Fatigue Life Prediction at the Microstructural Scale Considering Plastic Strain Localisation

During cyclic loading, particularly at low strain amplitudes, the time required for crack initiation can account for most of the fatigue life. Predicting the number of cycles to crack initiation is therefore a central issue in component design. In nickel-based superalloys such as Inconel 718, fatigue crack initiation is closely linked to the formation of Persistent Slip Bands (PSBs), within which plastic strain localizes. These bands result from the irreversible accumulation of dislocations, which gradually organize into structures leading to the formation of surface extrusions or intrusions, or stress concentrations at grain boundaries, along which cracks preferentially initiate. The formation of these bands strongly depends on the local microstructure. It is therefore necessary to work at this scale in order to predict crack initiation life. The current state of the art generally relies on crystal plasticity simulations performed on numerical polycrystalline microstructures. The results are then used to construct Fatigue Indicator Parameters (FIPs), which are subsequently related to fatigue life through empirical laws identified from experimental data. However, the models and spatial resolution commonly used in these simulations remain poorly suited to representing strain localisation in the form of slip bands. In addition, the associated life criteria do not always directly account for the physical mechanisms governing crack initiation, particularly the irreversibility of plastic deformation. This work aims to investigate the effect of plastic strain localisation on the prediction of fatigue life. A crystal plasticity model capable of capturing strain localisation in the form of persistent slip bands is employed. Numerical simulations are used to compute local ratcheting, which directly quantifies the irreversibility of plastic deformation along these bands. The locally accumulated irreversible energy, associated with dislocation accumulation in the PSBs, is evaluated as the dissipated energy weighted by the ratcheting quantity. This energy is then compared with the specific free surface energy, following the Tanaka–Mura–Wu formalism, to estimate fatigue crack nucleation life. This approach allows the consideration of plastic strain localisation, the accurate identification of likely crack initiation sites, and a direct link between the local microstructural mechanisms of localisation and fatigue life criteria. It is finally applied to numerical microstructures containing typical fatigue-sensitive features, such as carbides, coarse grains, and twin boundaries, to assess its ability to capture the criticality of these features and the competition between different crack initiation mechanisms.