Microscale characterization of small fatigue crack propagation behavior using in situ SEM-DIC

Characterizing small fatigue crack propagation behavior is essential for assessing structural integrity, and refining fatigue life models by incorporating critical factors like crack closure effects. The integration of scanning electron microscopy (SEM) with digital image correlation (DIC) techniques offers significant advantages for in situ microscale observation and analysis, enabling full-field and non-contact deformation measurement. In this study, the small fatigue crack growth behavior of nickel-based superalloy GH2036 is investigated through in situ SEM monitoring of single-edge notched tensile (SENT) specimens. The deformation field near the crack tip is quantified using SEM-DIC, based on which a previously proposed parameter termed the crack opening ratio (COR) is introduced and employed to evaluate crack closure effect at the microscale for the first time. The crack closure level, specifically the crack opening load ratio (COLR), determined via COR shows good agreement with results from the stress intensity factor (K)-based approach. This consistency underscores the reliability of the COR methodology for microscale fatigue characterization, providing a robust framework to advance understanding of crack closure mechanisms and refine fatigue life prediction models.