Superalloys are commonly used in aircraft manufacturing;however,the requirements for high surface quality and machining accuracy make them difficult to machine.In this study,a hybrid electrochemical discharge process ...Superalloys are commonly used in aircraft manufacturing;however,the requirements for high surface quality and machining accuracy make them difficult to machine.In this study,a hybrid electrochemical discharge process using variable-amplitude pulses is proposed to achieve this target.In this method,electrochemical machining(ECM)and electrical discharge machining(EDM)are unified into a single process using a sequence of variable-amplitude pulses such that the machining process realizes both good surface finish and high machining accuracy.Furthermore,the machining mechanism of the hybrid electrochemical discharge process using variable-amplitude pulses is studied.The mechanism is investigated by observations of machining waveforms and machined surface.It is found that,with a high-frequency transformation between high-and low-voltage waveforms within a voltage cycle,the machining mechanism is frequently transformed from EDM to pure ECM.The critical discharge voltage is 40 V.When pulse voltages greater than 40 V are applied,the machining accuracy is good;however,the surface has defects such as numerous discharge craters.High machining accuracy is maintained when high-voltage pulses are replaced by low-voltage pulses to enhance electrochemical dissolution.The results indicate that the proposed hybrid electrochemical discharge process using variable-amplitude pulses can yield high-quality surfaces with high machining accuracy.展开更多
The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength co...The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue. life predicted by this model is found to be in good agreement with the results of the experimental research.展开更多
Large-head variable-amplitude pump turbines(PTs) encounter serious transient hydraulic instability issues. To explore the evolution mechanisms of pressure fluctuations(PFs) and flow patterns inside large-head variable...Large-head variable-amplitude pump turbines(PTs) encounter serious transient hydraulic instability issues. To explore the evolution mechanisms of pressure fluctuations(PFs) and flow patterns inside large-head variable-amplitude PTs, the load rejection process(LRP) was investigated using a one-and three-dimensional coupled flow simulation approach. The temporal,spatial, and frequency characteristics of the fluctuating pressures were analyzed for four monitoring points using a combined time-frequency analysis approach. The results indicated that PFs during the LRP of large-head variable-amplitude PTs had a new fluctuation frequency component related to Dean vortices(DVs) in the volute, in addition to the common fluctuation frequency components related to rotor-stator interaction phenomena and local backflow vortices near the impeller inlet. The PF frequency component existed throughout the LRP and had a significant influence on the transient maximum pressure at the volute end. This study provides a useful theoretical guide for the design and optimization of large-head variable-amplitude PTs.展开更多
基金the National Natural Science Foundation of China(No.51705239)the Natural Science Foundation for Distinguished Young Scholars of Jiangsu Province(BK20170031)of China。
文摘Superalloys are commonly used in aircraft manufacturing;however,the requirements for high surface quality and machining accuracy make them difficult to machine.In this study,a hybrid electrochemical discharge process using variable-amplitude pulses is proposed to achieve this target.In this method,electrochemical machining(ECM)and electrical discharge machining(EDM)are unified into a single process using a sequence of variable-amplitude pulses such that the machining process realizes both good surface finish and high machining accuracy.Furthermore,the machining mechanism of the hybrid electrochemical discharge process using variable-amplitude pulses is studied.The mechanism is investigated by observations of machining waveforms and machined surface.It is found that,with a high-frequency transformation between high-and low-voltage waveforms within a voltage cycle,the machining mechanism is frequently transformed from EDM to pure ECM.The critical discharge voltage is 40 V.When pulse voltages greater than 40 V are applied,the machining accuracy is good;however,the surface has defects such as numerous discharge craters.High machining accuracy is maintained when high-voltage pulses are replaced by low-voltage pulses to enhance electrochemical dissolution.The results indicate that the proposed hybrid electrochemical discharge process using variable-amplitude pulses can yield high-quality surfaces with high machining accuracy.
基金This work was financially supported by the National Natural Science Foundation of China(Grant No.50078010)
文摘The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue. life predicted by this model is found to be in good agreement with the results of the experimental research.
基金supported by the National Natural Science Foundation of China(Grant Nos.52209108 and 52079034)Sichuan Science and Technology Program(Grant No.2023YFQ0021)+1 种基金the Natural Science Foundation of Heilongjiang Province,China(Grant No.LH2023E058)China Postdoctoral Science Foundation(Grant No.2022M720948)。
文摘Large-head variable-amplitude pump turbines(PTs) encounter serious transient hydraulic instability issues. To explore the evolution mechanisms of pressure fluctuations(PFs) and flow patterns inside large-head variable-amplitude PTs, the load rejection process(LRP) was investigated using a one-and three-dimensional coupled flow simulation approach. The temporal,spatial, and frequency characteristics of the fluctuating pressures were analyzed for four monitoring points using a combined time-frequency analysis approach. The results indicated that PFs during the LRP of large-head variable-amplitude PTs had a new fluctuation frequency component related to Dean vortices(DVs) in the volute, in addition to the common fluctuation frequency components related to rotor-stator interaction phenomena and local backflow vortices near the impeller inlet. The PF frequency component existed throughout the LRP and had a significant influence on the transient maximum pressure at the volute end. This study provides a useful theoretical guide for the design and optimization of large-head variable-amplitude PTs.
