. DOI: 10.19936/j.cnki.2096-8000.20221228.009

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3D printing technology provides a new method for composite material preparation. In order to study the effect of different printing layer thicknesses (t) on the tensile deformation and damage evolution of 3D printed composites, three kinds of composite materials with different printing layer thicknesses were printed by COMBOT-200 printer. The mechanical behavior and damage evolution of the printed composite specimens under tensile loading were studied by using acoustic emission (AE) and digital image correlation (DIC) techniques. Through AE signal analysis, cluster analysis and global strain analysis, the failure mechanism was clearly revealed. The results show that the failure load of 3D printed composites decreases, and the cumulative hits of AE signals per unit time increases rapidly with the increasing printing layer thickness. AE signals under tensile loading are divided into three categories which correspond to matrix cracking, fiber debonding and fiber fracture, respectively. In the same load range, the strain value on the specimen surface increases with the increase of print layer thickness. When the specimen approaches to tensile failure, the surface strains of such material increase sharply, indicating that the specimen is almost fractured. The complementary nondestructive testing technology of AE and DIC can be used to simultaneously obtain the AE response and surface strain field information of 3D printed composites under tensile load,and the damage evolution and fracture mechanism are revealed. The combination of AE and DIC technology provides a reference for the health monitoring of 3D printed composites.

Effect of molding methods on properties of high HGMS volume fraction solid buoyant materials

HUANG Wei, YI Ya-dong, WU Ping-wei, DAI Jin-hui

2022, 0(12): 75-78. DOI: 10.19936/j.cnki.2096-8000.20221228.010

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With epoxy resin E-4221 as matrix and hollow glass microsphere (HGMS) as filling material, composites with high HGMS volume fraction were prepared by molding, extrusion and vacuum tamping, respectively. The effects of forming methods on the density, compressive strength and water absorption of composites under different hydrostatic pressures were studied, and the application scope of different forming methods was determined. The results show that extrusion molding and vacuum tamping molding are suitable for the preparation of solid buoyant materials with high volume fraction of HGMS. Solid buoyant materials with density no less than 0.62 g/cm³ and safe use depth no less than 8 000 m can be obtained by extrusion molding method. The vacuum tamping method can obtain solid buoyant materials with a density of less than 0.57 g/cm³ and a safe use depth of 4 000 m.

Mechanical behavior of novel basalt fiber-reinforced polymer (BFRP) rock bolt

LUO Jin-biao, PENG Zhe-qi, WANG Xin, YAN Zong-xue, LIN Jing-hui, LIU Lang

2022, 0(12): 79-86. DOI: 10.19936/j.cnki.2096-8000.20221228.011

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In this study, three types of BFRP cable rock bolt, namely single-tendon, parallel-tendon and twisted-tendon cable rock bolt were studied in terms of tensile, anchoring and pullout behavior. By using epoxy resin adhesive, the parallel-tendon cable had highest strength of 1 319 MPa. Owing to twisting of tendons, the tensile capacity and stiffness were reduced in twisted-tendon cable. The adoption of an anchor with inner cylinder and cone can ensure a failure mode of fiber fracture at free length. The anchoring efficiencies were larger than 97% in all groups. The parallel-tendon cable had highest pullout capacity. For pullout stiffness, however, the twisted-tendon cable was better than other cables.

A study of interlaminar toughness enhancement of GFRP plate spring based on structural bionics

BAI Lu-yao, WANG Hong-xiao, YU Jun-an, CHEN Yan-lei, HUI Yan-bo

2022, 0(12): 87-92. DOI: 10.19936/j.cnki.2096-8000.20220128.031

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In this study, resin-based glass fiber reinforced composite (GFRP) plate spring was designed with a brick type toughening structure by the arrangement form of mother-of pearl mineral structural elements to improve the interlayer toughness of GFRP plate springs,thereby improving their fatigue life. The same size brick masonry type structure and non-toughened structure shrinkage plate spring samples were produced, and the ply laying method was based on the decreasing design principle of variable thickness ply laying. And the mechanical properties of brick structure and non-toughened structure GFRP plate spring were compared by stiffness test, fatigue test and residual stiffness test. The experimental results show that compared with the GFRP plate spring of the non-toughened structure, the stiffness of the GFRP plate spring of the brick structure is reduced by 8.4%, and the fatigue performance is improved by 28.99%. During the fatigue test, the non-toughened structure is completely broken without residual stiffness, and the GFRP plate spring of the brick toughened structure still has the original stiffness of 43.52%. The fatigue failure of GFRP plate springs without toughened structure is in the form of delamination between the layers and complete fracture of the structure. In contrast, the fatigue failure of the toughened structure GFPR plate springs is in the form of interlaminar delamination being blocked and the structure forming a local fracture, for which the fracture cross section is stepped after the residual stiffness test.

Situational awareness prediction model in bonding repair of composite materials

HE Qiang, CHENG Lin

2022, 0(12): 93-100. DOI: 10.19936/j.cnki.2096-8000.20221228.013

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The situational awareness loss and degradation is an important error causation in airworthiness maintenance of composite material components. The monitoring of situational awareness level has great significance to reduce maintenance errors and ensure flight safety. To monitor the situational awareness, the measurement and prediction method of situational awareness based on eye movement was proposed. Firstly, the situational awareness of composite materials bonding repair was analyzed according to the maintenance process and the theoretical model of situational awareness. Then, the situational awareness measurement experiment was designed. In this experiment, the eye movement data was recorded by Tobii and the SART was used to measure the situational awareness subjectively. Dependent on the correlation analysis between eye movement and situational awareness, the feature mode of eye movement was constructed to measure the situational awareness. Finally, the situational awareness prediction model was established by relevance vector machine with this pattern. The experimental results show that the prediction accuracy and fitting effect of the model can support the online monitoring and evaluation of the maintenance situational awareness.

Effect of lightning current parameters on damage of carbon fiber composites subjected to lightning strike

KUANG Cheng-zhao, ZHU Hui-xin, FU Kun-kun

2022, 0(12): 101-107. DOI: 10.19936/j.cnki.2096-8000.20220528.031

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In this paper, the damage mechanism of carbon fiber reinforced polymer (CFRP) composite under simulated lightning current was studied by the lightning simulation experiment and finite element (FE) method. An electrical-thermal coupled FE model considering the dielectric breakdown effect was established, and the damage area and depth calculated by the model were compared with the experimental results of lightning strike simulation, which verified the validity of the finite element model. After that, the influence of lightning current parameters on lightning damage of carbon fiber composites was explored by using the identified FE model. The results show that when the lightning current waveform is the same, the relationship between the action integral of lightning current and the damage area of the FE model is linear. When the action integral and intensity of lightning current are the same, the damage area of CFRP composite decreases with the decrease of the rise time of the current waveform, but the damage depth increases.

Intelligent structural design and self-healing performance of CFRP based on core-shell nanofibers

XIAO Hong-bo, MAO Chi, WANG Hong-wei, GAN Yu

2022, 0(12): 108-115. DOI: 10.19936/j.cnki.2096-8000.20221228.015

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An ultrathin core-shell nanofiber mats were prepared by encapsulating a mixture of the foaming agent diethyl azodicarboxylate (DEAD) and epoxy resin (EP) into polyacrylonitrile (PAN) nanofibers by coaxial electrostatic spinning technique and laying them on the interface of carbon fiber fabric, and forming CFRP with ultrathin self-healing smart sandwich after resin injection and curing. The self-healing effect of the ultrathin self-healing smart sandwich on the bending strength of CFRP laminates with damage failure was evaluated using a three-point bending test. The results showed that the thermal properties of PAN core-shell nanofibers and its encapsulated self-healing agent matched with the CFRP preparation and molding process. When the DEAD content was 5wt% and the healing temperature/healing time (T/t) was 150 ℃/30 min, the self-healing efficiency of CFRP reached 93.5% at 100%L primary bending damage, which was improved by 12.0% compared with that without DEAD in the self-healing agent. Under the bending load with loading displacement of 100%L, 90%L and 80%L, respectively, the self-healing efficiency still reached 71.2%, 70.2% and 84.0% after three damage repairs, indicating that the composite still has self-healing ability after multiple damages.

Microwave irradiation on CFs and its effects on the interfacial properties of CFRPs

ZHANG Jing-jing, LIANG Sen, TANG Chao

2022, 0(12): 116-123. DOI: 10.19936/j.cnki.2096-8000.20221228.016

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Microwave irradiation is seen as an effective modification technique for interface enhancement of carbon fiber reinforced plastics (CFRPs). In this study, a microwave system which can transfer single mode of TE₁₀ electromagnetic wave was used for CFs surface modification. The modification process of microwave on carbon fibers (CFs) were investigated in electromagnetic field theory, electromagnetic simulation and experiments. Three irradiating parameters of the angle between CFs and the electric field direction, irradiating area and irradiating time were optimized. The enhancing effect of microwave irradiation on composites were quantitatively characterized. Ansoft HFSS simulation results showed that the microwave absorption property of CFs is related to the direction of microwave polarization, exhibiting a good absorption of microwave that polarized perpendicular to the CFs with reflectivity of only 6.44%, while a high reflectivity of 98.44% to microwave that polarized parallel to the CFs which should be avoided. The temperature of the irradiated CF fabric was tested and analyzed, based on which an optimized irradiating method was obtained for uniform irradiation of the carbon fabric. Based on the micro drop debonding method, the relationship between the interfacial shear strength (IFSS) of CFRPs and the microwave irradiation time was established, and the maximum 22.6% increase of the IFSS of CFRPs was obtained at 80 s radiation.

Research progress on preparation methods and fiber orientation of directional fiber reinforced concrete

LU Ming-yu, LIAO Wei-zhang

2022, 0(12): 124-132. DOI: 10.19936/j.cnki.2096-8000.20221228.017

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Adding chopped fiber material into concrete can solve the problems of poor crack resistance and low ductility. However, due to the random distribution of chopped fibers in concrete, the fiber orientation is inconsistent with the stress direction, so the contribution rate of fibers to the mechanical properties is limited. Therefore, scholars have carried out the research on the influence of fiber orientation on the basic mechanical properties of fiber reinforced concrete, provided theoretical and experimental basis for improving the contribution rate of chopped fibers, and developed various preparation methods of directed fiber and studied its fiber orientation. In this paper, the preparation methods of directional fiber reinforced concrete and the research status of fiber orientation are sorted out and summarized in detail from the aspects of preparation principle, production equipment and fiber types. Then the application prospect of directional fiber reinforced concrete is further given.

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