Transient Non-linear FEA and TMF Life Estimates of Cast Exhaust Manifolds

A transient nonlinear Finite Element Analysis (FEA) method has been developed to simulate the inelastic deformation and estimate the thermo-mechanical fatigue life of cast iron and cast steel exhaust manifolds under dynamometer test conditions. The FEA uses transient heat transfer analysis to simulate the thermal loads on the manifold, and includes the fasteners, gasket and portion of the cylinder head. The analysis incorporates appropriate elastic-plastic and creep material models. It is shown that the creep deformation is the most single critical component of inelastic deformation for cast iron manifold ratcheting, gasket sealing, and crack initiation. The predicted transient temperature field and manifold deformation of the FEA model compares exceptionally well with two experimental tests for a high silicon-molybdenum exhaust manifold. Based on the predicted inelastic strain results, the thermo-mechanical fatigue lives are estimated using the inelastic strain range partitioning method. It is also shown that the model is capable of predicting the crack initiation site in all cases with the estimated lives correlating reasonably well with observed lives during testing. In addition, the method is capable of predicting the dynamic sealing pressure on the gasket and potential exhaust gas leak locations along with fastener load histories.