Influence of TiB2 particles on machinability and machining parameter optimization of TiB2/Al MMCs

In situ formed TiB₂ particle reinforced aluminum matrix composites (TiB₂/Al MMCs) have some extraordinary properties which make them be a promising material for high performance aero-engine blade. Due to the influence of TiB₂ particles, the machinability is still a problem which restricts the application of TiB₂/Al MMCs. In order to meet the industrial requirements, the influence of TiB₂ particles on the machinability of TiB₂/Al MMCs was investigated experimentally. Moreover, the optimal machining conditions for this kind of MMCs were investigated in this study. The major conclusions are: (1) the machining force of TiB₂/Al MMCs is bigger than that of non-reinforced alloy and mainly controlled by feed rate; (2) the residual stress of TiB₂/Al MMCs is compressive while that of non-reinforced alloy is nearly neutral; (3) the surface roughness of TiB₂/Al MMCs is smaller than that of non-reinforced alloy under the same cutting speed, but reverse result was observed when the feed rate increased; (4) a multi-objective optimization model for surface roughness and material removal rate (MRR) was established, and a set of optimal parameter combinations of the machining was obtained. The results show a great difference from SiC particle reinforced MMCs and provide a useful guide for a better control of machining process of this material.     

纤维增韧陶瓷基复合材料的比例极限应力与残余热应力关系

从细观力学角度分析并建立了纤维增韧陶瓷基复合材料从制备温度冷却到室温过程中产生的残余热应力与复合材料的比例极限应力的关系模型。该模型表明,减少复合材料的残余热应力或提高复合材料的纤维与基体的模量比,均可提高复合材料的比例极限应力。通过单调拉伸实验测试了先驱体浸渍裂解法(PIP)制备的2D SiC/SiC复合材料的比例极限应力,并采用文中建立的比例极限应力与残余热应力关系模型,计算出复合材料SiC基体的残余热应力为-19.5 MPa。分析表明,该结果是合理的。此外,引用了公开文献报道的5种复合材料体系数据,用于验证文中所建立的比例极限应力与残余热应力关系模型的适应性和可靠性,计算结果与实验结果最大误差为18.6%,表明该模型具有较好的适应性和可靠性,可为纤维增韧陶瓷基复合材料的研究提供新思路。     

Initial nighttime ionospheric observations with advanced ionospheric probe onboard FORMOSAT-5

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28° inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation.     

High temporal resolution global PWV dataset of 2005–2016 by using a neural network approach to determine the mean temperature of the atmosphere

Atmospheric water vapour plays an important role in phenomena related to the global hydrologic cycle and climate change. However, the rapid temporal–spatial variation in global tropospheric water vapour has not been well investigated due to a lack of long-term, high-temporal-resolution precipitable water vapour (PWV). Accordingly, this study generates an hourly PWV dataset for 272 ground-based International Global Navigation Satellite System (GNSS) Service (IGS) stations over the period of 2005–2016 using the zenith troposphere delay (ZTD) derived from global-scale GNSS observation. The root mean square (RMS) of the hourly ZTD obtained from the IGS tropospheric product is approximately 4 mm. A fifth-generation reanalysis dataset of the European Centre for Medium-range Weather Forecasting (ECMWF ERA5) is used to obtain hourly surface temperature (T) and pressure (P), which are first validated with GNSS synoptic station data and radiosonde data, respectively. Then, T and P are used to calculate the water vapour-weighted atmospheric mean temperature (Tm) and zenith hydrostatic delay (ZHD), respectively. T and P at the GNSS stations are obtained via an interpolation in the horizontal and vertical directions using the grid-based ERA5 reanalysis dataset. Here, Tm is calculated using a neural network model, whereas ZHD is obtained using an empirical Saastamoinen model. The RMS values of T and P at the collocated 693 radiosonde stations are 1.6 K and 3.1 hPa, respectively. Therefore, the theoretical error of PWV caused by the errors in ZTD, T and P is on the order of approximately 2.1 mm. A practical comparison experiment is performed using 97 collocated radiosonde stations and 23 GNSS stations equipped with meteorological sensors. The RMS and bias of the hourly PWV dataset are 2.87/?0.16 and 2.45/0.55 mm, respectively, when compared with radiosonde and GNSS stations equipped with meteorological sensors. Additionally, preliminary analysis of the hourly PWV dataset during the EI Niño event of 2014–2016 further indicates the capability of monitoring the daily changes in atmospheric water vapour. This finding is interesting and significant for further climate research.     

Theoretical aspects of selecting repeated unit cell model in micromechanical analysis using displacement-based finite element method

Repeated Unit Cell(RUC)is a useful tool in micromechanical analysis of composites using Displacement-based Finite Element(DFE)method,and merely applying Periodic Displacement Boundary Conditions(PDBCs)to RUC is almost a standard practice to conduct such analysis.Two basic questions arising from this practice are whether Periodic Traction Boundary Conditions(PTBCs,also known as traction continuity conditions)are guaranteed and whether the solution is independent of selection of RUCs.This paper presents the theoretical aspects to tackle these questions,which unify the strong form,weak form and DFE method of the micromechanical problem together.Specifically,the solution’s independence of selection of RUCs is dealt with on the strong form side,PTBCs are derived from the weak form as natural boundary conditions,and the validity of merely applying PDBCs in micromechanical Finite Element(FE)analysis is proved by referring to its intrinsic connection to the strong form and weak form.Key points in the theoretical aspects are demonstrated by illustrative examples,and the merits of setting micromechanical FE analysis under the background of a clear theoretical framework are highlighted in the efficient selection of RUCs for Uni Directional(UD)fiber-reinforced composites.     

基于计划到达时刻的四维航迹规划

为了加速空管自动化与智能化技术的应用与实施,提出了水平航迹与高度/速度剖面分阶段设计的四维航迹规划方法。基于动态时间规整与层次聚类方法实现历史雷达航迹聚类分析,完成水平航迹设计。建立速度剖面规划模型,通过航迹生成器与速度调整器交互协作,精细化控制进场时间,以满足计划到达时间要求。以上海浦东国际机场PINOT进场航班为例实施验证,结果表明,四维航迹规划方法可有效实现航班的计划到达时间,从而减少航班延误,提高终端空域运行效率。     

基于模型的系统工程概述

由于航空领域涉及的系统日益高度复杂,为更好推进基于模型的系统工程(Model Based System Engineering,MBSE)研发体系,通过从当前遇到的问题、推行基于模型的系统工程的必要性、优势、未来的挑战等方面进行了较为详细地阐述.基于模型的系统工程研发体系具有知识表示的无二义、系统设计的一体化、沟通交流的高效率等优势,是未来发展的大趋势.     

工业CT检测技术在5 kN发动机研制中的应用

针对5 kN发动机结构焊缝和复合材料喷管不可检不可测的难题,开展了工业CT检测技术研究。分析了复杂结构焊缝和复合材料喷管的常规方法检测难点,利用工业CT检测技术穿透能力强、不受工件复杂结构影响的优势,对5 kN发动机产品中的电子束焊缝、氩弧焊焊缝、复合材料喷管等结构进行了检测试验,得到了良好的检测效果。结果表明,工业CT检测技术可以识别5 kN发动机产品中的气孔、未焊透、分层、裂纹等缺陷,并实现对缺陷的定位与测量,可为焊缝质量评估提供依据。     

外挂物投放问题的二维非结构动态网格技术研究

以动态网格守恒型方程为控制方程,在二维非结构网格基础上发展了弹簧式光顺法和局部网格重构法两种动态网格变形技术。结合运动轨迹计算方程,将动态网格法应用于飞机外挂物投放问题中,成功模拟了导弹自由投放的整个过程,获得了详细的包括弹体运动速度、运动轨迹和受力情况等在内的丰富的流场信息。成功的应用不仅验证了动态网格法的可行性;而且也表明,动态网格法的进一步完善与发展将成为解决外挂物投放问题的有力工具。     

石墨填料对浸渗热解-熔融浸渗方法制备的C/SiC复合材料结构和性能的影响

采用石墨树脂浆料浸渍三维针刺碳毡增强体,热解后得到C/C多孔体,并采用反应熔体浸渗法制备C/SiC复合材料.研究了石墨填料对C/C多孔体的结构以及C/SiC复合材料力学性能的影响.结果 表明,石墨树脂浆料浸渍时树脂填充束间小孔形成结构致密的亚结构单元,而石墨可以有效填充胎网层等大孔隙,一次浸渍热解后碳产率有效提高.所得...