Multiscale analysis of fatigue crack growth using digital image correlation

To date, fatigue characterization has primarily consisted of lifetime versus loading curves (S-N curves) or crack growth rate curves (da/dN curves), usually obtained through far-field measurements. The optical technique of digital image correlation (DIC) allows quantitative full-field near-tip measurements to be made throughout the specimen lifetime, and can improve the fidelity of fatigue crack growth data. In this work, DIC is used to measure displacement fields throughout a typical cycle of Paris regime fatigue crack growth in a titanium specimen. In successive cycles, the area of observation is made progressively smaller around the crack tip in order to resolve displacements on the crack flanks better. As the scale of observation changes from a magnification of 258 to 3023 pixels/mm, effects such as higher order terms in the displacement field or plastically induced crack tip closure become evident. The plastic zone size, stress intensity factor, and T-stress are determined for each magnification. Differences observed in the stress intensity values are shown to be consistent with one another if crack face closure and field-of-view effects are accounted for.