High resolution digital image correlation measurements of strain accumulation in fatigue crack growth

Microstructure plays a key role in fatigue crack initiation and growth. Consequently, measurements of strain at the microstructural level are crucial to understanding fatigue crack behavior. The few studies that provide such measurements have relatively limited resolution or areas of observation. This paper provides quantitative, full-field measurements of plastic strain near a growing fatigue crack in Hastelloy X, a nickel-based superalloy. Unprecedented spatial resolution for the area covered was obtained through a novel experimental technique based on digital image correlation (DIC). These high resolution strain measurements were linked to electron backscatter diffraction (EBSD) measurements of grain structure (both grain shape and orientation). Accumulated plastic strain fields associated with fatigue crack growth exhibited inhomogeneities at two length scales. At the macroscale, the plastic wake contained high strain regions in the form of asymmetric lobes associated with past crack tip plastic zones. At high magnification, high resolution DIC measurements revealed inhomogeneities at, and below, the grain scale. Effective strain not only varied from grain to grain, but also within individual grains. Furthermore, strain localizations were observed in slip bands within grains and on twin and grain boundaries. A better understanding of these multiscale heterogeneities could help explain variations in fatigue crack growth rate and crack path and could improve the understanding of fatigue crack closure and fracture in ductile metals.