Weight optimised and safe component design is a fundamental requirement in the passenger car as well as the commercial vehicle industry. Under service conditions, combustion engine components like cylinder heads are subjected to cyclic mechanical and thermal loadings due to start-stop procedures and load changes. In this context, increasing specific power output leads to increasing internal pressures, and therefore to the request of optimal utilization of the materials’ fatigue strength. Due to the combination of beneficial mechanical and thermal properties, cast irons are suitable to meet these requirements.
To characterize the strain rate dependency of the cast iron EN-GJS-600 at ambient and elevated temperatures, dynamic strain ageing (DSA) effects have to be considered. To establish an efficient and reliant fatigue life calculation, the physically based fatigue life calculation approach PHYBAL-SIT (SIT = Strain Increase Test) was used at AT, the temperature of relative fatigue life minimum, the temperature of relative fatigue life maximum due to DSA and one temperature clearly above DSA being effective. With this efficient test procedure, the temperature- and load-dependent lifetime behavior as well as the cyclic deformation behavior of the ductile cast iron EN-GJS-600 was characterized for the testing frequencies 0.005 Hz, 5 Hz, 47 Hz and 92 Hz in total strain control. A comparison of the resulting four strain-temperature-lifetime-diagrams allows characterizing how temperature range of DSA and lifetime behavior are affected by the strain rate.
With the fatigue parameters obtained from one strain increase test (SIT) and two constant amplitude tests (CAT), the HCF (LCF)-Woehler (ε-N) curve can be calculated for the frequencies 5 Hz, 47 Hz and 92 Hz (0.005 Hz). Because PHYBAL-SIT follows a two-parameter concept, either the LCF- or the HCF-regime can be described mathematically by using the corresponding morrow equation. Hence, the cyclic total strain - plastic strain curve (CStSp) according to Morrow derived from the SIT shows the transition between LCF- and HCF-regime. The results show a good accordance of Nf evaluated experimentally and the PHYBAL-SIT calculated Woehler curves.