| The goal of the work is to carry out an experimental analysis to predict the kinetics of stage II fatigue crack growth based on the strain field around a fatigue crack tip in polycrystalline nickel as a model material. The strains ahead of the crack tip were measured by digital image correlation (DIC) with standard compact tension (CT) specimens. Cracks were grown under constant stress intensity factor range (SIFR), in the stage II region of fatigue crack growth, to prevent excessive stretch of the plastic zone. Two specimen sizes, one that experienced large scale yielding and another that did not, were used to assess the consistency of the model. The strain field showed a 'V' shape, corresponding to "forward" strains ahead of the crack tip and strongly depended upon the microstructure. Deformation was sometimes concentrated along a particular slip system resulting in a very strong ridge in the 'V'. In most cases the local crack tip followed this stronger ridge and did not change its direction after several in-situ loading cycles.; The maximum opening strain ahead of a crack tip shows a power law relationship with applied SIFR regardless of specimen size. However, it experiences too much scatter due to the microstructure to be used as a reliable parameter. A "volumetric strain," was defined to address this issue. This volumetric strain also shows a power law relationship with applied SIFR. The combination of this result with the Paris relationship indicated that the crack growth rate had an almost linear relationship with the volumetric strains.; Crack growth rates were estimated in-situ using half the measured crack tip opening displacement (CTOD). The in-situ measurements were higher than the average crack growth rate obtained by standard fatigue crack growth tests. This was due mainly to differences in constraint. To compensate, a constraint factor was introduced via 3D elastic-plastic finite element analysis (FEA). When the in-situ measurement data were multiplied by this constraint factor both data agreed well.; Finally, misorientation measurements using orientation imaging microscopy (OIM) revealed that there was a substantial amount of cumulative lattice rotation after fatigue crack growth, with large strains corresponding to large lattice rotations. Lattice rotation caused by one loading cycle was measurable at high loads and was correlated with the opening strain measured by the DIC, indicating that strain localization persisted for several cycles. |
