In recent years,there have been important developments in the joint analysis of the travel behavior based on discrete choice models as well as in the formulation of increasingly flexible closed-form models belonging t...In recent years,there have been important developments in the joint analysis of the travel behavior based on discrete choice models as well as in the formulation of increasingly flexible closed-form models belonging to the generalized extreme value class.The objective of this work is to describe the simultaneous choice of shopping destination and travel-to-shop mode in downtown area by making use of the cross-nested logit(CNL) structure that allows for potential spatial correlation.The analysis uses data collected in the downtown areas of Maryland-Washington,D.C.region for shopping trips,considering household,individual,land use,and travel-related characteristics.The estimation results show that the dissimilarity parameter in the CNL model is 0.37 and significant at the 95% level,indicating that the alternatives have high spatial correlation for the short shopping distance.The results of analysis reveal detailed significant influences on travel behavior of joint choice shopping destination and travel mode.Moreover,a Monte Carlo simulation for a group of scenarios arising from transportation policies and parking fees in downtown area,was undertaken to examine the impact of a change in car travel cost on the shopping destination and travel mode switching.These findings have important implications for transportation demand management and urban planning.展开更多
针对2024铝合金自冲铆接头底部易出现裂纹的问题,通过铆接实验来分析裂纹形成机理,并结合2024铝合金板材先退火再铆接的工艺开展裂纹抑制研究;同时,对比了接头剖面的显微组织和力学性能,并采用扫描电镜(scanning electron microscope, S...针对2024铝合金自冲铆接头底部易出现裂纹的问题,通过铆接实验来分析裂纹形成机理,并结合2024铝合金板材先退火再铆接的工艺开展裂纹抑制研究;同时,对比了接头剖面的显微组织和力学性能,并采用扫描电镜(scanning electron microscope, SEM)对接头断口特征进行分析,以探究其失效机理,进而分析热处理工艺对接头力学性能和失效形式的影响。结果表明,2024铝合金板材经360℃退火处理后,其塑性和延伸率显著提高,硬度下降了23.6%。当2024铝合金板材未经退火处理直接进行铆接时,裂纹萌生点位于铆钉管腿尖区域,接头底部出现沿径向分布的宏观裂纹;退火处理后接头底部无明显裂纹,连接点的密封性、抗腐蚀性能显著增强。未退火处理组接头剖面的晶粒结构粗大、不规则,且铆钉管腿尖区域的变形较大,而退火处理组接头剖面的晶粒结构细化且更匀称。退火处理组接头的静强度下降了12.93%,但失效位移和能量吸收值分别提高了27.3%和19.31%,其失效形式由上板完全断裂转变为下板撕裂,且连接点底部被铆扣拉穿,断口由脆性断裂转变为韧性断裂。研究结果可为自冲铆接工艺在汽车、航空航天等领域的应用提供重要参考。展开更多
基金Projects(JCYJ20120615145601342,JCYJ20130325151523015)supported by Shenzhen Science and Technology Development Funding-Fundamental Research Plan,ChinaProject(2013U-6)supported by Key Laboratory of Eco Planning & Green Building,Ministry of Education(Tsinghua University),China
文摘In recent years,there have been important developments in the joint analysis of the travel behavior based on discrete choice models as well as in the formulation of increasingly flexible closed-form models belonging to the generalized extreme value class.The objective of this work is to describe the simultaneous choice of shopping destination and travel-to-shop mode in downtown area by making use of the cross-nested logit(CNL) structure that allows for potential spatial correlation.The analysis uses data collected in the downtown areas of Maryland-Washington,D.C.region for shopping trips,considering household,individual,land use,and travel-related characteristics.The estimation results show that the dissimilarity parameter in the CNL model is 0.37 and significant at the 95% level,indicating that the alternatives have high spatial correlation for the short shopping distance.The results of analysis reveal detailed significant influences on travel behavior of joint choice shopping destination and travel mode.Moreover,a Monte Carlo simulation for a group of scenarios arising from transportation policies and parking fees in downtown area,was undertaken to examine the impact of a change in car travel cost on the shopping destination and travel mode switching.These findings have important implications for transportation demand management and urban planning.
文摘针对2024铝合金自冲铆接头底部易出现裂纹的问题,通过铆接实验来分析裂纹形成机理,并结合2024铝合金板材先退火再铆接的工艺开展裂纹抑制研究;同时,对比了接头剖面的显微组织和力学性能,并采用扫描电镜(scanning electron microscope, SEM)对接头断口特征进行分析,以探究其失效机理,进而分析热处理工艺对接头力学性能和失效形式的影响。结果表明,2024铝合金板材经360℃退火处理后,其塑性和延伸率显著提高,硬度下降了23.6%。当2024铝合金板材未经退火处理直接进行铆接时,裂纹萌生点位于铆钉管腿尖区域,接头底部出现沿径向分布的宏观裂纹;退火处理后接头底部无明显裂纹,连接点的密封性、抗腐蚀性能显著增强。未退火处理组接头剖面的晶粒结构粗大、不规则,且铆钉管腿尖区域的变形较大,而退火处理组接头剖面的晶粒结构细化且更匀称。退火处理组接头的静强度下降了12.93%,但失效位移和能量吸收值分别提高了27.3%和19.31%,其失效形式由上板完全断裂转变为下板撕裂,且连接点底部被铆扣拉穿,断口由脆性断裂转变为韧性断裂。研究结果可为自冲铆接工艺在汽车、航空航天等领域的应用提供重要参考。
