Short crack propagation in LCF regime at room and high temperature in Q & T rotor steels

Crack propagation in full plastic zones is one of the main factors for life assessment of a component subjected to high strain concentrations in critical regions. Residual life evaluation can be obtained considering a crack growing from component first load cycle: life assessment for those components parts where Low Cycle Fatigue (LCF) design is adopted, especially for those critical regions where high stress concentrations cause cyclic yielding of the material, can be made only considering an appropriate crack growth model. The scope of this paper is to discuss the application of existing models to three different types of quenched and tempered structural steel. In particular, a series of experimental tests in LCF regime at room and high temperature (up to T = 500 degrees C) were performed in order to determine epsilon - N diagrams and to determine crack growth rates in the LCF regime. During fatigue tests interruptions, crack advancement was detected using the plastic replica technique with a thin foil of acetate. Experimental results were examined in terms of crack growth rates and a series of models based on elastic-plastic J-Integral. Results showed that at high temperature there is a significant increase of crack growth rates respect to models usually valid at room temperature. This increase is related to the presence of a diffused damage ahead of the crack tip.