Low-cycle Thermal Fatigue and High-cycle Vibration Fatigue Life Estimation of a Diesel Engine Exhaust Manifold

This paper aims at estimating the low-cycle and high-cycle fatigue life of a turbocharged Diesel engine exhaust manifold. First, a decoupled thermo-structural Finite Element analysis has been performed to investigate low-cycle fatigue phenomena due to the thermal loadings applied to the exhaust manifold. High/low temperature cycles causes stress-strain hysteresis loops in the manifold material whose related dissipated energy can be directly correlated to low-cycle thermal fatigue. Afterwards, a dynamic harmonic analysis has been performed aiming at investigating the existence of high-cycle fatigue phenomena due to vibrational loading applied to the exhaust manifold during the duty cycle. Three direction acceleration experimental loadings have been applied to the model. An ad-hoc methodology has been developed to superimpose thermo-structural results to dynamic harmonic analysis results. In particular, quasi-static thermo-structural results have been employed to identify the mean stress values of vibration fatigue cycles, while alternate stress values have been derived from harmonic analysis. Different combinations of frequencies and phases of the acceleration input signals have been considered to create different high-cycle fatigue loadings. Each cyclic load case has been processed employing the multiaxial Dang Van fatigue criterion.