Fatigue behaviour and fatigue life calculation of the cast iron EN-GJS-600 under isothermal and thermo-mechanical loading

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A weight optimised and safe design of components is a fundamental requirement of the passenger as well as commercial vehicle industry. Under service conditions components like cylinder heads are subjected to mechanical as well as cyclic thermal loadings due to start-up procedures. In this context, increasing internal pressures consequently request higher fatigue strength. Due to the combination of good mechanical and thermal properties cast irons are suitable to meet these requirements. This research project is focused on the nodular cast iron EN-GJS-600 (ASTM 80-55-06) currently used for various applications in the commercial vehicle industry. Besides the individual microstructure and the monotonic properties the fatigue behaviour at ambient as well as elevated temperatures will be investigated. Mechanical stress-strain hysteresis, temperature (only at ambient temperature) and electrical resistance measurements will be applied to characterise the fatigue behaviour. In addition to total-strain-controlled strain increase and constant amplitude tests with testing frequencies from 0.005 Hz to 100 Hz, temperature increase tests will be performed to determine the temperature leading to maximum dynamic strain-ageing effects. Compared to the conventional determination of dynamic strain-ageing, this short-time procedure leads to a significant reduction of experimental time. Furthermore, temperature increase tests offer the possibility to select appropriate temperatures as a function of the deformation-rate and testing frequency respectively for further strain increase and constant amplitude tests. On the basis of the measured quantities the physically based fatigue life calculation method PHYBAL developed at the Institute of Materials Science and Engineering at the University of Kaiserslautern will be modified for the application at total-strain-controlled fatigue tests additionally the influence of the testing temperature and frequency will be incorporated.Based on the results of the first two years, in the third year thermo-mechanical fatigue tests will be performed. A characteristic temperature-time-function emerging in vehicle engines will be superimposed in-phase and out-of-phase (with a phase shift of 180 Degree) by a mechanical load-time-function. For a reliable dimensioning of vehicle components a lifetime calculation which considers complex thermo-mechanical loading conditions is of high interest. In the first and second year, PHYBAL should be modified with respect to total-strain-controlled fatigue tests at ambient and elevated temperatures. In the third year, the advanced short-time-procedure will be modified to meet additionally the specific requirements of thermo-mechanical loading. Thereby, it must be taken into account that the thermal and mechanical loading fractions can not simply be superimposed because they strongly affect each other.