For aluminum automotive cylinder head designs, one of the concerning failure mechanisms is thermo-mechanical fatigue from changes in engine operating conditions. After an engine is assembled, it goes through many different operating conditions such as cold start, through warm up, peak power, and intermediate cycles. Strain alternation from the variation in engine operation conditions change may cause thermo-mechanical fatigue (TMF) failure in combustion chamber and exhaust port. Cylinder heads having an integrated exhaust manifold are especially exposed to this failure mode due to the length and complexity of the exhaust gas passage.
First a thermo-mechanical fatigue model is developed to simulate a known dynamometer/bench thermal cycle and the corresponding thermo-mechanical fatigue damage is quantified. Additionally, strain state of the cylinder head and its relation to thermo-mechanical fatigue are discussed. The bench test was used to verify the TMF analysis approach.
For field risk assessment, the customer usage profiles (customer image) are analyzed and corresponding duty cycles are built. A thermo-mechanical fatigue analysis approach based on transient cylinder head temperature history is developed for each customer usage duty cycle, and the corresponding strain alternation is translated to a thermo-mechanical fatigue life/damage.
The risk of a cylinder head thermo-mechanical fatigue crack can be predicted from a customer image and related duty cycles.