Low cycle fatigue life analysis for exhaust manifold considering Elastoviscoplastic effect

Due to the complex geometry and loading conditions, exhaust manifolds are the most challenging components among all parts of internal combustion engines. They must withstand severe cyclic thermo-mechanical loading throughout their lifetime. Thus, simulation and analysis of fatigue cracks are essential. In this paper, low cycle fatigue (LCF) life analysis of the exhaust manifold is performed by using the finite element method and ABAQUS software to predict the temperature and stresses and then LCF life by using Smith-Watson-Topper theory and FE-SAFE software. Mechanical properties of silicon-molybdenum ductile cast irons obtained by LCF tests at different temperatures. The combination of the Chaboche nonlinear isotropic-kinematic hardening model with viscous stress law is used to consider the effect of viscosity. The results of finite element analysis (FEA) showed that the maximum temperature and stress values in the exhaust manifold are 757.7 °C and 395.2 MPa and the position is at the confluence region. According to the fatigue life analysis results, neglecting the elastoviscoplastic effect caused an estimation of 619 cycles or about 5.7% higher than the limit. Therefore, it is necessary to consider the elastoviscoplastic effect in the analysis of the low cycle fatigue life of the exhaust manifold. The results of the fatigue life analysis showed that the minimum LCF life of the exhaust manifold occurs in the confluence area. Thermo-mechanical analysis and LCF life results are compared with experimentally damaged exhaust manifolds to evaluate the results appropriately. It has been shown that the critical identified area match well with the area of failure in the experimental sample.