In situ SEM-DIC technique and its application to characterize the high-temperature fatigue crack closure effect

In situ high-temperature(maximum: 650℃) experiments for fatigue crack growth on single edge-notched GH4169 superalloy specimens have been performed in scanning electron microscopy(SEM). In order to investigate the crack closure behavior,digital image correlation(DIC) method was applied to measure the full-field displacement and strain fields around the crack tip.The deformation carrier(speckle) was fabricated on the polished specimen surface with Zr O2 powders having a good hightemperature resistance, and the speckle patterns were recorded during the fatigue test by SEM. In consideration of the distortion of SEM images, the unabridged model and the bicubic model were used to correct the spatial distortion, and the distortion parameters can be determined by a calibration test. It is shown that the unabridged model makes a better contribution towards improving SEM-DIC measurement accuracy. Based on the deformation field, the mode Ι stress intensity factor range ?K and the crack opening displacement(COD) can be calculated for evaluating the crack closure effect. Additionally, the crack propagation was monitored using SEM and optical microscope. It indicates that the cracks grew initially in trans-granular mode at room temperature and 650°C, then propagated in both trans-granular and inter-granular modes.

In situ high-temperature(maximum: 650°C) experiments for fatigue crack growth on single edge-notched GH4169 superalloy specimens have been performed in scanning electron microscopy(SEM). In order to investigate the crack closure behavior,digital image correlation(DIC) method was applied to measure the full-field displacement and strain fields around the crack tip.The deformation carrier(speckle) was fabricated on the polished specimen surface with Zr O2 powders having a good hightemperature resistance, and the speckle patterns were recorded during the fatigue test by SEM. In consideration of the distortion of SEM images, the unabridged model and the bicubic model were used to correct the spatial distortion, and the distortion parameters can be determined by a calibration test. It is shown that the unabridged model makes a better contribution towards improving SEM-DIC measurement accuracy. Based on the deformation field, the mode Ι stress intensity factor range ?K and the crack opening displacement(COD) can be calculated for evaluating the crack closure effect. Additionally, the crack propagation was monitored using SEM and optical microscope. It indicates that the cracks grew initially in trans-granular mode at room temperature and 650°C, then propagated in both trans-granular and inter-granular modes.