The asymmetric semi-circular bend(ASCB)specimen has been proposed to investigate the cracking behavior in different geo and construction materials and attracted the attention of researchers due to its advantages.Howev...The asymmetric semi-circular bend(ASCB)specimen has been proposed to investigate the cracking behavior in different geo and construction materials and attracted the attention of researchers due to its advantages.However,there are few studies on the fracture toughness determination of rock materials.In this work,a series of fracture tests were performed with the ASCB specimens made of granite.The onset of fracture,crack initiation angle and crack propagating trajectory was analyzed in detail combined with several mixed mode fracture criteria.The influence of the crack length on the mode Ⅰ/Ⅱ fracture toughness was studied.A comparison between the fracture toughness ratios predicted by varying criteria and experimental results was conducted.The relationship between experimentally determined crack initiation angles and curves of the generalized maximum tangential stress(GMTS)criterion was obtained.The fracture process of the specimen was recorded with the high-speed camera.The shortcomings of the ASCB specimens for the fracture toughness determination of rock materials were discussed.The results may provide a reference for analysis of mixed mode I and II fracture behavior of brittle materials.展开更多
Stable or nearly stable orbits do not generally possess well-distinguished manifold structures that assist in designing trajectories for departing from or arriving onto a periodic orbit.For some potential missions,the...Stable or nearly stable orbits do not generally possess well-distinguished manifold structures that assist in designing trajectories for departing from or arriving onto a periodic orbit.For some potential missions,the orbits of interest are selected as nearly stable to reduce the possibility of rapid departure.However,the linearly stable nature of these orbits is also a drawback for their timely insertion into or departure from the orbit.Stable or nearly stable near rectilinear halo orbits(NRHOs),distant retrograde orbits(DROs),and lunar orbits offer potential long-horizon trajectories for exploration missions and demand eficient operations.The current investigation focuses on leveraging stretching directions as a tool for departure and trajectory design applications.The magnitude of the state variations along the maximum stretching direction is expected to grow rapidly and,therefore,offers information for efficient departure from the orbit.Similarly,maximum stretching in reverse time enables arrival with a minimal maneuver magnitude.展开更多
With the development of Unmanned Aerial Vehicles(UAVs), the applications of UAVs have been extensively explored. In the field of wireless communications, the relay nodes are often used to extend network coverage. Howe...With the development of Unmanned Aerial Vehicles(UAVs), the applications of UAVs have been extensively explored. In the field of wireless communications, the relay nodes are often used to extend network coverage. However, traditional fixed ground relays cannot be flexibly deployed due to their low heights and fixed locations. Hence, deploying UAV as relay node is a promising solution and has become a research hotspot. In this paper, we consider an UAVenabled relaying network in which a fixed-wing UAV is deployed between the Base Station(BS)and Ground Users(GUs). We study the energy-efficiency gap between the link “BS-UAV-GUs”and the link “BS-GUs”, and jointly optimize UAV relay transmission power and flight radius to achieve the highest energy-efficiency. Firstly, the UAV/BS-GUs channels models and the UAV energy consumption model are built. Secondly, the optimization objective function is formulated to maximize the energy-efficiency gap. Then, the solution of the optimization problem is divided into a two-step iteration process, in which the UAV relay transmission power and flight radius are adjusted to maximize the energy-efficiency gap. Finally, the experimental results under different simulation scenarios(such as cities, forests, deserts, oceans, etc.) are shown to illustrate the effectiveness of the proposed algorithm. The results show that the proposed algorithm can always find the optimal UAV relay transmission power and flight radius settings, and achieve the largest energy-efficiency gap. The convergency speed of the proposed algorithm is fast, and can obtain the optimal solution within only a few iterations.展开更多
基金Projects(52004182,51804110,51904101)supported by the National Natural Science Foundation of ChinaProject(2020JJ5188)supported by the Natural Science Foundation of Hunan Province,China。
文摘The asymmetric semi-circular bend(ASCB)specimen has been proposed to investigate the cracking behavior in different geo and construction materials and attracted the attention of researchers due to its advantages.However,there are few studies on the fracture toughness determination of rock materials.In this work,a series of fracture tests were performed with the ASCB specimens made of granite.The onset of fracture,crack initiation angle and crack propagating trajectory was analyzed in detail combined with several mixed mode fracture criteria.The influence of the crack length on the mode Ⅰ/Ⅱ fracture toughness was studied.A comparison between the fracture toughness ratios predicted by varying criteria and experimental results was conducted.The relationship between experimentally determined crack initiation angles and curves of the generalized maximum tangential stress(GMTS)criterion was obtained.The fracture process of the specimen was recorded with the high-speed camera.The shortcomings of the ASCB specimens for the fracture toughness determination of rock materials were discussed.The results may provide a reference for analysis of mixed mode I and II fracture behavior of brittle materials.
文摘Stable or nearly stable orbits do not generally possess well-distinguished manifold structures that assist in designing trajectories for departing from or arriving onto a periodic orbit.For some potential missions,the orbits of interest are selected as nearly stable to reduce the possibility of rapid departure.However,the linearly stable nature of these orbits is also a drawback for their timely insertion into or departure from the orbit.Stable or nearly stable near rectilinear halo orbits(NRHOs),distant retrograde orbits(DROs),and lunar orbits offer potential long-horizon trajectories for exploration missions and demand eficient operations.The current investigation focuses on leveraging stretching directions as a tool for departure and trajectory design applications.The magnitude of the state variations along the maximum stretching direction is expected to grow rapidly and,therefore,offers information for efficient departure from the orbit.Similarly,maximum stretching in reverse time enables arrival with a minimal maneuver magnitude.
基金supported in part by Shanghai Rising-Star Program(No.19QA1409100)in part by the National Natural Science Foundation of China(Nos.62071332,61631017 and U1733114)+1 种基金in part by the Fundamental Research Funds for the Central Universities,China。
文摘With the development of Unmanned Aerial Vehicles(UAVs), the applications of UAVs have been extensively explored. In the field of wireless communications, the relay nodes are often used to extend network coverage. However, traditional fixed ground relays cannot be flexibly deployed due to their low heights and fixed locations. Hence, deploying UAV as relay node is a promising solution and has become a research hotspot. In this paper, we consider an UAVenabled relaying network in which a fixed-wing UAV is deployed between the Base Station(BS)and Ground Users(GUs). We study the energy-efficiency gap between the link “BS-UAV-GUs”and the link “BS-GUs”, and jointly optimize UAV relay transmission power and flight radius to achieve the highest energy-efficiency. Firstly, the UAV/BS-GUs channels models and the UAV energy consumption model are built. Secondly, the optimization objective function is formulated to maximize the energy-efficiency gap. Then, the solution of the optimization problem is divided into a two-step iteration process, in which the UAV relay transmission power and flight radius are adjusted to maximize the energy-efficiency gap. Finally, the experimental results under different simulation scenarios(such as cities, forests, deserts, oceans, etc.) are shown to illustrate the effectiveness of the proposed algorithm. The results show that the proposed algorithm can always find the optimal UAV relay transmission power and flight radius settings, and achieve the largest energy-efficiency gap. The convergency speed of the proposed algorithm is fast, and can obtain the optimal solution within only a few iterations.
