发动机排气歧管在动态热负荷与整机振动载荷耦合作用的恶劣环境中工作,在热负荷单独作用时其部分区域就已经发生塑性变形,而整机振动载荷的耦合作用将使其疲劳失效问题更为严峻.为量化整机振动载荷对排气歧管低周疲劳损伤的影响,以某三缸增压发动机为研究对象,首先,基于流固耦合方法获得了排气歧管在标定工况和怠速工况下内外流场的换热边界,并联合增压器、催化器等部件温度和换热边界对两工况下排气系统的温度场进行计算.然后,根据温度场计算结果,耦合螺栓预紧力的作用,对怠速工况下的弹性应力场以及标定工况下的弹性和弹塑性应力场进行了计算,并基于标定工况下的弹塑性应力场,应用模态瞬态动力学对标定工况下的整机振动载荷作用下的动应力进行分析.最后,依据标定和怠速工况下的弹性应力场、标定工况下的动应力场结果,参照发动机低周疲劳试验标准分别建立了排气歧管常规高温低周疲劳与整机振动 热耦合低周疲劳分析模型,引入Neuber准则对两者的载荷谱进行应力—应变修正后采用主应变法进行疲劳寿命评估.结果表明:排气歧管疲劳破坏风险点主要位于高温拉应力区域,叠加振动载荷会使整体疲劳寿命下降接近25.2%,部分区域下降幅度甚至高达57%.研究结果为排气歧管整机瞬态振动 热耦合低周疲劳寿命预测提供了一定的理论依据和参考.%Engine exhaust manifold works in the harsh environment with the coupling effect of dy-namic thermal load and vibration load .It is found that plastic deformation has occurred in some areas with the thermal load alone,and the fatigue failure problem will be more serious coupling with the vibration load .A three-cylinder supercharged engine was taken as the research object to quantify the effect of vibration load on low-cycle fatigue damage of exhaust manifold .Firstly,the heat transfer boundary of the exhaust manifold internal and external flow fields under idle and ra-ted conditions were calculated through the fluid-solid coupling method .The temperature field of the whole exhaust system under two conditions were respectively calculated by combining the temperature and the thermal exchange boundary of the turbocharger,catalyst and other compo-nents with that of the exhaust manifold above .Secondly, the elastic and elasto-plastic stress fields under the rated condition,and the elastic stress field under idle condition were calculated coupling with the temperature field and the bolt pretightening force .Based on the results of the elasto-plastic stress field under the rated condition,the dynamic stress field under the engine vi-bration load was calculated through the modal transient dynamics .Finally,according to the re-sults of elastic stress field under rated and idle conditions as well as dynamic stress field under ra-ted condition,the analysis models of the exhaust manifold about low-cycle thermal fatigue and vi-bration-thermal coupling fatigue were established with reference to the engine low cycle fatigue test standard and the load spectra from the two fatigue models corrected by the Neuber's rule was used to evaluate the fatigue life with the principal strain method .The result indicated that the risk region was mainly located in the high temperature and tensile stress regions .Compared with low-cycle thermal fatigue,the entire life with vibration load decreased by 25.2% and in some regions that came up to 57% .The study result provides theoretical basis and reference for the prediction of transient vibration-thermal coupling low-cycle fatigue life of exhaust manifold .
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