Investigating the effect of confluence geometry on optimizing the low-cycle fatigue life of the exhaust manifold

Fatigue caused by thermomechanical loading plays a decisive role in causing damage and reducing the life of the exhaust manifold. The aim of this research was to evaluate the effect of geometric modification and optimization of the manifold confluence on its low-cycle fatigue (LCF) life. In this research, the LCF life analysis of the exhaust manifold was performed using the finite element method and ANSYS software to predict temperature and stress, and then the low-cycle fatigue life by using Smith-Watson-Topper theory and ANSYS NCode Design Life software. Mechanical properties of gray cast irons obtained by LCF tests at different temperatures. Numerical results showed that the maximum temperature in the original and modified exhaust manifold is 706.67 and 673.09°C, respectively, which can be seen in the area of the exhaust manifold confluence. Based on the results of thermo-mechanical analysis, the optimized exhaust manifold withstands about 24MPa less stress than the original exhaust manifold. The results of the low-cycle fatigue life analysis showed that optimizing the exhaust manifold will increase its fatigue life by about 5980 cycles or 50%. To verify the accuracy of the results of the thermo-mechanical analysis and LCF life, the simulated results were compared with the real sample of the damaged exhaust manifold and it was shown that the critical region corresponds well to the rupture region in the real sample.