MODELS FOR SMALL CRACK GROWTH IN LCF AT ROOM TEMPERATURE AND HIGH TEMPERATURE

The life assessment for a component subjected to high strain concentration in critical regions can be treated as a crack growth estimation starting from the first cycle of the component life to- gether with an appropriate crack growth model. This approach assumes that the presence of small defects or slip bands causes a quick crack nucleation in critical regions where Low Cycle Fa- tigue (LCF) design is adopted. Such a tool is useful to assess the residual life of a component in the presence of a crack in critical regions where high stress concentrations cause cyclic yielding of the material. The scope of this paper is to discuss the application of existing models to a quenched and tempered structural steel. In particular, a series of experimental tests in the LCF regime at room and at high temperature (up to T = 500◦C) have been per- formed in order to analyze the crack propagation in the presence of plastic deformations. Two types of specimens have been used. Smooth cylindrical specimens have been used to obtain the cyclic stress-strain curve and the Manson-Coffin curve under constant strain amplitude loading. Moreover, in order to observe the initi- ation and growth of a fatigue crack, additional cylindrical speci- mens with an artificial defect of 100 or 400 μm depth have been tested. For these specimens, during the interruption of fatigue tests, the crack advancement has been detected using the plastic replica technique with a thin foil of acetate. The experimental results have been examined in terms of crack growth rates and a series of parameters have been used to correlate these data with the external applied load. Two different approaches, i.e. frac- ture mechanics approach (based on ∆K and ∆J) and analytical model based on Manson-Coffin law, have been analyzed in order to define a model able to predict the crack growth rate for high constant strain amplitudes. Finally, a new analytical model able to predict the crack growth rates is presented.