Thermal Fatigue Life Prediction And Structure Modification Of An Exhaust Manifold Made Of Ni-resist Cast Iron

Exhaust manifold is a key component of automotive exhaust system.Long-term working in high temperature conditions can lead to failures of manifold,affecting vehicle lifetime and driving safety.Due to lacking of test data of D5 S material under high temperature,the thermal fatigue life prediction of D5 S Ni-resist cast iron exhaust manifold in the product design stage cannot successfully guide the engineering design to pass the endurance tests.Therefore,it is meaningful to study the thermal fatigue life of exhaust manifold.This article considers the fracture of a D5 S Ni-resist exhaust manifold occurred in an engineering test stage.Firstly,failure analysis on the failure sample was conducted to determine the main reason of the manifold fracture,where the results showed that the major factor of manifold fracture was thermal fatigue;Coupling analysis approach of ABAQUS and STARCCM+ was applied to simulate the thermal fatigue condition,by which the distribution of flow field inside the manifold,the distribution of the temperature field and the thermal stress deformation of the manifold were calculated.Finally,the equivalent plastic strain distribution of the dangerous zone was obtained(?PEEQ=0.548%);High-temperature tensile tests were conducted to obtain the mechanical properties of D5 S under high-temperature.The life prediction parameters of Manson-Coffin formula were estimated by applying the mechanical parameters.In order to validate the formula,the manifold crack test was performed;The test results(2116 cycles)showed that the result of modified formula(1678 cycles)was more accurate than the result of empirical formula(625 cycles).According to the simulation and experimental results,the main reason for the fracture of the manifold was that the transition radius of the inlet and outlet was so small that the stress concentration phenomenon occurred.To solve this problem,stiffening rib was added at the transition zone to solve the problem.Simulation method and modified formula were used again to predict thermal fatigue life of modified manifold.Corresponding results showed that the prediction life of modified manifold reached 5658 cycles,which exceeded the requirement for durability test by 2400 cycles.Durability test was run to verify the validity of structure modification.In the end,modified manifold passed the 400 h durability test,which guaranteed product development progress.