New fatigue life calculation method for quenched and tempered steel SAE 4140

In stress-controlled constant amplitude and service loading tests at ambient temperature mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed to characterize the fatigue behavior of the quenched and tempered steel SAE 4140. The applied measurement methods use deformation-induced changes of the microstructure in the bulk material and represent the actual fatigue state. A new test procedure combines any kind of load spectra with periodically inserted constant amplitude sequences to measure the plastic strain amplitude, the change in temperature and the change in electrical resistance at the same time. 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 service loading. On the basis of generalized Morrow and Basquin equations the physically based fatigue life calculation method “PHYBAL” was developed for constant amplitude and service loading. With only three fatigue tests, Woehler (S–N) and fatigue life curves can be calculated in very good agreement with experimental ones determined in a conventional manner. The application of “PHYBAL” provides an enormous saving of experimental time and costs.