COMPOSITES SCIENCE AND ENGINEERING

Effect of ply and dimension on tensile properties of composite laminates with holes

YANG Jun-chao, ZOU Peng, DENG Fan-chen, CHAI Ya-nan, XIONG Mei-rong

2021, 0(11): 5-11. DOI: 10.19936/j.cnki.2096-8000.20211128.001

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Based on the failure theory of fracture surface, a numerical model for progressive damage analysis of composite laminates with holes was established. After the failure of the fiber, the stiffness of the damaged area was directly reduced to 0.01% of the original value. After inter-fiber failure, the material properties in the damaged zone were softened linearly based on energy release rate. The effect of shear non-linearity on failure analysis was also considered in the model. Subsequently, the tensile failure process of composite laminates with different diameters and layers was simulated, and verified by physical tests. The results show that the calculation error is less than 10%, and the predicted failure mode is consistent with the test. Finally, the effects of layers and structure size on the failure mode and strength of laminates with holes were discussed. Under the same aperture, the failure modes of laminates with different layers are similar. Under the same layers, when the ratio of hole diameter to width is constant, the tensile strength decreases rapidly as the hole diameter increases.

Analysis of structural strength and progressive damage of countersunk bolt lap joint with CFRP

YU Fen, LIU Guo-feng, HE Zhen-peng, LI Bai-chun

2021, 0(11): 12-20. DOI: 10.19936/j.cnki.2096-8000.20211128.002

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Carbon fiber reinforced polymer (CFRP) has been widely used in aviation industry due to its excellent mechanical properties. It is of great significance to evaluate the ultimate load and damage propagation of composite countersunk head bolt joints for the economy and safety of aircraft joints. In this paper, a three-dimensional model of composite countersunk head bolt lapping is established, and the pre-tightening force of the bolt is applied by virtual thermal deformation method, then the VUMAT user-defined program is written in FORTRAN language though finite element simulation. The selected stiffness degradation mode and 3D Hashin failure criterion are embedded into the progressive damage model to establish the finite element damage prediction model, and study different lap end distance on the composite material single screw countersunk bolt connection structure failure. The results show that the lap end distance mainly affects the shear strength of the connection structure. The smaller the end distance is, the smaller the ultimate bearing strength is. When the short diameter ratio of the lap joint reaches 2.5, the ultimate bearing strength changes little with the increase of the end diameter ratio. The tensile damage of the matrix first appears in the 0° ply of the bottom layer, the fiber compression damage first appears in the turning point of the countersink and straight hole, and the failure element expands along the loading direction until the free boundary fails.

Analysis of relaxation time spectrum of PVC coated membrane

WANG Ze-xing, LI Shuai, TAN Dong-yi, MENG Shuo, LIU Chao

2021, 0(11): 21-25. DOI: 10.19936/j.cnki.2096-8000.20211128.003

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To further study stress relaxation characteristics in theory, the generalized Maxwell models were employed to simulate the measured stress relaxation modulus of Polyvinyl chloride (PVC) coated membrane, and the approximate solutions of relaxation time spectrum were obtained based on second-order approximation method. Meanwhile, the effect of unit numbers in generalized Maxwell model on fitting accuracy and relaxation time spectrum characteristics were also compared and analyzed. The results show that increasing unit number in generalized Maxwell model, is beneficial to improve fitted accuracy of generalized Maxwell model, and also detailed describe the relaxation time spectrum characteristics, but it will increase the difficulty of model solving. For the PVC coated membrane selected in this work, the stress relaxation behavior can be better described by generalized Maxwell models with 5 Maxwell units, while it is more reasonable to solve approximate solutions of relaxation time spectrum based on generalized Maxwell model with 6 Maxwell units. The results can provide a theoretical reference for investigation on stress relaxation characteristics of such materials in the engineering field.

Study on the elastic property of honeycomb aluminum based on the generalized method of micromechanics

YANG Ran, ZHANG Ji-gang, SHI Cheng-long, YAN Qing-feng, ZHAO Di

2021, 0(11): 26-30. DOI: 10.19936/j.cnki.2096-8000.20211128.004

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With the rapid development of all kinds of new composite materials, there are more and more methods to study the material properties of complex structures. Based on the method of composite micromechanics, the concept of representative volume element is extended, and the elastic parameters of honeycomb aluminum are numerically studied. 135 RVE finite element models of honeycomb aluminum with different geometric parameters are built and calculated. The fitting formula of elastic modulus was obtained by multiple regression analysis. At the same time, a statistical method is proposed to analyze the finite element calculation results, obtain the general law of complex material properties, predict the material properties with complex structure, and guide the application of materials in the engineering field. The feasibility of this method is verified by SHPB experiment at a certain strain rate.

Research on reinforcement scheme of solid rocket motor case based on prepreg winding

WANG Xin, ZHANG Qian, FEI Yang, ZU Lei, JIN Shu-ming, XU Hui

2021, 0(11): 31-38. DOI: 10.19936/j.cnki.2096-8000.20211128.005

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In this paper, a study on the mechanical properties of solid rocket motor composite shell samples was carried out using prepreg winding process. The mechanical behavior of composite shells was investigated and a refined numerical model of the composite dome using cubic-spline thickness prediction method was established. The burst pressure was predicted using Hashin failure criterion. With the aid of the test results, the failure mechanism of the composite shell was revealed. The influence of various reinforcement methods on the burst behavior of the composite shell was evaluated. The results show that the fine reinforcement of the dome can greatly improve the load carrying capacity of the shell. The burst pressure of the shell is increased by 50% with the reinforcement layup angle of 40°. The burst pressure increases with the increase of the reinforcement layer in a certain range. The local reinforcement for the damaged position will increase the hydraulic burst pressure of the shell to a certain extent. However, the unreinforced area at the dome of the shell will be damaged due to insufficient strength. This paper provides an important reference for design and production of composite solid rocket motor shell fabricated using prepreg winding.

Study on debonding damage of carbon fiber composite based on electromechanical impedance

HUANG Fei, ZHENG Yan-ping, WANG Xu, LI Ming-kun

2021, 0(11): 39-43. DOI: 10.19936/j.cnki.2096-8000.20211128.006

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Debonding damage is one of the most common damage in composite structures, which is not easily detected but poses a serious threat to structural safety, so its detection is very important to evaluate the safety of composite structures. In this paper, based on the electro-mechanical impedance technique (EMI), the root mean square (RMS) and the peak corresponding frequency (f_max) of the real part of the impedance curve are used as discriminators to distinguish the debonded and non-debonded regions in carbon fiber composites with the help of piezo-structural coupling field analysis. Next, the different degrees of debonding of carbon fiber composites were quantified and compared using the root mean square deviation (RMSD), mean absolute deviation (MAPD) and correlation coefficient deviation (CCD) of damage metrics. The results show that the discriminant indexes RMS and f_max can well distinguish the debonded and non-debonded regions in carbon fiber composites, and the damage indexes RMSD and CCD can better characterize the degree of debonding.

The combined effects of hydrostatic pressure and temperature on the performances of polymer syntactic foams

LIU Gang, QU Yan, JU Er-fan, WANG Zhao, ZOU Ke

2021, 0(11): 44-48. DOI: 10.19936/j.cnki.2096-8000.20211128.007

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An experimental investigation on the aging of a syntactic foam with a nominal density of 475 kg/m³ was carried out under the combined effects of a hydrostatic pressure of 30 MPa and a variety of water temperatures ranging from 15 ℃~60 ℃. The samples were aged for a variety of time periods ranging from 1 day to 30 days. And their water absorption rate, compressive strength and modulus of elasticity were tested. The results indicate that the water absorption rate of the polymer syntactic foams increases with increasing water temperature and aging time, while their compressive strength decreases with the same environmental factors. Their modulus of elasticity shows different trends under lower and higher water temperatures: It slightly increases with aging time under a relatively low water temperature, but decreases with aging time under the higher water temperature. The mechanisms driving the change of the performances of the syntactic foam under the combined effects of pressure and temperature were analyzed. And a characteristic relationship between the compressive strength and the water absorption rate was shown. A method for the prediction of the long-term performance of the syntactic foam through extrapolation from the short-term test data has also been established.

Experimental study on the mechanical properties of light-aggregate fiber shotcrete attacked by chloride salt

JI Cheng-fei

2021, 0(11): 49-53. DOI: 10.19936/j.cnki.2096-8000.20211128.008

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In this paper, the application of Lightweight Aggregate Fiber Shotcrete (LAFS) sprayed layer support in an environment with high groundwater chloride ion content and significant influence of dynamic pressure is used as a background. The orthogonal tests with different erosion ages of NaCl solution and erosion concentrations, ceramic particle admixture and fiber type as influencing factors were designed, compressive, splitting tensile and dynamic compression one-dimensional SHPB impact tests were conducted, and the chloride erosion and fiber reinforcement mechanisms were analyzed with the aid of scanning electron microscopy. The results show that the factors affecting the compressive, splitting tensile and impact performance of LAFS by chloride erosion were mainly the erosion age and the amount of ceramic particles, which showed a trend of increasing and then decreasing with the increase of the erosion age and the amount of ceramic particles. The appropriate amount of pottery granules can make LAFS have certain elastic buffer performance in the chloride salt erosion environment, so that its mechanical and chloride salt erosion resistance can be improved. Under this test condition, the best amount of pottery granules replaces 14% volume rate of stone LAFS. Combined with the LAFS impact damage morphology, it is appropriate to use steel fibers to enhance the mechanical properties of LAFS, followed by basalt and polypropylene fibers.

Influence of hot pressing process on energy dissipation and residual strain characteristics of airship envelope material under cyclic stretching

GU Zhan-hua, SHAO Hui-qi, JIANG Jin-hua, CHEN Nan-liang

2021, 0(11): 54-61. DOI: 10.19936/j.cnki.2096-8000.20211128.009

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In order to explore the influence mechanism of the process parameters of the hot-pressing compound on the energy dissipation and residual strain characteristics of the airship envelope material, the polyvinyl fluoride (PVF) film is used as the weather resistant layer, the TPU film as the helium barrier layer, and the polyarylate Vectran fiber woven plain weave as load-bearing layer, and the TPU film as the adhesive layer. The airship envelope materials with various hot-pressing process parameters were prepared by hot-pressing composite method, and the mechanical properties of airship envelope materials are tested and analyzed after cyclic loading-unloading. The influence of hot pressing temperature and hot pressing time on dissipation energy and residual strain is analyzed, and theoretical formulas are proposed. The results show that the hot-pressing temperature and hot-pressing time have the same influence on the energy dissipation and residual strain characteristics of the airship envelope materials, and the fitting degree of the fitting equation is good, and the coefficients are positively correlated with the hot-pressing temperature and hot-pressing time. The energy dissipation and residual strain characteristics of the materials show a two-stage linear characteristic, which is not only dependent on the hot pressing process, but also closely related to the times of cyclic loading and unloading. By comparing the dissipated energy and residual strain of the airship envelope materials prepared under different hot pressing process parameters after cyclic loading and unloading, the optimal preparation process was obtained as follows: Hot pressing temperature is 160 ℃, hot pressing time is 600 s.

Study on Z-pin enhancement technology with low in-plane damage

YAN Bin, LI Xiang-qian, YIN Ming-xin, LIU Wei-wei, CHEN Wei, MA Cheng-yan

2021, 0(11): 62-68. DOI: 10.19936/j.cnki.2096-8000.20211128.010

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This article presents an experimental investigation on the efficiency of the Z-pin pre-hole insertion process in reducing the in-plane damage while improving the delamination resistance of carbon fibre/epoxy laminates. The results show that the Z-pin pre-hole insertion process could reduce the fiber crimp, flow dilution and fracture. Specifically, the tensile strength and flexural strength of the laminates reinforced with Z-pin(0.28 mm in-diameter) were only reduced by 3.02% and 3.33%, respectively. It shows that Z-pin pre-hole insertion process could significantly reduce the in-plane damage of Z-pinned laminates. Furthermore, the tensile strength and flexural strength of the laminates reinforced with Z-pin(0.18 mm in-diameter) were only decreased by 1.69% and 1.12%, respectively. It shows that fine Z-pin could further reduce the in-plane damage of Z-pinned laminates while achieving interlaminar toughening. In addition, through the analysis of the failure mechanism of Z-pinned laminates, it is concluded that the in-plane damage of the laminates could be reduced by reasonably setting the arrangement of Z-pin, using fine Z-pin and reducing the use of 0° ply in design of layer.

Comparative study and equivalence of domestic T800H carbon fiber composites manufactured by automated tape laying and manual layup method

PENG Gong-qiu, ZHONG Xiang-yu, SHI Feng-hui, LI Guo-li, BAI Yu, BAO Jian-wen

2021, 0(11): 69-74. DOI: 10.19936/j.cnki.2096-8000.20211128.011

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Domestic T800H carbon fiber composites were prepared by automated tape laying and manual layup method using one batch prepreg. Mechanical properties of composites were compared, and equivalence evaluation of composites manufactured by automated tape laying and manual layup method was performed. The results show that average value deviation of mechanical properties manufactured by automated tape laying and manual layup method are -2.42%~4.50%. Strength average value, strength minimum individual and modulus average value of composites manufactured by automated tape laying and manual layup method are equally evaluated. There are 42 evaluation items, among which 39 evaluation items are equivalent. The three inequality properties exhibit little difference, and have little influence on the subsequent application of composites. In summary, composite manufactured by automated tape laying is basically equivalent to the composite manufactured by manual layup.

Motive trajectory design of one-sided stitching mechanism for carbon fiber composite

WANG Li-wen, DONG Jiu-zhi, CHEN Yun-jun, JIANG Xiu-ming

2021, 0(11): 75-81. DOI: 10.19936/j.cnki.2096-8000.20210528.032

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The single-sided stitching technology is widely used in composite curved surface preforms and large-scale structural parts because it does not require a stitching table to be installed at the bottom of the fabric, and it also makes an important contribution to the three-dimensional reinforcement of carbon fiber composites. In order to obtain the desired stitches in the carbon fiber preform with the unilateral stitching technology, and avoid the problems of skipped stitches, over-tight or too loose stitches in the stable loop of the stitches, the principle of the unilateral stitching equipment is analyzed and the original in some OSS stitch stitching, the action and time coordination relationship of the lead, hook thread and thread take-up mechanism are redesigned and planned, and the chute type lever thread pulling mechanism is added to the thread loop to be penetrated by the hook thread needle. The added thread take-up mechanism does not cause any thread loss and has a certain thread take-up function. On the basis of effectively avoiding the loop failure problem, the difficulty of designing the thread take-up mechanism is reduced. This paper proposes an idea of suture consumption at each instant of the mechanism, analyzes the matching relationship at each time point, and establishes an improved OSS stitch demand model. The motion cycle diagram is used to visually express the changing laws of each mechanism, and finally verify by stitching samples of different thicknesses. The experimental results show that the various mechanisms are well matched, the sample stitches are formed stably, and the improved mechanism coordination relationship does not have the problem of skipping stitches, which verifies the reliability of the movement trajectory of the unilateral stitching mechanism.

Optimization design of autoclave forming die based on temperature field

LU Jia-mei, MA Gui-chun, GUO Jing-yu, LI Yun-hao, ZHENG Jun-wang, WANG Zheng

2021, 0(11): 82-87. DOI: 10.19936/j.cnki.2096-8000.20210625.031

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The temperature gradient of autoclave mold will increase the uncertainty of composite curing. Improving the uniformity of die surface temperature distribution can ensure the curing quality and forming accuracy of composite components. Most of the existing mold optimization schemes are difficult to operate and can not be applied to engineering practice. This paper proposes to add a straight plate with a certain angle along the flow direction as the guide plate in the mold support structure. By controlling the inclined angle of the guide plate and adjusting the compression degree of the gas at the back of the mold, the best position of the guide plate is finally determined. It can be used to guide the optimization of autoclave mold for composite components in practical engineering.

Development of a broadband radome

ZHANG Chen-hui, ZHAO Wen-zhong, QU Yan-jie, GAO Qi, XU Pei-lun

2021, 0(11): 88-93. DOI: 10.19936/j.cnki.2096-8000.20211128.014

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In this paper, FiberSIM composite software was used to simulate the best lay-up method, and the results of software simulation were used to make the inner contour cutting samples of the inner and outer skin. The design angle alignment of the tooling accurately controls the inner and outer skin lay-up joints to ensure the uniformity of the space distribution of the inner and outer skin. The honeycomb cutting template is used to cut the honeycomb to ensure that the honeycomb joint is tightly stitched, which avoids the influence of styrofoam in the honeycomb joint on the electric thickness uniformity. Suitable curing parameters were used to shape the inner and outer skin, bonding honeycomb and block respectively to ensure the uniform distribution of adhesives on the curved radome. The radome made by the above method has good uniformity of electric thickness. The amplitude consistency can be controlled within ±0.1 dB, and phase consistency can be controlled within ±3°, which is better than the design requirements. Under the simulated wind speed of 45 m/s and 65 m/s,the maximum shape variable of the radome meets the design requirements.

Non-geodesic winding strength analysis and burst test of type Ⅳ 70 MPa pressure vessel

HU Zheng-yun, CHEN Ming-he, PAN Bo

2021, 0(11): 94-101. DOI: 10.19936/j.cnki.2096-8000.20211128.015

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The non-geodesic winding and bursting strength of type Ⅳ 70 MPa gas cylinder were studied in this paper. Firstly, the non-geodesic winding angle differential equation was solved by differential geometry and Runge-Kutta method, and the non-geodesic winding was divided into two modes: fixed inflection point and fixed angle. Then the experimental method was used to characterize the non-geodesic winding of the two modes, and the stable winding range was obtained. Finally, the predicted blasting pressure of geodesic and non-geodesic processes is compared and verified by experiments. The results show that the sliding-line coefficient of the winding mode with fixed inflection point can reach 0.35, while the sliding-line coefficient of the winding mode with fixed angle is within the range of [-0.1,0.1]. The non-geodesic winding technology can enhance the head and cylinder body and increase the cylinder bursting pressure.

Synthesis technology of lignin modified phenolic resin and properties of its moulding compounds

HUANG Shi-jun, TONG Yue-jin, SU Zhi-zhong, CHEN Yin-gui, AI Sheng-er, ZHANG Zhen-wei

2021, 0(11): 102-107. DOI: 10.19936/j.cnki.2096-8000.20211128.016

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The effects of the molar ratio of the raw materials, quantity of lignin, amount of catalyst on the properties of the modified phenolic resin were studied by orthogonal test. The phenolic moulding compounds was prepared based on lignin modified phenolic resin and its properties were tested. The orthogonal results showed that the amount of lignin was the key influencing factor of the curing time, free phenol of resin and waste water, the response rate of phenol. While the quantity of catalyst was the key influencing factor of the viscosity and soften point. For this research, the optimum synthetic parameters of lignin modified phenolic resin suitable for composite were as follows: the molar ratio was 0.84, the amount of lignin was 10%, the quantity of catalyst was 0.9%. The moulding compounds based on lignin modified phenolic resin had better physical performance and thermotolerant performance.

Experimental study on the environmental adaptability of 044B/BA9918 aramid fiber reinforced prepreg

SU Zhen-yu

2021, 0(11): 108-115. DOI: 10.19936/j.cnki.2096-8000.20211128.017

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The tensile, compressive, shear tests of 044B/BA9918 aramid fiber laminates were carried out at room temperature, low temperature and high temperature. The basic mechanical properties of aramid fiber laminates in different environments and the mechanical parameters of notched specimens were obtained, such as the strength and elastic modulus of fiber and matrix, in-plane shear strength and modulus, shear strength of short beam of aramid fiber specimens. The results show that the mechanical properties of 044B/BA9918 aramid fiber laminate have strong temperature dependence. Compared with the mechanical properties in normal temperature environment, the strength and modulus values related to the matrix are enhanced in different degrees under low temperature environment, such as the strength values of 0° compression and 90° compression, while in high temperature environment, due to the degradation of matrix material performance after moisture absorption, the mechanical parameter values of the material are significantly reduced, such as in-plane shear strength.

Application progress of power fiber reinforced composite materials

WU Xiong, CAI Wei, LI Jian, SUN Qi-gang, ZHU Xiao-dong, HE Chang-lin, KE Rui

2021, 0(11): 116-128. DOI: 10.19936/j.cnki.2096-8000.20211128.018

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The technology and application of fiber-reinforced resin composite materials is one of the key technical fields developed in our country. It has the advantages of light weight, high strength, corrosion resistance and high insulation technology, which is especially suitable for the application in power industry. In order to promote the development and application of fiber-reinforced insulation composite materials in the power industry, the application and progress of fiber-reinforced composite materials in power transmission poles, composite insulation cross arms, fan blades, composite insulators and high-voltage casings in recent years are reviewed. The structural design, technical characteristics and technical advantages of composite materials in the corresponding electric equipment are briefly described. Finally, combined with the development of corresponding electric composite materials and national policy trends, the application direction and development trends in future of fiber reinforced insulation composite materials in the power industry are also summarized.

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