Wind-tunnel tests of a large-scale sectional model with synchronous measurements of force and vibration responses were carried out to investigate the nonlinear behaviors of vertical vortex-induced forces （VIFs） on t...Wind-tunnel tests of a large-scale sectional model with synchronous measurements of force and vibration responses were carried out to investigate the nonlinear behaviors of vertical vortex-induced forces （VIFs） on three typical box decks （i.e., fully closed box, centrally slotted box, and semi-closed box）. The mechanisms of the onset, development, and self-limiting phenomenon of the vertical vortex-induced vibration （VlV） were also explored by analyzing the energy evolution of different vertical VIF components and their contributions to the vertical VIV responses. The results show that the nonlinear components of the vertical VIF often differ from deck to deck; the most important components of the vertical VIF, governing the stable amplitudes of the vertical VIV responses, are the linear and cubic components of velocity contained in the self-excited aerodynamic damping forces. The former provides a constant negative damping ratio to the vibration system and is thus the essential power driving the development of the VIV amplitude, while the latter provides a positive damping ratio proportional to the square of the vibration velocity and is actually the inherent factor making the VIV amplitude self-limiting. On these bases, a universal simplified nonlinear mathematical model of the vertical VIF on box decks of bridges is presented and verified in this paper; it can be used to predict the stable amplitudes of the vertical VIV of long-span bridges with satisfactory accuracy.展开更多
To simulate the fatigue characteristics of the pile-board structure under long-term dynamic load, using the in-situ dynamic testing system DTS-1, the forced vibration loading was repeated one million times at differen...To simulate the fatigue characteristics of the pile-board structure under long-term dynamic load, using the in-situ dynamic testing system DTS-1, the forced vibration loading was repeated one million times at different cross-sections of the pile-board structure for high-speed railway. The dynamic deformation, permanent deformation and dynamic stress of main reinforcements were measured. The test results show that the dynamic responses of the pile-board structure almost did not vary with the forced vibration times under the simulated trainload. After one million times of forced vibration, the permanent deformations of the midspan section of intermediate span and midspan section of side span were 0.7 mm and 0. 6 mm, respectively, and there was no accumulative plastic deformation at the bearing section of intermediate span.展开更多
An innovative semisolid technique termed as vibrating cooling slope(VCS)has been applied to producing in-situ Al-25%Mg2Si(mass fraction)composite.The molten Al-16.5Mg-9.4%Si(mass fraction)alloy with 100°C superhe...An innovative semisolid technique termed as vibrating cooling slope(VCS)has been applied to producing in-situ Al-25%Mg2Si(mass fraction)composite.The molten Al-16.5Mg-9.4%Si(mass fraction)alloy with 100°C superheat was poured on the surface of an inclined copper plate(set at 45°inclined angle)while it was vibrated at a frequency of 40 Hz and an amplitude of 400μm.After travelling the length of 40 cm on the slope,the resultant semisolid alloy was cast into a steel mold.For the purpose of comparison,reference composite samples were made by gravity casting(GC)and conventionally still cooling slope casting(CS)methods using the same alloy under identical conditions.The samples were hot extruded at 500°C.It was concluded that the size of Mg2Si particles was decreased by about 50%and 70%for the CS and VCS produced samples respectively when compared to that of the GC produced sample.Despite of their higher porosity contents,both the as-cast and hot-extruded VCS processed samples exhibited higher hardness,shear yield stress(SYS)and ultimate shear strength(USS)values as compared with their GC produced counterparts.These results were attributed to the refined and modified microstructure obtained via this newly developed technique.展开更多
Ultrasonic vibration-assisted milling has been widely applied in machining the difficultto-cut materials owing to the remarkable improvements in reducing the cutting force.However,analytical models to reveal the mecha...Ultrasonic vibration-assisted milling has been widely applied in machining the difficultto-cut materials owing to the remarkable improvements in reducing the cutting force.However,analytical models to reveal the mechanism and predict the cutting force of ultrasonic vibrationassisted milling metal matrix composites are still needed to be developed.In this paper,an analytical model of cutting force was established for ultrasonic vibration-assisted milling in-situ TiB_(2)/7050 Al metal matrix composites.During modeling,change of motion of the cutting tool,contact of toolchip-workpiece and acceleration of the chip caused by ultrasonic vibration was considered based on equivalent oblique cutting model.Meanwhile,material properties,tool geometry,cutting parameters and vibration parameters were taken into consideration.Furthermore,the developed analytical force model was validated with and without ultrasonic vibration milling experiments on in-situ TiB_(2)/7050 Al metal matrix composites.The predicted cutting forces show to be consistent well with the measured cutting forces.Besides,the relative error of instantaneous maximum forces between the predicted and measured data is from 0.4%to 15.1%.The analytical model is significant for cutting force prediction not only in ultrasonic-vibration assisted milling but also in conventional milling in-situ TiB_(2)/7050 Al metal matrix composites,which was proved with general applicability.展开更多
分段估计法作为一种重要的非线性振型结构高频天平(high-frequency force balance,HFFB)试验模态力计算方法,可以对结构脉动风荷载的空间分布进行估计。该文章抛弃了分段估计法中无量纲风荷载谱沿高度不变的假定,提出了一种可以考虑荷...分段估计法作为一种重要的非线性振型结构高频天平(high-frequency force balance,HFFB)试验模态力计算方法,可以对结构脉动风荷载的空间分布进行估计。该文章抛弃了分段估计法中无量纲风荷载谱沿高度不变的假定,提出了一种可以考虑荷载谱沿高度变化的方法。以Kaimal脉动风谱为例提出并推导了无量纲风荷载谱沿高度变化的修正系数,并以某真型输电塔架为算例,结合HFFB试验,分析了修正系数对结构响应计算的影响。研究结果表明,修正前后位移响应的背景分量计算值差异很小,但共振分量差别明显,偏差幅度达25%。对柔度较大的高耸结构,其共振分量在总响应中占比较高,需考虑荷载谱沿高度的变化;对于沿高度减小或增大的风速谱而言,现有荷载谱沿高度不变的分段估计法将高估或低估结构的位移响应。本文方法具有更高精度和更广的适用性。展开更多
基金The work described in this paper was jointly supported by the National Natural Science Foundation of China （51478360, 51323013, and 50978204）.
文摘Wind-tunnel tests of a large-scale sectional model with synchronous measurements of force and vibration responses were carried out to investigate the nonlinear behaviors of vertical vortex-induced forces （VIFs） on three typical box decks （i.e., fully closed box, centrally slotted box, and semi-closed box）. The mechanisms of the onset, development, and self-limiting phenomenon of the vertical vortex-induced vibration （VlV） were also explored by analyzing the energy evolution of different vertical VIF components and their contributions to the vertical VIV responses. The results show that the nonlinear components of the vertical VIF often differ from deck to deck; the most important components of the vertical VIF, governing the stable amplitudes of the vertical VIV responses, are the linear and cubic components of velocity contained in the self-excited aerodynamic damping forces. The former provides a constant negative damping ratio to the vibration system and is thus the essential power driving the development of the VIV amplitude, while the latter provides a positive damping ratio proportional to the square of the vibration velocity and is actually the inherent factor making the VIV amplitude self-limiting. On these bases, a universal simplified nonlinear mathematical model of the vertical VIF on box decks of bridges is presented and verified in this paper; it can be used to predict the stable amplitudes of the vertical VIV of long-span bridges with satisfactory accuracy.
基金Key Subject for Science Research and De-velopment Plan of Railway Ministry (No.2006G004-B)
文摘To simulate the fatigue characteristics of the pile-board structure under long-term dynamic load, using the in-situ dynamic testing system DTS-1, the forced vibration loading was repeated one million times at different cross-sections of the pile-board structure for high-speed railway. The dynamic deformation, permanent deformation and dynamic stress of main reinforcements were measured. The test results show that the dynamic responses of the pile-board structure almost did not vary with the forced vibration times under the simulated trainload. After one million times of forced vibration, the permanent deformations of the midspan section of intermediate span and midspan section of side span were 0.7 mm and 0. 6 mm, respectively, and there was no accumulative plastic deformation at the bearing section of intermediate span.
文摘An innovative semisolid technique termed as vibrating cooling slope(VCS)has been applied to producing in-situ Al-25%Mg2Si(mass fraction)composite.The molten Al-16.5Mg-9.4%Si(mass fraction)alloy with 100°C superheat was poured on the surface of an inclined copper plate(set at 45°inclined angle)while it was vibrated at a frequency of 40 Hz and an amplitude of 400μm.After travelling the length of 40 cm on the slope,the resultant semisolid alloy was cast into a steel mold.For the purpose of comparison,reference composite samples were made by gravity casting(GC)and conventionally still cooling slope casting(CS)methods using the same alloy under identical conditions.The samples were hot extruded at 500°C.It was concluded that the size of Mg2Si particles was decreased by about 50%and 70%for the CS and VCS produced samples respectively when compared to that of the GC produced sample.Despite of their higher porosity contents,both the as-cast and hot-extruded VCS processed samples exhibited higher hardness,shear yield stress(SYS)and ultimate shear strength(USS)values as compared with their GC produced counterparts.These results were attributed to the refined and modified microstructure obtained via this newly developed technique.
基金sponsored by National Natural Science Foundation of China(No.51775443)National Science and Technology Major Project of China(No.2017-Ⅶ-0015-0111)。
文摘Ultrasonic vibration-assisted milling has been widely applied in machining the difficultto-cut materials owing to the remarkable improvements in reducing the cutting force.However,analytical models to reveal the mechanism and predict the cutting force of ultrasonic vibrationassisted milling metal matrix composites are still needed to be developed.In this paper,an analytical model of cutting force was established for ultrasonic vibration-assisted milling in-situ TiB_(2)/7050 Al metal matrix composites.During modeling,change of motion of the cutting tool,contact of toolchip-workpiece and acceleration of the chip caused by ultrasonic vibration was considered based on equivalent oblique cutting model.Meanwhile,material properties,tool geometry,cutting parameters and vibration parameters were taken into consideration.Furthermore,the developed analytical force model was validated with and without ultrasonic vibration milling experiments on in-situ TiB_(2)/7050 Al metal matrix composites.The predicted cutting forces show to be consistent well with the measured cutting forces.Besides,the relative error of instantaneous maximum forces between the predicted and measured data is from 0.4%to 15.1%.The analytical model is significant for cutting force prediction not only in ultrasonic-vibration assisted milling but also in conventional milling in-situ TiB_(2)/7050 Al metal matrix composites,which was proved with general applicability.
文摘分段估计法作为一种重要的非线性振型结构高频天平(high-frequency force balance,HFFB)试验模态力计算方法,可以对结构脉动风荷载的空间分布进行估计。该文章抛弃了分段估计法中无量纲风荷载谱沿高度不变的假定,提出了一种可以考虑荷载谱沿高度变化的方法。以Kaimal脉动风谱为例提出并推导了无量纲风荷载谱沿高度变化的修正系数,并以某真型输电塔架为算例,结合HFFB试验,分析了修正系数对结构响应计算的影响。研究结果表明,修正前后位移响应的背景分量计算值差异很小,但共振分量差别明显,偏差幅度达25%。对柔度较大的高耸结构,其共振分量在总响应中占比较高,需考虑荷载谱沿高度的变化;对于沿高度减小或增大的风速谱而言,现有荷载谱沿高度不变的分段估计法将高估或低估结构的位移响应。本文方法具有更高精度和更广的适用性。
