关键词： 翻转课堂   机械动力学   教学改革  

摘要： 机械动力学是很多工科专业本科和研究生的重要课程,并已成为工程技术人员必须具备的理论知识。本文针对以往教学中,研究生主观能动性发挥不足,很多研究生不会提问题和提不出问题的现象进行了教学改革,采用翻转课堂的方式,结合案例教学法,取得了良好的效果。

关键词： annealing treatment   Avrami exponent   blue brittleness temperature   prestrain temperature   rolling process   static recrystallization   tensile properties  

摘要： In this research, the samples of a low carbon steel sheet were rolled up to a thickness prestrain of 67% at three different temperatures consisted of room, blue brittleness, and subzero temperature. Microhardness, SEM, and tensile tests were carried out to evaluate the static recrystallization kinetics defined by the Avrami equation, microstructural evolution, and mechanical properties. It was found that the Avrami exponent is altered with change in prestrain temperature and it achieves the value of 1 to 1. 5. Moreover, it was indicated that prestraining at subzero temperature followed by annealing at 600 A degrees C leads to considerable enhancement in tensile properties and kinetics of static recrystallization compared to room and blue brittleness temperatures. The prestraining at blue brittleness temperature followed by annealing treatment caused, however, a higher strength and faster kinetics compared with that at room temperature. It was concluded that although from the steel ductility point of view, the blue brittleness temperature is called an unsuitable temperature, but it can be used as prestraining temperature to develop noticeable combination of strength and ductility in low carbon steel.

关键词： cable   crystallization kinetics   electron beam   fluoride precipitation   mechanical performance   XETFE  

摘要： The cross-linking effects of cross-linked ethylene-tetrafluoroethylene copolymer (XETFE)-insulated cables at different electron beam radiation doses were analyzed in this paper. Evaluation of the mechanical performance of the cables revealed that the highest tensile and breaking elongation was achieved at a radiation dose of 8x10(4)Gy and that XETFE had a good resistance to extreme electron beam irradiation. This is attributed to the cross-linking effects induced by electron beam irradiation, and this takes full advantage of the strength of molecular chain crosslink to each other. The crystallization kinetics of XETFE at different electron beam radiation doses were studied in detail in terms of the non-isothermal and isothermal crystallization processes. The results indicated that the crystallinity of the XETFE domain increased with an increase in the radiation dose as a result of heterogeneous nucleation. Moreover, the highest E-a was obtained, indicating that XETFE absorbed some energy at a radiation dose of 8x10(4)Gy. These kinetic parameters had help in carrying out a comprehensive evaluation of the performance of XETFE-insulated cables for aerospace applications. Moreover, the fluoride precipitation observed in this study indicated that upon electron beam irradiation, XETFE could internally produce hydrogen fluoride, which is corrosive to metals. Thus, optimizing the radiation dose was necessary to achieve the desired performance. We could believe that the improvement for properties of electron beam XETFE insulation cables would expand their range of applications in the aerospace field.

关键词： High Density Polyethylene   Composite   Chemical Treatment   Mechanical Behavior   Water Absorption Kinetics  

摘要： The objective of this work was to investigate the effect of surface improvement on human hair fiber (HF) based high-density polyethylene (HDPE) reinforced polymer composites. A universal testing machine was used for the characterization of the mechanical behaviors of different types of HF base reinforced HDPE composites, and studies were conducted for the optimization of male/female fiber percentage (15% HF) in polymer composites. The alkali treatment (AT) and acrylic acid treatment (AAT) of HF reinforced HDPE composites showed a remarkable improvement in tensile strength (upto 15.487MPa and 15.638MPa, respectively), which was significantly changed in comparison to the tensile strength of untreated composites. FTIR and SEM test were used to characterize the fiber surface and HF/HDPE reinforced composites. Water absorption kinetics was investigated for the study of diffusion mechanism and kinetics of composites materials, which would be useful to boost the applications of the composite in different areas.

关键词： Self-power   Optical wireless transmission   Mechanical sensing   Triboelectric nanogenerators  

摘要： The ubiquitous sensors have accelerated the realization of Internet of Things (IoT) but also raised challenges to the current overcrowding radio frequency (RF) based communications. The optical wireless communication (OWC) that utilizes the wide optic bandwidth can well solve the spectrum crisis and is an appealing complementary solution to the IoT applications. However, the additional direct current (DC) power supply and complicated modulating and power management circuits may limit the large-scale deployment of OWC systems. In this paper, by integrating with triboelectric nanogenerators (TENGs), the light-emitting diode (LED) could be directly transformed into a wireless transmitter that conveys the information associated with mechanical stimuli without additional power supply. With the customized TENG devices and the help of advanced image processing and machine learning techniques, three demonstrations with functions of optical remote control, pressure sensing, and security authentication, were demonstrated. The concept and results in this paper may greatly broaden the application of IoT through the integration of OWC and TENG.

关键词： Annealing   Microstructure evolution   Steel/aluminum interface   Intermetallic compound layer   Mechanical property  

摘要： In this paper, an annealing treatment was performed to eliminate the work hardening effect of cold roll bond clad sheets, and the effect of annealing temperature and time on the microstructure evolution at steel/aluminum interface and mechanical properties of aluminum-steel clad sheet were investigated. Results showed that the longer the time and the higher the temperature are, the thicker the inter diffusion zone become. Fe2Al5 phase preferentially formed at the inter-diffusion layer at 600 degrees C. As the annealing temperature increased, another FeAl3 phase gradually formed. The thickness of intermetallic compound layers over 10 mu m would seriously deteriorated the mechanical properties of aluminum-steel clad sheet. Furthermore, the relationship of the thickness of inter-diffusion layer with the annealing time and temperature was established, which played a vital role in predicting the working time of aluminum steel clad sheet. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： dissipative particle dynamics   mechanical properties   molecular dynamics   nanocomposites   rubber  

摘要： Dissipative particle dynamics (DPD) is used to predict the mechanical properties of crosslinking-polyisoprene (PI) reinforced by single-walled carbon nanotube (SWCNT). The role of the increasing concentration of SWCNT up to 8% in morphology of PI composite is investigated. The carbon nanotube (CNT) restricts the polymer movement as indicated by the reduction of mean square displacement and the end-to-end distance. The analysis of CNT bundle reveals that the CNT forms bundle of many sizes depending on its concentration. The mechanical reinforcement of the PI composites is attributed to the restricted motion of crosslinked entangled polyisoprene and to the alignment of aggregated CNT along the elongation direction. However, the DPD alone fails to account for the change in Young's modulus of the composite system as a function of the SWCNT concentration. Using the modified segmental repulsive potential (mSRP), which prevents polymeric chain crossing and enhances polymer entanglement, can address this shortcoming. The mSRP is necessary for correct description of mechanical reinforcement in a polymer composite system. To obtain quantitative agreement with the experimental modulus, the polyisoprene interaction parameter is modified from the published value by 10%. This modified parameter is found to work well for the simulation of polyisoprene composites.

关键词： Diffusion   Ketones   Evaporation   Solvents   Water  

摘要： The formation of hard agglomerates during drying has impacts on the product quality, particle size distribution, and downstream formulation. Hence, understanding the cause of hard agglomerates and determining a remedy for them or preventing their formation are necessary. The integrated sorption chamber (ISC) has been successfully applied to the study of hard agglomerate formation. The design and development of this ISC are briefly described. The vapor-phase profile of a binary mixture of acetone and water without active pharmaceutical ingredient (API) was predicted using an analytical solution of a batch distillation and verified via mass spectrometry (MS). The drying of the API wet cake, which is very soluble in water, using a starting point of 5 wt % water in acetone was conducted in the ISC. A significant delay in water removal was observed from both the weight measurement and the MS signal. The slow removal of water was attributed to diffusion of water from a thin crust layer induced by the solution of the API in water and drying, resulting in the formation of hard agglomerates at the end of drying. A wet-cake solvent composition change following a cake wash with acetone reduced the initial water content from 5.5 to 0.5 wt %. This altered wet-cake solvent composition changed the behavior of water removal, leading to a significant reduction of the diffusion time. The dried API contained soft lumps without hard agglomerates.

关键词： Cable galloping   Co-rotational finite element   Quasi-steady theory   Aerodynamic finite element  

摘要： Cables are widely used lightweight and efficient structural members affected by various nonlinearities. This paper is devoted to the development of a finite element approach to the study of the nonlinear dynamic behavior of cable structures under wind loading. Firstly, the formulation of a co-rotational beam-cable element is presented to account for cable geometrical nonlinearities within the context of an Updated Lagrangian approach. The Euler-Bernoulli kinematics is adopted in the co-rotated frame assuming small or moderate displacements and strains. Secondly, the formulation of an aerodynamic element, meant to be superimposed to the mechanical beam element, is developed to fully account for the nonlinearity of the aerodynamic forces. The wind interaction forces are computed within the framework of the quasi-steady theory. Appropriate procedures for applying the aforementioned elements in static and dynamic analyses are presented and applied to the study of the static configuration and the galloping vibrations of a suspended cable under steady and turbulent wind conditions. The results point out the importance of the first anti-symmetric in-plane mode on the galloping response of the iconic selected structure, and reveal the role of the mechanical and aerodynamic model by comparison with a different discretization based on cable finite elements. A mechanical model, such as the proposed co-rotational finite element formulation, which includes the description of torsional rotations is required to properly model the aerodynamic loads in dynamic galloping analyses under steady wind. The mechanical and aerodynamic assumptions, instead, play a less important role in the case of turbulent wind conditions, leading to an eminently buffeting response entailing a strong effect of the swing of the structure.