Table of Content

25 August 2024, Volume 52 Issue 8

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Traffic & Transportation Engineering

Calculation and Spatial Distribution Characteristics of Carbon Emissions from New Energy Vehicles on Expressways



WEN Huiying, HE Ziqi, HU Yuqing, et al

2024, 52(8):  1-13.  doi:10.12141/j.issn.1000-565X.230293

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In order to overcome the difficulty in estimating the indirect carbon emissions from new energy vehicles due to the energy consumption in a large-scale expressway network, this paper proposes a new carbon emission calculation method of new energy vehicles based on the toll data in expressway ETC network. Firstly, the traffic flow data of new energy vehicles on expressways were cleansed and processed. Then, according to the distribution of toll gantries, road sections were defined and segmented. On this basis, the energy consumption of different types of new energy vehicles was analyzed. Furthermore, a carbon emission quantification model of new energy vehicles in the operational stage was established. Finally, by taking Guangdong Province as an example, the carbon emissions of new energy vehicles on expressways were calculated, with its spatial distribution characteristics being also analyzed. The results show that (1) Category 1 pure electric passenger cars are the main source of carbon emissions from new energy vehicles on expressways, accounting for 74.20% of the total carbon emissions of new energy vehicles, followed by Category 1 pure electric trucks, whose carbon emissions account for 14.18% of the total, and Category 1 plug-in hybrid passenger cars contribute 11.62% of the total; (2) the cities in the Pearl River Delta urban agglomeration are the main contributors to high carbon emissions from new energy vehicles, and the less developed regions in eastern, western and northern Guangdong account for less than 13% of the total carbon emissions from new energy vehicles; (3) the road segments with high carbon emissions from new energy vehicles mainly locate in the transportation hubs and inner-city expressway rings of developed cities, and Guangzhou metropolitan area as well as Shenzhen metropolitan area as the center has a radiating effect on the expressway network of surrounding cities; and (4) in less developed areas, due to the low density of expressway network and relatively lagging infrastructure, the penetration rate of new energy vehicles is low, the corresponding carbon emissions from new energy vehicles are generally low.

Differentiated Highway Toll Pricing Model Considering Carbon Emission Reduction Benefits



WANG Jiangfeng, LI Xiudong, SONG Zhifan, et al

2024, 52(8):  14-22.  doi:10.12141/j.issn.1000-565X.230225

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Differentiated highway toll strategy can promote carbon emission reduction in the road field and boost the realization of the “double-carbon” goal. In this study, the carbon reduction benefits are considered as a part of the highway’s benefits. With the goal of simultaneously considering highway benefits and logistics enterprise costs, the study constructed a differentiated toll pricing model based on dual-layer programming. Firstly, the factors influencing truck route choices were analyzed; then Logit model was used to construct transfer traffic flow measurement method based on differentiated highway charging and carbon emission measurement method based on transferred traffic flow, and it proposed carbon trading pricing model under shadow price; finally, considering the partial maximization of carbon emission reduction benefits and the cost minimization of logistics enterprises, it proposed the dual-layer programming model of differentiated charging, and at the same time, the traffic flow data of highway A and parallel national and provincial roads in Shandong province were used for example analysis. The results indicate that, through the adjustment of differentiated highway toll strategy, the dual-layer programming model ensures optimal benefits for highway management while minimizing logistics enterprise costs. Furthermore, adjusting toll standards around the optimal toll rate has minimal impact on the benefits of Highway A, with benefit fluctuations remaining below 1.5%, demonstrating good revenue stability. The dual-layer programming model for differentiated toll pricing model proposed in this study can maximize the increase in highway carbon reduction benefits and minimize logistics enterprise costs.

Travel Carbon Emission Prediction Model Based on Resident Attribute Data



SU Yuejiang, WEN Huiying, YUAN Minxian, WU Dexin, ZHOU Lulu, QI Weiwei

2024, 52(8):  23-33.  doi:10.12141/j.issn.1000-565X.230355

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It is an important basis for precise formulation of transportation emission reduction measures to accurately analyze the importance of factors influencing residents’ travel mode and the sensitivity of carbon emissions. According to the comprehensive analysis of the influencing factors such as family attributes, personal attributes, travel attributes and environmental attributes of the residents’ travel survey, the prediction model of residents’ travel mode was constructed based on LightGBM (Light Gradient Boosting Machine) and verified. Combined with the travel activity level, the carbon emission coefficient of various energy types, the standard coal coefficient and other parameters, the travel carbon emission prediction model based on the resident attribute data was constructed. Finally, taking Guangzhou as an example, the carbon emission intensity and total amount of residents’ travel mode were predicted, and importance of factors influencing travel mode and sensitivity was analyzed.The results indicate that the carbon emission prediction model constructed based on the attribute data of residents can more accurately predict the carbon emission of various modes of travel, better analyze the importance and sensitivity of the influencing factors of carbon emission, and comprehensively reveal the relationship between travel behavior, travel mode and travel carbon emission. Among them, the distance between the start and the end and the nearest bus station or the distance from the nearest subway station, the cost of self-driving and travel distance are important factors affecting the choice of residents’ travel mode. The competitiveness of subway travel increases significantly with the decrease of distance when the distance between the starting and the end point and the nearest subway station drops by 55%. In the area with high density of bus stops, the distance between the start and the nearest bus station is not sensitive to residents’ travel mode choice. It is the turning point of the residents’ travel mode and carbon emission when the carbon emission cost increases by 400%. After passing the turning point, the car travel mode is difficult to transfer. The carbon emissions fell the fastest, with a maximum reduction of 90.4% when the reduction in travel distance was within 90%.

Traffic & Transportation Engineering

Speed Prediction for Road Around Large Scale Activities Venues Considering Multiple Factors Synergism



WENG Jiancheng, WU Mingzhu, WEI Ruicong, et al

2024, 52(8):  34-44.  doi:10.12141/j.issn.1000-565X.230386

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Large scale activities can cause a sudden increase and dissipation of traffic flow in the area around the venue, resulting in occasional and uncertain fluctuations of the road network operation in the surrounding area. The existing methods are insufficient to capture the evolution mechanism of traffic flow under the influence of multi-dimensional factors in special events at the prediction scale. In order to fully exploit the information of time series and influencing factor features of road section speed and effectively deal with the coupling mechanism between different influencing features in speed prediction, this paper proposed a speed prediction model (MC-LSTM) combining Interpretable Machine Learning and Long Short-Term Memory network. Firstly, the study combined the characteristics of large scale activities to construct the set of influencing factors. Then it used the XGBoost algorithm to evaluate the relative importance of the impact of activity scale, nature and other factors characteristics on the speed of road sections around the venue. It quantified the synergistic utility of multiple factors on the operation state of the road network around the venue, fused LSTM networks, considered the time-dependent relationship of traffic state, captureed the temporal correlation of different historical periods, and accurately predicted the speed of road sections around the venue during the activity. MC-LSTM was validated by taking the road network around large scale activities venues in Beijing for six consecutive months. The results indicate that the prediction accuracy of the MC-LSTM model can reach more than 94.5%, which is better than that of XGBoost model considering multiple factors synergism, LSTM model considering only single factor features and the LSTM model not considering external features. It proved that the model proposed in this paper has better validity and stability. This study can provide a decision basis for optimizing the traffic organization of the road network around the large scale activities venues and formulating traffic control and security measures.

Intelligent Vehicle Object Detection Algorithm Based on Lightweight CenterNet



YUE Yongheng, NING Ruihou

2024, 52(8):  45-55.  doi:10.12141/j.issn.1000-565X.230513

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Aiming at the problem that the current object detection algorithm has many parameters and larger computation amount, resulting in slower response and difficulties of application in intelligent vehicle systems, this paper proposed an improved CenterNet object detection algorithm. That is, the lightweight MobileNetV3 network was applied to replace the original ResNet-50 network to reduce the amount of computation; the depth-separable PANet was applied to replace the feature enhancement network to obtain the features after the fusion of the multi-scale feature information, and the SimAM attention mechanism was introduced to strengthen the attention of the target features before the fusion of the features, then the SiLU activation function was used to replace the original object detection network in the ReLU activation function to enhance the network learning. Finally, the CPAN-ASFF module was proposed to fuse the depth-separable PANet output multi-scale feature maps to improve the object detection accuracy. The optimized KITTI dataset was applied to train and detect the improved CenterNet object detection algorithm for validation. And the results show that its mean average precision is 80.7%, which is 12 percentage points higher than that of the original CenterNet object detection algorithm; its detection speed is 65 f/s, and its number of parameters is 8.91 M, which is 72.73% less than the original algorithm. The improved algorithm performs better in the detection of occluded objects, overlapped objects and objects similar to the background. In the SODA10M dataset, the detection accuracy and speed of the algorithm are better than the current mainstream algorithms. The optimization of the algorithm and the experiments in the paper laid the technical support for the application of intelligent vehicles in practical engineering.

Analysis of Water Accumulation Characteristics on S-Curve of Highway Based on Numerical Simulation



WU Wenliang, ZENG Weikai, LI Zhi, WANG Xiaofei

2024, 52(8):  56-64.  doi:10.12141/j.issn.1000-565X.230505

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The S-curve pavement formation is susceptible to waterlogging due to its distinctive linear geometric characteristics. Therefore, the mastery of the waterlogging pattern is beneficial for the prevention and management of the hazards associated with waterlogged road sections, particularly in the design and construction phases. Using computational fluid dynamics software Fluent and the Eulerian Wall Film (EWF) which is based on the Eulerian-Lagrange method in computational fluid dynamics analysis, this paper carried out simulation calculations on the the typical S-curve bidirectional multilane road model with refinement treatment. The simulation included different variables, such as geometric features and rainfall, and obtained the film thickness and flow velocity distribution of the ponded water. The simulation results indicate that during the rainfall stage, 0% to 1% of the cross slope of the S-curve represents a waterlogged road section, 1% to 2% of the cross slope represents a waterlogged road section, and the waterlogging situation of the road section with a cross slope exceeding 2% is influenced by the intensity of rainfall. During the drainage stage, a cross-slope of 0%~1% on the S-curve represents a challenging road section, while a cross-slope of 1%~2% indicates a poorly drained road section. A cross-slope of more than 2% indicates a road section that is smoothly drained. The drainage of the road section at the end of the drainage time can remove the water film to less than 2 cm under different geometric conditions and rainfall conditions. The dimensions and gradient of the cross slope influence the lateral distribution of water as well as the longitudinal distribution. As the drainage stage progresses, the water film thickness distribution law changes to a longitudinal high in the middle and low on both sides with the overall distribution of S pattern, while the water film flow rate distribution law becomes longitudinal low in the middle and high on both sides. The width of the roadway exerts a significant influence on the total amount of water that falls onto the roadway from rainfall. This, in turn, affects the water film thickness. The ultra-high retardation rate primarily increases the number of road sections that are waterlogged or prone to water damage, thereby influencing the range of water distribution.

Operational Characteristics and Eco-Safe Influence of Connected Mixed Platoon in Car-Following Event



FU Qiang, ZHAO Xiaohua, LI Haijian, REN Wenhao, DAI Yibo

2024, 52(8):  65-75.  doi:10.12141/j.issn.1000-565X.230388

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Considering the current maturity level of autonomous technology and the general public’s acceptance of this technology, the connected mixed platoon mode may be a practical transitional strategy prior to the widespread deployment of fully autonomous platoons. To study the influence of connected condition and platoon mode on the operational characteristics and eco-safe influence of connected mixed platoon, this paper constructed a connected mixed platoon mode consisting of five vehicles: a human-driven leader vehicle equipped with an L2 connected driving assistance system, followed by connected autonomous vehicles. Thirty-six participants were invited to participate in driving simulation experiments using the developed test platform. Given that time-domain analysis focuses on the variation of indicators in the time dimension, while frequency-domain analysis can explore the frequency distribution characteristics of indicators, this paper analyzed vehicle operation characteristics from the time and frequency domain dimensions with indicators of speed, acceleration, fuel consumption and speed safety entropy. The results show that both the connected condition and the platoon mode contribute to stable vehicle operation and significantly enhance their eco-safety. Compared with the traditional single-vehicle mode, the connected mixed platoon has the best comprehensive efficiency and can reduce the fuel consumption by 10.67% and the speed safety entropy by 73.25%. The research results can provide scheme reference and platform support for autonomous driving enterprises and industries in executing connected HMI design, navigate on autopilot (NOA) development, and the feasibility and effectiveness test of connected mixed platoon.

TsGAN-Based Automatic Generation Algorithm of Lane-Change Cut-in Test Scenarios on Expressways for Autonomous Vehicles



ZHU Yu, XU Zhigang, ZHAO Xiangmo, WANG Runmin, QU Xiaobo

2024, 52(8):  76-88.  doi:10.12141/j.issn.1000-565X.230229

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The event wherein vehicles from the adjacent lane execute a lane-change maneuver, cutting into the lane occupied by autonomous vehicles, epitomizes a typical high-risk scenario on expressways within the domain of autonomous driving. Replicating such scenarios for testing on actual expressways involves significant safety risks. Virtual simulation test is one of the best approaches to addressing this issue. In order to automatically generate mass high-fidelity expressway lane-change cut-in test scenarios, this paper presents an automatic generation algorithm of lane-change cut-in test scenarios for autonomous vehicles based on TsGAN (Time-Series Generative Adversarial Network). In this algorithm, the time headway and lateral gap at the cut-in moment are taken as the evaluation metrics for scenario risk assessment, and 2 853 instances of lane-change cut-in scenarios with four different risk levels are extracted from the real expressway trajectory dataset highD. A model for generating lane-change cut-in test scenarios is established based on TsGAN, and is trained with the extracted real trajectory data. Then, the model is employed to generate the trajectories for the lane-change vehicle and the tested vehicle before the cut-in moment. To authenticate the generated trajectories, the distribution similarity and spectral error between the generated and the real trajectories are compared. Furthermore, an in-depth analysis of the kinematic interplay between the two vehicles at the cut-in moment and the distribution of trajectory parameters in the generated scenarios is performed to validate the coverage of the generated scenarios within naturalistic settings. The findings can be summarized as follows: (1) as illustrated by the distribution of key trajectory parameters, the average similarity between the generated and the real lane-change trajectories is 79.7%, with an average spectral error less than 8%, and more than 83.2% of the generated trajectories are within the buffer of the most analogous real trajectories, indicating a notable fidelity of the generated trajectories; (2) as compared with the collected real scenarios, the generated instances exhibit a more expansive coverage and a more even distribution within parameter intervals, and the time headway and lateral gap between the lane-change vehicle and the tested vehicle at the cut-in moment decrease by 17.83% and 16.37% in average, respectively, the distribution range of trajectory parameters expands by 19.44%, signifying a heightened coverage of the generated scenarios; and (3) the proposed TsGAN-based generation model has the capability of emulating lane-change cut-in test scenarios with four different risk levels, exhibiting pronounced specificity.

Architecture & Civil Engineering

Mechanism and Mechanical Characteristics of Cable-Catenary Arch Combined Structure



HAO Tianzhi, LI Chunhua, YANG Tao, LONG Xiayi, DENG Nianchun

2024, 52(8):  89-102.  doi:10.12141/j.issn.1000-565X.230566

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With the increase of traffic volume and overloading of vehicles, the mechanical properties of early arch bridges under live load can no longer satisfy the needs of modern traffic due to low design load grade, lack of effective maintenance and long term operation. To improve the mechanical performance of this kind of arch bridges, this paper proposed a cable-catenary arch combined structure. The cables were symmetrically arranged on both sides of the arch rib to form a new force system, which could change the force transmission path of the catenary arch structure and reduce the internal force of the arch structure. Under the mechanical diagram of cable-arch combined structure, the force equation of the cable-arch combined structure was established based on the elastic center method, and the analytical solution of the internal force of the arch rib was deduced by the approximate curve integration method under the vertical moving load. The reliability of the analytical solution was verified by ANSYS finite element analysis software, and the influences of design parameters such as cable constraint position, arch-axis coefficient, rise-span ratio, and axial stiffness ratio on the internal force of the arch structure under lane loading were analyzed. The mechanical properties and internal variation rules of the cable-arch combined structure were revealed. The results show that the relative error between the analytical solution of the internal force and the finite element result is within 1%. The setting of the cable changes the positive and negative interval distribution of the bending moment influence line value of the arch structure, and effectively reduces the peak value of the bending moment influence line of the arch structure, which can greatly reduce the overall bending moment of the arch structure and make the bending moment distribution more uniform. From the arch foot to the arch crown, the reduction of the bending moment and the growth of the axial force of the arch structure gradually decrease. As the axial stiffness ratio increases from 0.02 to 0.10, the negative bending moment of the arch foot decreases nonlinearly, with the maximum decrease of 63.7%; the increase of the cable force is inversely proportional to its value, and when the cables are set at 0.3L (L is the span of the arch) from the arch crown, the cable force can be increased to 1.9 times that at the axial stiffness ratio of 0.02. The influence of the rise-span ratio on the internal force of the arch structure cannot be ignored. The greater the rise-span ratio is, the greater the change in internal force of the arch structure is. The effect of arch-axis coefficient on the internal force of the arch structure can be neglected.

Simulation and Optimization Analysis of Natural Lighting in the Lower Space of Elevated Sports Venues in Primary and Secondary Schools



GUO Haoxu, LIN Duobing, DENG Mengren

2024, 52(8):  103-114.  doi:10.12141/j.issn.1000-565X.230626

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Under the background of insufficient school places and shortage of construction land in large and medium-sized cities in China, the three-dimensional design of primary and secondary school campus sports fields can form a vertical superposition of space, thereby releasing a large amount of construction land. However, the three-dimensional design of sports fields will also lead so some new design problems. Among them, the most complicated is the natural lighting of the lower space of the sports field, which facing both the problem of whether the illumination is sufficient and the problem of whether the light distribution is uniform. Domestic and foreign literature research shows that the current research on the lighting of three-dimensional sports venues at home and abroad is almost blank, which makes the current construction somewhat blind. Based on the above considerations, the study simulated the natural lighting environment of the lower area of different forms of campus three-dimensional sports venues with the parametric light environment simulation technology of the lighting simulation platform built based on Rhino, Grasshopper and Ladybugtools. And it extracted the factors affecting the lighting of the lower space of the three-dimensional sports field, including the size of the stadium, the number of overhead layers, the depth of the space, the functional layout, the location of the lighting port, and the sunshade and reflection measures. By simulating and analyzing the influencing factors, the influencing mechanisms of different factors were obtained, and the design strategies for three-dimensional sports fields to create a good natural light environment were further refined. The research feedback was formed based on the practice of optimizing the natural light environment of the student activity center of the Guangzhou Experimental Middle School project, thereby verifying the effectiveness of the research.

Urban Design Strategies Based on Compact Block Concept



HUANG Yeqing

2024, 52(8):  115-125.  doi:10.12141/j.issn.1000-565X.230052

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At present, the expansion of Chinese cities in a sprawl way in the current rapid construction process has caused the lack of humanized quality in the new urban central space environment. The huge development scale and rapid construction process of the new urban central areas lead to the prominent “isolation” phenomenon, mainly reflected in the isolation of the block and the isolation of life. In the face of those problems, guiding the future urban spatial environment to develop in the direction of compactness has become one of the important concerns of urban design. It aims to promote the transition of cities from the expansive development of serving rapid economic growth to the quality construction of social, economic, cultural and other comprehensive benefits by creating a dense urban space environment that can carry a greater density of social, economic and cultural activities and realize the continuous cycle and symbiosis of multiple activity flows. Therefore, the paper put forward the idea of creating humanized urban space through urban design based on compact blocks. Then, taking the construction of compact blocks of urban central district as the starting point, and through the analysis of some design projects, it discussed some strategies of the urban design on the construction of human-oriented, walkable and vigorous compact block environment in the central district. The strategies are as follows: making the structure compact so that the new central area of the city can maintain a block-pattern in a walkable and suitable scale; making the texture compact so that small-scale blocks can construct block space carriers with more humanized quality; and making the functions compact so as to provide people with a variety of choices to use block space and convenient space experience, etc. By doing so, it aims to shape a compact central district block environment that is people-oriented, walkable and full of vitality, in order to realize the ultimate goal of urban design to improve the quality of space environment and provide people with a colorful and humanized urban lifestyle.

Materials Science & Technology

Study on the Modification Mechanism of Warm Asphalt Rubber Based on Stage-Extraction Method



YU Huayang, ZHANG Zheng, DENG Yihao, et al

2024, 52(8):  126-137.  doi:10.12141/j.issn.1000-565X.230692

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To investigate the performance impact mechanism of warm asphalt rubber (WAR), 2 types of asphalt rubber (AR) and 4 types of warm asphalt rubber were prepared by using 70# base asphalt, 2 types of warm mixing agents, and 2 grain sizes of crumb rubber modifier. Liquid phase and insoluble crumb rubber modifier for prepared asphalt samples were obtained by the solid-liquid phase separation. The staged extraction method was innovatively employed to systematically extract components in the insoluble crumb rubber modifier blending zone layer by layer. Through experiments, the study established the connection between the preferential absorption of crumb rubber modifier and high- and medium-temperature rheological properties represented by asphalt rubber and clarified the modification mechanism of warm asphalt rubber. Test results indicate that the improvement of asphalt rubber’s high-temperature rutting resistance and low-temperature cracking resistance performance are attributed to the particle effect generated by insoluble crumb rubber modifier in asphalt and the modification effect of the soluble components of crumb rubber modifier. The improvement of fatigue resistance at medium temperature and the increase in viscosity of asphalt rubber is mainly attributed to the particle effect generated in the insoluble crumb rubber modifier. Different types of thermal mixture have different effects on the rheological properties of asphalt rubber. The addition of the organic warm mix additive is more significant in enhancing the high-temperature resistance to rutting and medium-temperature resistance to fatigue performance of asphalt rubber. The addition of the chemical warm mix additive proves to be more effective in improving the low-temperature resistance to cracking performance of asphalt rubber. The lighter components with smaller relative molecular mass in the asphalt rubber liquid phase are more easily absorbed by the crumb rubber modifier into its blending zone. The organic warm mix additive has almost no significant effect on the preferential absorption of crumb rubber modifier, while the addition of the chemical warm mix additive not only reduces the proportion of large relative molecular mass components in the liquid phase but also weakens the preferential absorption of crumb rubber modifier. The preferential absorption of crumb rubber modifier is beneficial for improving the high-temperature and fatigue resistance properties of asphalt rubber.

Preparation and Properties of Bacterial Cellulose Based CNFs/ZnO Microwave Absorbing Materials



LIU Pingan, LIN Baoshun, DING Huiling, et al

2024, 52(8):  138-145.  doi:10.12141/j.issn.1000-565X.230743

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With the continuous development of electronic information technology, electromagnetic pollution is becoming increasingly serious, and the research of high-efficiency absorbing materials has received more and more attention. Using bacterial cellulose as carbon source, the study prepared CNFs/ZnO composites based on bacterial cellulose with carbonization-modification and hydrothermal two-step method, and studied the effects of zinc acetate dihydrate concentration on the microwave absorption properties of CNFs/ZnO composites. The structure, morphology and microwave absorption performance of the composites were characterized by X-ray diffraction (XRD), cold field emission scanning electron microscopy (FESEM) and vector network analysis (VNA). The results show that CNFs/ZnO composites are successfully prepared, and the carbon nanofibers (CNFs) show an amorphous state without obvious diffraction peaks. Both carbonized and modified CNFs maintain the fine microscopic morphology of nanofibers with a three-dimensional network and porous structure of bacterial cellulose, but CNFs become crimped and the diameter of the fibers decreases significantly. In CNFs/ZnO composites, ZnO is closely attracted to the surface of CNFs or randomly inserted into the gap of CNFs. By changing the concentration of zinc acetate hydrate, the content of ZnO in the composite can be controlled, which in turn regulates the electromagnetic parameters of the composite and obtains good impedance matching. When the concentration of zinc acetate hydrate is 0.25 mol/L, ZnO is most uniformly dispersed on CNFs. At this time, the resistance loss, dielectric loss and interface polarization of CNFs and ZnO are synergistic on the three-dimensional porous network structure, which increases the multiple reflection, scattering and long-range dissipation of electromagnetic waves. The CNFs/ZnO composite prepared under this condition is a reliable composite absorbing material with the best reflection loss of -57.5 dB and an effective absorption bandwidth of 7.1 GHz, at a coating thickness of 2.8 mm and a frequency of 15.1 GHz.

Crack Extension Analysis of Rock-Like Material Based on the Improved Contact Model



DING Xiaobin, XIE Yuxuan, SHI Yu

2024, 52(8):  146-158.  doi:10.12141/j.issn.1000-565X.230416

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To explore the mechanism of extension and coalescence of the defect (flaw) in rocks under complex stress state, the study simulated the cracking process of rock-like material containing double pre-existing flaws in the biaxial compression test based on 3D discrete element software (PFC3D) and the improved parallel-bond model (IPBM). The microscopic parameters of the numerical model were calibrated by matching both macroscopic mechanical properties (uniaxial compressive strength, tensile strength, and elastic modulus) and cracking pattern with the laboratory experiments. The results show that: the first cracks are tensile cracks which vary with the inclination angle of flaw，and the secondary cracks are shear; the confining stress will suppress the first cracks and promote the development of the secondary cracks; the coalescence patterns of samples are closely related to the geometries of the flaw and the confining stress. Two new shear coalescence patterns (New1 and New2) can be observed at the confining stress σ_c > 0 MPa. The number of microcracks in the numerical sample is greatly affected by the confining stress, but not by the flaw geometries. The initial stress and peak stress of crack are correlated with confining stress and flaw inclination angle, but not with bridging length and bridging angle. The crack extension is closely related to the stress distribution in the sample, and the cracks first occur in the stress concentration area around the pre-existing flaws. The study provides a reliable reference for the subsequent calibration of discrete element model parameters and the crack extension mechanism based on discrete element.

Contents



2024, 52(8):  159. 

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