Low cycle fatigue study of medium silicon molybdenum ductile iron exhaust manifold

Diesel engine exhaust manifolds are subjected to repeat thermal cycling during start-stop operations, making them prone to fatigue damage in critical areas. To evaluate the service life of key components during design, QTRSi4Mo ductile iron is investigated in this study for its low-cycle fatigue behavior, the stress-strain characteristics of QTRSi4Mo exhibit low-temperature hardening and high-temperature softening. Based on experimental results, a fatigue life prediction model was developed. The constitutive behavior of the material at various temperatures was accurately characterized using the Chaboche model. Through AVL-FIRE software, we obtained time-averaged temperature and heat transfer coefficient distributions of the exhaust manifold. The results indicate that the highest temperature occurs in the main pipe confluence section, Low-cycle fatigue (LCF) damage is caused by the temperature gradient and external constraints between the inlet and straight pipe sections. A simulation of the exhaust manifold temperature and stress fields was carried out using the ABAQUS software. Finally, by integrating the Ostergren model with the UVARM subroutine, the low-cycle fatigue (LCF) life of the exhaust manifold was estimated to be 1,880 cycles.