In situ SEM observation of microscale strain fields around a crack tip in polycrystalline molybdenum

Abstract

In situ scanning electron microscopy was employed to investigate the crack initiation and propagation in polycrystalline molybdenum under uniaxial tensile load at room temperature. The microscale grid pattern was fabricated using the sputtering deposition technology on the specimen surface covered with a fine square mesh copper grid. The microscale strain fields around the crack tip were measured by geometric phase analysis technique and compared with the theoretical solutions based on the linear elastic fracture mechanics theory. The results showed that as the displacement increases, the crack propagated mainly perpendicular to the tensile direction during the fracture process of molybdenum. The normal strain exx and shear strain exy are relatively small, and the normal strain eyy holds a dominant position in the deformation fields and plays a key role in the whole fracture process of molybdenum. With the increase in displacement, the eyy increases rapidly and the two lobes grow significantly but maintain the same shape and orientation. The experimental eyy is in agreement with the theoretical solution. Along the x-axis in front of the crack tip, there is minor discrepancy between the experimental eyy and theoretical eyy within 25 μm from the crack tip, but the agreement between them is very good far from the crack tip (>25 μm).