High temperature tensile ductilities and deformation mechanisms of an extruded and rolled AZ31 Mg alloy were investigated.Elongation-to-failure tests were conducted under constant T-head velocity and constant temperat...High temperature tensile ductilities and deformation mechanisms of an extruded and rolled AZ31 Mg alloy were investigated.Elongation-to-failure tests were conducted under constant T-head velocity and constant temperatures ranging from 300℃ to 450℃.Strain-rate-change tests were conducted under varying strain rate from 5×10-5s-1to 2×10-2s-1and constant temperature from 300℃ to 450℃.Experimental results show that the maximum elongation of the AZ31 alloy with an average grain size of about 19μm is 117%at strain rate of 10- 3s-1 and temperature of 450℃.Stress exponent and activation energy were characterized to clarify the deformation mechanisms.The enhanced ductility is dominated by solute drag dislocation creep,and the major failure mechanism is cavity growth and interlinkage.展开更多
The flight control systems, designed in order to assure the necessary safety level even in failure conditions, are generally characterized by a proper redundant layout. The redundancies must be designed in order to as...The flight control systems, designed in order to assure the necessary safety level even in failure conditions, are generally characterized by a proper redundant layout. The redundancies must be designed in order to assure an adequate system behavior when some failures are present; in fact an incorrect layout may cause serious shortcomings concerning the response when some component is not operational. Therefore the usual correct design activities request the complete analysis of the system behavior in failure condition. The work analyses the response of a redundant secondary flight control hydraulic servo-mechanism equipped with some proper equalization devices, when some of the most probable and representative failures are present. It must be noted that the redundancy layout, designed in order to assure the necessary safety level even in failure conditions, may behave improperly during normal operations, if the system architecture is unsuitable, when manufacturing defects are present. The improper behavior, generally consisting of force fighting or speed fighting caused by different offsets or asymmetries between the two sections of the system, may be usually overcome by means of a suitable equalization device. Therefore, the system behavior during and following the failure transient greatly depends on both its redundancy architecture and related equalization device. The above mentioned problems have been studied by means of an appropriate physical-mathematical model of a typical electro-hydraulic servo-mechanism prepared to the purpose, performing a certain number of simulations of representative actuations in which different types of failures are accurately modeled. In the opinion of the authors, this paper concerns a topic quite neglected but important in the technical literature. At the best of the authors＇ knowledge, no specific scientific work in this field is available, excepting some industrial technical reports.展开更多
Au-MgO-ZnO （AMZ） ultraviolet （UV） photodetectors were fabricated to enhance their sensitivities by an inserting ultrathin insulating MgO layer. With the insulating layer, the sensitivities of the UV photodetectors...Au-MgO-ZnO （AMZ） ultraviolet （UV） photodetectors were fabricated to enhance their sensitivities by an inserting ultrathin insulating MgO layer. With the insulating layer, the sensitivities of the UV photodetectors were improved via the reduction of the dark current. Furthermore, strain modulation was used to enhance the sensitivities of the AMZ UV photodetectors. The sensitivities of the photodetectors were enhanced by the piezo-phototronic effect. However, there was a limiting value of the applied strains to enhance the sensitivity of the photodetector. When the external strains exceeded the limiting value, the sensitivity decreased because of the tunneling dark current. The external strains loaded on the photodetectors result in the degradation of the photodetectors, and an applied bias can accelerate the process. This work presents a prospective approach to engineer the performance of a UV photodetector. In addition, the study on the service behavior of the photodetectors may offer a strain range and theoretical support for safely using and studying metal-insulator-semiconductor （MIS） UV photodetectors.展开更多
Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor w...Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor with a vaned diffuser,a discrete frequency noise induced by interaction between the impeller-discharge flow and the diffuser vane,which appears most notably in the power spectra of the radiated noise,can be reduced using a tapered diffuser vane(TDV) without affecting the performance of the compressor.Twin longitudinal vortices produced by leakage flow passing through the tapered portion of the diffuser vane induce secondary flow in the direction of the blade surface and prevent flow separation from the leading edge of the diffuser.The use of a TDV can effectively reduce both the discrete frequency noise generated by the interaction between the impeller-discharge flow and the diffuser surface and the broadband turbulent noise component.In the case of low-flow operation,a leading-edge vortex(LEV) that forms on the shroud side of the suction surface near the leading edge of the diffuser increases significantly in size and blocks flow in the diffuser passage.The formation of an LEV may adversely affect the performance of the compressor and may cause the diffuser to stall.Using a one-side tapered diffuser vane to suppress the evolution of an LEV,the stable operating range of the compressor can be increased by more than 12 percent,and the pressure-rise characteristics of the compressor can be improved.The results of a supplementary examination of the structure and unsteady behavior of LEVs,conducted by means of detailed numerical simulations,are also presented.展开更多
基金Project(50801034)supported by the National Natural Science Foundation of ChinaProject(20060425)supported by the Scientific and Technological Research Key Lab Foundation of Liaoning Education Department,China
文摘High temperature tensile ductilities and deformation mechanisms of an extruded and rolled AZ31 Mg alloy were investigated.Elongation-to-failure tests were conducted under constant T-head velocity and constant temperatures ranging from 300℃ to 450℃.Strain-rate-change tests were conducted under varying strain rate from 5×10-5s-1to 2×10-2s-1and constant temperature from 300℃ to 450℃.Experimental results show that the maximum elongation of the AZ31 alloy with an average grain size of about 19μm is 117%at strain rate of 10- 3s-1 and temperature of 450℃.Stress exponent and activation energy were characterized to clarify the deformation mechanisms.The enhanced ductility is dominated by solute drag dislocation creep,and the major failure mechanism is cavity growth and interlinkage.
文摘The flight control systems, designed in order to assure the necessary safety level even in failure conditions, are generally characterized by a proper redundant layout. The redundancies must be designed in order to assure an adequate system behavior when some failures are present; in fact an incorrect layout may cause serious shortcomings concerning the response when some component is not operational. Therefore the usual correct design activities request the complete analysis of the system behavior in failure condition. The work analyses the response of a redundant secondary flight control hydraulic servo-mechanism equipped with some proper equalization devices, when some of the most probable and representative failures are present. It must be noted that the redundancy layout, designed in order to assure the necessary safety level even in failure conditions, may behave improperly during normal operations, if the system architecture is unsuitable, when manufacturing defects are present. The improper behavior, generally consisting of force fighting or speed fighting caused by different offsets or asymmetries between the two sections of the system, may be usually overcome by means of a suitable equalization device. Therefore, the system behavior during and following the failure transient greatly depends on both its redundancy architecture and related equalization device. The above mentioned problems have been studied by means of an appropriate physical-mathematical model of a typical electro-hydraulic servo-mechanism prepared to the purpose, performing a certain number of simulations of representative actuations in which different types of failures are accurately modeled. In the opinion of the authors, this paper concerns a topic quite neglected but important in the technical literature. At the best of the authors＇ knowledge, no specific scientific work in this field is available, excepting some industrial technical reports.
文摘Au-MgO-ZnO （AMZ） ultraviolet （UV） photodetectors were fabricated to enhance their sensitivities by an inserting ultrathin insulating MgO layer. With the insulating layer, the sensitivities of the UV photodetectors were improved via the reduction of the dark current. Furthermore, strain modulation was used to enhance the sensitivities of the AMZ UV photodetectors. The sensitivities of the photodetectors were enhanced by the piezo-phototronic effect. However, there was a limiting value of the applied strains to enhance the sensitivity of the photodetector. When the external strains exceeded the limiting value, the sensitivity decreased because of the tunneling dark current. The external strains loaded on the photodetectors result in the degradation of the photodetectors, and an applied bias can accelerate the process. This work presents a prospective approach to engineer the performance of a UV photodetector. In addition, the study on the service behavior of the photodetectors may offer a strain range and theoretical support for safely using and studying metal-insulator-semiconductor （MIS） UV photodetectors.
文摘Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor with a vaned diffuser,a discrete frequency noise induced by interaction between the impeller-discharge flow and the diffuser vane,which appears most notably in the power spectra of the radiated noise,can be reduced using a tapered diffuser vane(TDV) without affecting the performance of the compressor.Twin longitudinal vortices produced by leakage flow passing through the tapered portion of the diffuser vane induce secondary flow in the direction of the blade surface and prevent flow separation from the leading edge of the diffuser.The use of a TDV can effectively reduce both the discrete frequency noise generated by the interaction between the impeller-discharge flow and the diffuser surface and the broadband turbulent noise component.In the case of low-flow operation,a leading-edge vortex(LEV) that forms on the shroud side of the suction surface near the leading edge of the diffuser increases significantly in size and blocks flow in the diffuser passage.The formation of an LEV may adversely affect the performance of the compressor and may cause the diffuser to stall.Using a one-side tapered diffuser vane to suppress the evolution of an LEV,the stable operating range of the compressor can be increased by more than 12 percent,and the pressure-rise characteristics of the compressor can be improved.The results of a supplementary examination of the structure and unsteady behavior of LEVs,conducted by means of detailed numerical simulations,are also presented.
