Relating fatigue crack growth to microstructure via multiscale digital image correlation.

摘要

This work investigates fatigue crack growth at multiple scales and links material deformation to microstructural properties. The optical technique of digital image correlation (DIC) is well suited for this task since it provides quantitative full-field deformation measurements at multiple length scales.;The first part of this work introduces two DIC-based techniques for measuring fatigue crack closure levels and demonstrates them on titanium. At the macroscale, a least squares regression was performed on full field measurements of displacement near the crack tip. At the microscale, two-point virtual extensometers were placed on the crack flanks behind the crack tip. These two measurement techniques provided crack closure level measurements from multiple scales that were consistent with one another.;In the second part of this work, fatigue crack growth was observed in a nickel based superalloy, Hastelloy X, at the grain level. A novel high resolution ex situ DIC technique was developed and used to measure accumulated strain fields in the plastic wake of a fatigue crack. Additionally, the evolution of the strain field as the fatigue crack grew through the imaging region was captured using an in situ real time DIC technique. Finally, these strain fields (from both ex situ and in situ measurements) were related to microstructural geometry and orientation obtained through electron backscatter diffraction (EBSD). By linking accumulated fatigue strain to microstructure, a deeper understanding of fatigue crack growth was obtained.;Plastic deformation in fatigue crack growth was found to follow lobes of high strain that emanate from the crack tip at angles of 45°. These lobes leave behind an inhomogeneous strain field in which slip bands and strain localizations on grain boundaries were observed. Fatigue crack growth was observed to be transgranular following lines of slip bands as dictated by global loading and local microstructure.