Fatigue crack initiation modeling of 316LN steel based on non local plasticity theory

Numerous studies have pointed out the major role of microstructural phenomena in the nucleation of intra-granular crack nucleation during low cycle fatigue tests. Complex dislocation arrangements and rearrangements have been observed and different dislocation structures such as vein, ladder and/or cell structures have been identified in cyclically loaded steel specimens. These dislocation structures are related to a heterogeneous localization of plastic strain which is mostly accommodated by ladder structure of dislocations, also named Persistent Slip Bands (PSBs). These regions of intensive slip generate on the material surface intrusions and extrusions called Persistent Slip Markings (PSMs). The emergence of this rough relief leads to the initiation of fatigue cracks and is commonly seen as the first sign of fatigue damage. For a better understanding of fatigue crack nucleation in 316LN stainless steel, interrupted low cycle fatigue tests with constant loading amplitude were carried out on cylindrical specimens with polished shallow notches. Observations have been made at different stages to monitor the specimen surface. Development of PSMs and the initiation of fatigue cracks were observed. In parallel a three-dimensional finite elements model of crystalline plasticity, named CristalECP, has been developed in both ABAQUSTM and CAST3MTM finite elements codes. Compared to classical approaches, the hardening law has been modified to take into account a physically motivated measure of lattice incompatibility. This measure is introduced through Geometrically Necessary Dislocations (GNDs) which are directly related to the gradient of the lattice distortion and supposed to model the resistance to plastic flow provided partially by lattice defects and grain boundaries. The numerical studies performed on various polycrystalline aggregates of 316LN steel have shown that the inverse relationship between the macroscopic plastic flow stress and the grain size can be reproduced. An influence of the grain size on the localization of the computed mechanical fields has been observed.