Study On Low Cycle Fatigue Of Exhaust Manifold Based On Transient Vibration-Thermal Coupling

With the restrictions on fuel consumption getting harsher,supercharged engines with three cylinder have been widely applied to the small passenger cars.However,its increasing degree of enhancement and inherent imbalance have made the exhaust manifold bearing more dynamic thermal load and structural dynamic load.Traditional low cycle fatigue analysis which considers the thermal load only has been unable to meet the engineering design requirements.Thus,it is necessary to analyze the low cycle fatigue with the consideration of the thermal load and the transient vibration excitation and the plastic deformation,which is close to the actual state of the engine.In this paper,the vibration-thermal coupling low-cycle fatigue is studied based on a three-cylinder supercharged engine exhaust manifold to quantitatively analyze the influence of the vibration load on low cycle fatigue life with considering the plastic deformation.First,the exhaust manifold's temperature and heat transfer coefficient of the internal and external flow fields under idle and rated conditions are obtained with the fluid-solid coupling method and the temperature fields and stress fields under these two conditions are calculated.The result indicates that the stress level under the rated condition significantly increases compared with that under the idle condition.The high stress regions under these two conditions are both mainly located at the joint of the flange face and the exhaust branch.The first and third branch bear compressive stress and the second branch bears tensile stress.Second,the multi-body dynamics system of the engine under the excitation of the valve train and the pressure in cylinder is calculated.The result shows that the large energy section of the acceleration excitation load of the exhaust system is mainly of low frequency,whose impact needs to be taken seriously.In addition,the constrained modes of the elastoplastic stress field under normal temperature and rated conditions are calculated.Results show that high temperature decreases the natural frequency of the exhaust system,and the deformations under these two conditions present an opposite trend from the fourth-order mode.The dynamic stress is greatly influenced by the second-order mode according to the transient dynamics calculation result.Third,based on the transient dynamic stress field of exhaust manifold under rated condition as well as the thermal stress fields under idle and rated conditions,the traditional low cycle thermal fatigue and the transient vibration-thermal coupling low cycle fatigue analysis models are respectively established according to the test flow.The elastoplastic stress strain of the load spectras is corrected by the Neuber's rule and the fatigue life is evaluated with the principal strain method.The result indicates that the risk region of fatigue damage is mainly located in the high temperature and tensile stress region.Compared with traditional low-cycle thermal fatigue,the entire life with vibration load can decrease by 25.2% and in some regions that can come up to 75%.From the point of structural optimization,we should focus on the regions with high temperature and high tensile stress and the regions where tensile stress is the dynamic stress.The suspension stiffness is slightly optimized with the optimal Latin Hypercube method combined with the BP neural network method,decreasing the dynamic stress borne by the exhaust manifold,making the fatigue life increase by 8.69%.Finally,the fatigue test of the engine is carried out.After 500 hours of durability test,the engine works well and the exhaust manifold without failure after the cold and hot impact fatigue test,verifying the rationality of the transient vibration-thermal coupling low cycle fatigue method.