The paper proposes a comparison between classical two-dimensional (2D) finite elements (FEs) and an exact three-dimensional (3D) solution for the free vibration analysis of one-layered and multilayered isotropic, composite and sandwich plates and cylinders. Low and high order frequencies are analyzed for thick and thin simply supported structures. Vibration modes are investigated to make a comparison between results obtained via the finite element method and those obtained by means of the exact three-dimensional solution. The 3D exact solution is based on the differential equations of equilibrium written in general orthogonal curvilinear coordinates. This exact method is based on a layer-wise approach, the continuity of displacements and transverse shear/normal stresses is imposed at the interfaces between the layers of the structure. The geometry for shells is considered without any simplifications. The 2D finite element results are obtained by means of a well-known commercial FE code. The differences between 2D FE solutions and 3D exact solutions depend on the considered mode, the order of frequency, the thickness ratio of the structure, the geometry, the embedded material and the lamination sequence.

Desktop 3D FDM (Fused Deposition Modelling) printers are usually employed for the production of nonstructural objects. In recent years, the present authors tried to use this technology also to produce structural elements employed in the construction of small UAVs (Unmanned Aerial Vehicles). Mechanical stresses are not excessive for small multirotor UAVs. Therefore, the FDM technique combined with polymers, such as the ABS (Acrylonitrile Butadiene Styrene) and the PLA(Poly Lactic Acid), can be successfully employed to produce structural components. The present new work is devoted to the production and preliminary structural analysis of sandwich configurations. These new lamination schemes could lead to an important weight reduction without significant decreases of mechanical properties. Therefore, it could be possible, for the designed application (e.g., a multifunctional small UAV produced via FDM), to have stiffener and lighter structures easy to be manufactured with a low-cost 3D printer. The new sandwich specimens here proposed are PLA sandwich specimens embedding a PLA honeycomb core produced by means of the same extruder, multilayered specimens with ABS external layers and an internal homogeneous PLA core using different extruders for the two materials, sandwich specimens with external ABS skins and an internal PLA honeycomb core using different extruders for the two materials, and sandwich specimens where two different extruders have been employed for PLA material used for skins and for the internal honeycomb core. For all the proposed configurations, a detailed description of the production activity is given.Moreover, several preliminary results about three-point bending tests, different mechanical behaviors and relative delamination problems for each sandwich configuration will be discussed in depth.

Abstract:The Fused Deposition Modelling (FDM) technology is widely used in rapid prototyping. 3D printers for home desktop applications are usually employed to make non-structural objects. When the mechanical stresses are not excessive, this technology can also be successfully employed to produce structural objects, not only in prototyping stage but also in the realization of series pieces. The innovative idea of the present work is the application of this technology, implemented in a desktop 3D printer, to the realization of components for aeronautical use, especially for unmanned aerial systems. For this purpose, the paper is devoted to the statistical study of the performance of a desktop 3D printer to understand how the process performs and which are the boundary limits of acceptance. Mechanical and geometrical properties of ABS (Acrylonitrile Butadiene Styrene) specimens, such as tensile strength and stiffness, have been evaluated. ASTM 638 type specimens have been used. A capability analysis has been applied for both mechanical and dimensional performances. Statistically stable limits have been determined using experimentally collected data.

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In this paper, polymeric specimens are produced via the Fused Deposition Modelling (FDM) technique. Then, experimental tensile and compression tests are conducted to evaluate the main mechanical properties of elements made of PolyLacticAcid (PLA) material. A standardized characterization test method for FDM 3D printed polymers has not been developed yet. For this reason, the ASTM D695 (usually employed for polymers produced via classical methods) has been here employed for FDM 3D printed polymers after opportune modifications suggested by appropriate experimental checks. A statistical analysis is performed on the geometrical data of the specimens to evaluate the machine process employed for the 3D printing. A capability analysis is also conducted on the mechanical properties (obtained from the experimental tests) in order to calculate acceptable limits useful for possible structural analyses. The Young modulus, the proportional limit and the maximum strength here defined for PLA specimens allow to confirm the different behavior of FDM printed PLA material in tensile and compressive state. These differences and the calculated acceptable limits for the found mechanical properties must be considered when this technology will be employed for the design of small structural objects made of PLA, as in the present study, or ABS (Acrilonitrile Butadiene Stirene). From the statistical and capability analysis, the employed printing process appears as quite stable and replicable. These types of research together with other similar ones that will be conducted in the future will allow to use polymeric materials and the FDM technique to produce small structural elements and also to carry out the appropriate verifications.

Immersive media services, such as augmented reality and virtual reality (AR/VR), a 360degree video, and free-viewpoint video (FVV), are popular today. They require massive data storage, ultrahigh computing power, and ultralow latency. It is hard to fulfill these requirements simultaneously in a conventional communication system using a cloud/centralized radio access network (C-RAN). Specifically, due to centralized processing in such a system, the end-to-end latency, as well as the burden on the fronthaul network, are expected to be high. Fog computing-based radio access networks (F-RAN), in contrast, have been widely considered as an enabler for immersive media. Our contribution in this paper is threefold: First, we propose various service scenarios reflecting the characteristics of immersive media. Second, we identify the technologies that are required to support the proposed service scenarios under F-RAN and discuss how they can support the proposed scenarios efficiently. Third, we discuss possible research opportunities.a A list of acronyms can be found in the Appendix.INDEX TERMS Fog computing, radio access networks, immersive media, free-viewpoint video, 360-degree video, virtual reality, augmented reality.

Massive content delivery is in the spotlight of the research community as both data traffic and the number of connected mobile devices are increasing at an incredibly fast pace. The enhanced mobile broadband (eMBB) is one of the main use cases for the fifth generation of mobile networks (5G), which focuses on transmitting greater amounts of data at higher data rates than in the previous generations, but also on increasing the area capacity (given in bits per second per square meter) and reliability. However, the broadcast and multicast implementation in 5G and presents several drawbacks such as unexpected disconnections and the lack of device-specific QoS guarantees. As a result, whenever the exact same content is to be delivered to numerous mobile devices simultaneously, this content must be replicated. Hence, the same number of parallel unicast sessions as users are needed. Therefore, novel systems that provide efficient massive content delivery and reduced energy consumption are needed. In this paper, we present a network-coded cooperation (NCC) protocol for efficient massive content delivery and the analytical model that describes its behavior. The NCC protocol combines the benefits of cooperative architectures known as mobile clouds (MCs) with Random Linear Network Coding (RLNC). Our results show the benefits of our NCC protocol when compared to the establishment of numerous parallel unicast sessions are threefold: offload data traffic from the cellular link, reduce the energy consumption at the cooperating users, and provide throughput gains when the cellular bandwidth is insufficient. INDEX TERMS Cooperation, fifth generation of mobile networks (5G), massive content delivery, random linear network coding (RLNC).

Massive content delivery in cellular networks is in the spotlight of the research community as data traffic is increasing at an incredibly fast pace. The existing LTE-A implementation for content broadcast presents several issues such as indoor coverage, along with low energy and spectral efficiency. Therefore, novel systems that provide efficient massive content delivery and reduced energy consumption are needed. In this paper we present a massive content distribution protocol that combines the benefits of cooperative mobile clouds (CMCs) with Random Linear Network Coding (RLNC) through multicast WiFi links. Our main goal is to offload data traffic from the LTE-A link and to reduce the energy consumption at the cooperating UEs. We solve the problem of excessive signaling that oftentimes arises in cooperative approaches by eliminating feedback messages within the CMCs. Instead, we provide a simple but accurate analytic model to correctly configure the number of coded transmissions to be performed within the CMCs. Results show that energy savings of more than 37 percent can be achieved with our protocol when compared to direct content download from the cellular base station. Furthermore, bandwidth utilization at the LTE-A link is sharply reduced.