A novel method for measurements of microstructurally small fatigue crack growth using digital image correlation

Although several models addressing microstructurally small fatigue crack behavior have been presented, there is work to be done in developing models that are based on the directly measured crack driving forces. The current study proposes a new method that combines high-resolution and high sample-rate digital image correlation analyses, optical crack length measurements, and microstructural information from electron backscatter diffraction analysis (EBSD). Microstructurally small fatigue crack growth was studied using a dual-microscope setup. The crack growth rate, cyclic crack-tip opening displacement (DELTA_CTOD), and full-field shear strain were measured over multiple grains, starting from sub-grain crack length up to seven times the grain size. A novel crack length analysis method was developed for the high- resolution optical measurement data, and the DELTA_CTOD was defined at a length scale smaller than the grain size. The grain structure from EBSD was combined with the strain measurements, exposing the influence of microstructural features on the shear strain accumulation during the fatigue crack growth. The developed methodology was used to study the propagation of a microstructurally small crack in Fe-18%Cr ferritic stainless steel. The results show that the developed method enables high-fidelity investigations on the effect of the grain boundaries and the shear strain localization on the microstructurally small fatigue crack behavior.