Cylinder Heads for High Power Gasoline Engines - Thermomechanical Fatigue Life Prediction

Increasing demands on engine efficiency and specific power have resulted in progressively higher loadings on internal components of combustion engines. Therefore the durability assessment of such components is increasingly in demand, triggered by both reliability and economic requirements. Within this context the TMF cylinder head simulation process established at BMW is presented in the following article. The numerical model is able to account for thermo-mechanical loading histories. These lead to a transient evolution of the material characteristics during the lifetime due to aging in aluminum alloys. Therefore a viscoplastic constitutive model is coupled with an aging model to handle the change in precipitation structure and the effect on the material properties, especially for non heat-treated secondary aluminum alloys. The local damage evolution is modeled based on the growth of micro cracks. Cyclic isothermal and an-isothermal fatigue tests were conducted in order to calibrate the lifetime model. A conjugate heat transfer analysis is used to calculate the temperature distribution, which is verified using measurements of transient thermal engine tests. Predictions of the aging model were validated using micro structural investigations and hardness measurements on cylinder heads run under different test conditions. Examples for the good agreement found between measured and calculated lifetimes for different engines are shown.