Thermomechanical Fatigue of Exhaust Manifolds Radwan Hazime, ADACS Inc. ABSTRACT A thermomechanical fatigue (TMF) assessment methodology for HiSiMo cast iron exhaust manifolds has been developed and applied to the life prediction of a component. This type of material and application require the use of a temperature dependent plasticity constitutive model, which takes into account both viscous effects and non-linear kinematic hardening. For this purpose, results from the two-layer viscoplasticity (2LVP) model in Abaqus and the unified Chaboche model (implemented as a UMAT) are compared and validated versus isothermal and non-isothermal uniaxial tests. In addition, a fracture mechanics based model for TMF life prediction is developed and extended to three-dimensional stress states under transient nonproportional loading conditions using the critical plane concept. It is shown that the 2LVP model in Abaqus correlates well with uniaxial test data under isothermal and non-isothermal uniaxial testing conditions. Thus, the model is used for the transient structural Finite Element Analysis (FEA) of a component. Damage computations are performed on the results of the FEA using a TMF post-processor. Life predictions using this approach correlate very well with observed lives from three key life tests of an exhaust manifold.
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