The problem of cracks in cylinder heads due to low cycle fatigue (thermal fatigue) is well known for engines with high specific power output. However it is still difficult to predict the lifetime of a new cylinder head due to the number of influencing parameters and the complexity of material behavior. Better understanding of cylinder head fatigue can improve the development process of a new engine concerning CAE as well as mechanical testing efficiency.
Therefore a CAE tool which can calculate strains and stresses as a function of time for a defined operating cycle of the engine was developed. In parallel a measuring technique was developed which allows to measure strains on the surface of the combustion chamber side of the cylinder head during fired engine operation.
For different Aluminum-Silicon casting alloys the material behavior was described in the Finite Element Program ABAQUS by a nonlinear kinematic / isotropic hardening model. The numerical parameters were determined by measurement results. The cyclic thermal load (55°C - 210°C.) was applied to the assembled cylinder head FE-model.
A strain gage application was developed which allows dynamic strain and surface temperature measurements during fired engine operation. The results are used for comparison to and calibration of the calculations. More-over they can serve to optimize thermo-shock-test cycles. The measured time behavior of the material strains makes it possible to optimize the hot and cold cycle duration of thermo-shock tests regarding complete test duration and lifetime validity.
In this paper, the potential for improving of the numerical model is discussed and the successful use of conventional strain gages during fired engine operation is described. A comparison of calculated and measured strain in a temperature cycle validates the results.