PHYBAL—A new method for lifetime prediction based on strain, temperature and electrical measurements

Stress-controlled fatigue tests with constant and variable amplitudes were performed with quenched and tempered specimens of the steel SAE 4140 (42CrMoS4) and specimens of the railway wheel steel R7 (SAE 1050) specified in standard 812-3V of the International Railway Union UIC on servohydraulic testing systems. In addition to mechanical stress–strain hysteresis measurements, the changes of temperature and electrical resistance were regarded. All measured values depend on microstructural changes due to plastic deformation processes, and therefore, show a clear interaction with the actual fatigue state. Consequently, a physically based lifetime prediction (PHYBAL) according to Morrow and Manson/Coffin can be derived. A new testing procedure was applied allowing hysteresis measurements for the characterization and description of the fatigue behavior under random and even service loading. The basic idea is to combine any kind of load spectrum with constant load sequences using an amplitude below the endurance limit. This offers the possibility to apply lifetime prediction methods on the basis of cyclic deformation data for random or service loading equivalent to single step tests. Scanning and transmission electron microscopy allow to interpret the measured physical data on the basis of microstructural details.