Strain field measurement around the crack tip in Ni-base superalloy using Digital Image Correlation

Abstract

Digital Image Correlation (DIC) method was applied to measure the strain fields around the crack tip in a Ni-base single crystal superalloy. DIC systems were assembled to appropriately measure the strain fields at room temperature, 700˚C700˚C and 900˚C900˚C. Influences of crystallographic orientation and temperature on the crack tip strain fields were investigated. A series of measurement at room temperature revealed that cracks propagated in shearing mode and the strain fields around the crack tip were strongly affected by anisotropic plastic deformation along the octahedral slip system. Based on the shear strain components along the slip systems, effect of the crystal orientation on the strain field was visualized, and the visualized strain field provided a reasonable explanation on the crack propagation path and propagation rate. At 700˚C700˚C, on the contrary, cracks propagated in opening mode and symmetric shear strain fields were measured around the crack tip. Temperature dependent strain fields and resultant cracking modes were explained by the slip system activity which was also influenced by temperature. The present DIC systems could measure the strain field even at 900˚C900˚C, and it was found that the strain around the crack tip was higher at 900˚C900˚C than that at 700˚C700˚C.