A Phenomenological Model for Fatigue Life Prediction of Highly Loaded Cylinder Heads 2006-01-0542
Modern automobile diesel engines make use of aluminium cylinder heads that experience both high pressure and thermal loads. Maximum temperatures are above 250°C in the valve bridge area, generating microstructural transformations in the material and thus local evolution of the mechanical properties.
To be able to predict the life time of this component with a reasonable amount of confidence, it is therefore necessary to describe these changes in the material.
This has been done on a variety of casting materials, with various amount of silicon and copper. Two of them have been taken as references, namely the A356 and 319 type of alloys, making extensive use of Transmission Electron Microscope (TEM) associated with Automatic Image Analysis for quantitative analysis of the precipitation stages during different heat treatments, from the as-received state to saturated aging state.
These data, together with the results of isothermal low cycle fatigue tests at temperatures ranging between 20 and 300°C have been used to derive a phenomenological model for crack initiation in thermal fatigue. This model has then been validated by means of an original anisothermal fatigue test, replicating the thermo mechanical stresses exerted upon the valve bridge area of the cylinder head.
Finally, life time of a real 16 v diesel cylinder head has been calculated for different materials, making use of this model with regards to the microstructural evolutions. Results are in good agreement with crack initiation times experienced on hot and cold test routines, commonly used by car manufacturers and at MONTUPET.