F2010C062
Thermal Fatigue Lifetime Prediction of Exhaust Manifold
This paper describes the overall procedures for the lifetime prediction of the exhaust manifold. Since the exhaust systems operate at the extreme conditions under which the thermal and mechanical cyclic loads are imposed throughout their lifetime, the damage accumulated by thermo-mechanical fatigue mechanism is known to be the predominant cause of the failure of the exhaust manifold. The tests with specimens were performed to obtain the accurate temperature dependent material properties such as density, specific heat, thermal expansion coefficient and conductivity. The obtained material properties were used for the computer simulation. The temperature distributions for the exhaust manifold were calculated by CFD(computational Fluid Dynamics) and they were used as the boundary conditions to obtain the plastic strain range, delta-epsilon-p, based on FEM(Finite Element Method). The lifetime curves from the out-of-phase TMF (Thermo Mechanical Fatigue) and LCF (Low Cycle Fatigue) tests were developed to predict the number of cycle to failure, Nf, of the exhaust manifold on engine bench tests. Nf was determined by delta-epsilon-p and the lifetime curve. The comparison between the calculated and tested data shows that the current methods are practically useful. In order to obtain the correlation between engine bench and vehicle tests, the strain measurements were carried out. The strain gauges available for the temperature up to 750 °C were attached to the surface of the exhaust manifold and the tests for the engine bench and vehicle driving were conducted. With practical approach simply using the cyclic stress strain curve at constant temperature 600°C, the stress-strain hysteresis loop were constructed and the damages were computed with the rainflow cycle counting algorithm and the linear damage accumulation(Minor's Rule). The analysis of the measured data provides the information on the correlation between the severity of engine bench test and that of vehicle driving. The distribution of the field failed mileage data shows the results are reasonable. The developed procedures can be used as a useful tool not only for developing and modifying durability test codes for the components of the exhaust systems but also for predicting the lifetime of the exhaust manifold in design stage, thus reducing development time and cost.
This abstract is supplemented by a PDF, which can be viewed here.