Methods for LCF life predictions in presence of defects

In this Ph.D. thesis, fatigue crack growth in presence of plastic strains is discussed. This work has the aim to provide an accurate description of short crack propagation starting from defects, in order to develop elastic-plastic crack propagation models that can be included in a damage tolerant design frame. In the first part of the work, a general overview of the propagation models present in the literature is provided. Particular attention is given to models that describe propagation in regions in which plastic strains are present, together with an accurate description of crack closure phenomenon. In the second part of the work, the general formulation of the effective cyclic J- integral is discussed and applied to assess fatigue life of notched specimens. In this section, the effects of crack closure and temperature on short crack propagation are discussed and analyzed with the state-of-the-art models present in the literature. In the third chapter, an experimental campaign is developed to observe crack propagation in presence of a plastic strain gradient. The activity is performed to check short crack propagation in conditions similar to those experienced by real components, such as turbine disks. A crack propagation model, obtained taking into account numerical simulations of material cyclic response, is presented and discussed. The assessment results are employed to highlight the limits of actual crack closure models. Accordingly, the focus is shifted to the development of an innovative technique, based on digital image correlation, employed to measure crack opening levels. The technique is applied on single crystal specimens, made of a Ni-based superalloy, Haynes 230. A regression algorithm is developed to extract crack propagation driving forces from the displacement registered around the crack tips. These results are employed to measure the extent of the irreversibility present at the tip and are compared to those numerically calculated, obtained considering a single crystal plasticity code. Finally, the experimental method based on DIC is applied to LCF. Experimental measurements are implemented in a Jeff -based model. The model is employed to describe crack propagation in presence of very high plastic strains, like those experienced by undersea pipelines. The effects of material transient response on fatigue life assessment are discussed, together with the effects of the defect shape on crack opening and closing levels.