**5.8 Influence of HCF interaction**

Superimposed HCF-loading has an important influence on the fatigue life of TMF-loaded components. Experiments with superimposed HCF strain amplitudes from 0.01% to 0.1% show a significant decrease in fatigue life depending on the amplitude of the HCF-loads, wherein a small influence of HCF-frequency is found. Metallographic investigations show crack propagation due to HCF-loading and TMF-loading, wherein a combination of HCFamplitude and the shift in mean-stress causes crack propagation. Good correlation between striations (~crack propagation) and strain amplitude for different loadings is found.

HCF-loadings in a combustion engine occur especially during the heating of the component with maximum ignition pressure. Therefore additional experiments were conducted with superimposed HCF-loadings only during heating period and dwell time. In this case most of the HCF-cycles appear within the compression region of the hysteresis loop. Therefore the effect is obviously reduced. With regard to the typical ignition pressure in a diesel engine, the influence is small compared to the tests without superimposition; see figure 7 (Minichmayr et al., 2005).
