Fatigue life calculation of SAE 1050 and SAE 1065 steel under random loading

In this investigation, stress-controlled fatigue tests with SAE 1050 and SAE 1065 specimens were performed under single step and random loading to study fatigue mechanisms with particular attention to microstructural details. The applied plastic strain amplitude, temperature and electrical resistance measurements depend on deformation-induced changes of the microstructure and represent the actual fatigue state of the investigated steels. A new test procedure combines any kind of load spectra with periodically inserted single step sequences to measure the plastic strain amplitude, the temperature and the electrical resistance. The average values of the measuring sequences are plotted as function of the number of cycles in cyclic ‘deformation’ curves and represent the summation of microstructural changes caused by random loading. Electrical resistance measurements allow to detect the proceeding fatigue damage even in the load-free state. On the basis of comprehensive experimental fatigue data the physically based lifetime calculation method “PHYBAL” using generalized Morrow, Coffin–Manson and Basquin equations was developed for single step and random loading. S–N (Woehler) curves calculated with “PHYBAL” agree very well with experimentally determined lifetimes.