Out-of-Phase TMF lifetime calculation of EN-GJS-600 (ASTM 80-55-06) ductile cast iron based on strain increase tests and evaluation of cyclic deformation behavior in isothermal measuring intervals

Abstract

Components of internal combustion engines not only undergo mechanical high and low-cycle fatigue but also thermally induced loadings during start-stop and load changes. Accordingly, an efficient lifetime calculation method of the ductile cast iron EN-GJS-600 for Out-of-Phase thermomechanical fatigue (OP TMF) loading was developed based on Morrow’s relation between cyclic deformation and fatigue lifetime behavior. As an alternative to established fracture mechanics based approaches, the slope of the Wöhler curve is calculated from data determined in at least one each of a TMF strain increase and TMF constant amplitude test. Based on the Physically Based Lifetime calculation (PhyBaL) method previously published for isothermal low cycle fatigue (LCF) and high cycle fatigue (HCF) loadings, and data taken from only one strain increase and at least one constant amplitude OP TMF test, the Wöhler curve can be described as according well with constant amplitude OP TMF tests performed for validation. In the present work, triangular TMF cycles were applied at 5 mHz (0.005 Hz) with a temperature range from 50 to 350 °C. Cyclic deformation behavior as input data for calculation of the slope of the Wöhler curve was determined by stepwise increases of mechanical strain amplitude in OP TMF tests. By constant amplitude tests at different mechanical strain amplitudes, the lifetime and cyclic deformation behavior at OP TMF loading were determined for validation. Moreover, applicability of measurement cycles at constant temperature and low stress amplitude applied in defined intervals of regular OP TMF cycles proved to match the OP TMF cyclic deformation behavior well. This opens up the prospect of characterizing cyclic deformation behavior in more complex, near service TMF cycles as a baseline for lifetime assessment using the PhyBaL approach outlined above.