**8. Comparative consideration and application to components**

Both energy criteria and the model according to Neu-Sehitoglu were used for fatigue life modelling. In contrast to empirical approaches this model distinguishes between different damage mechanisms. The adequate method is chosen according to the type of application; in doing so, it is important to indicate the model limits. Energy criteria seem to represent the best compromise between accuracy and complexity in their application. Major differences occur if the damage mechanisms involved are changing.

Practical application to cylinder heads shows that the fatigue life, calculated on the basis of the dissipated plastic energy, largely depends on the chosen state of ageing. The solution to this problem is an ageing-dependent cumulative damage model.

The commercial fatigue lifetime prediction software FEMFAT (FEMFAT Manual, 2005) features, since version 6.5, a module for calculating the damage under thermo-mechanical loading according to the damage rate model by Neu-Sehitoglu.

With this model it is possible to calculate the local damage portions caused by fatigue, oxidation and creep. The calculation is based on shear strains, which are determined by a critical plane method and are therefore also applicable for multiaxial loading. The predominant part of the overall damage is caused by pure fatigue. The portion of oxidation damage amounts to some 10% in the regions of maximum loading. The fatigue life computation by means of FEMFAT-Sehitoglu provides realistic results concerning the critical areas and fatigue lives.

### **9. Conclusion**

TMF energy criteria is a suitable tool for TMF lifetime assessment of aluminium, provided the limitations of the application are known. They are representative for the cyclic material behaviour and good damage indicators, since they are associated with the macroscopic crack initiation. The damage rate model of Sehitoglu is powerful to describe more influences, albeit with the major disadvantage being the need of an extensive data basis for every specific material. Depending on the application, one specific lifetime calculation method should be preferred.
