Method for successive photographing of rapid crack bifurcation by means of high-speed holographic microscopy

High-speed holographic microscopy is applied to take three successive photographs of rapidly bifurcating cracks. A crack propagates in a PMMA specimen at a speed more than 600 m/s, and bifurcates into two cracks in the observation area at the center of the specimen. The position at which the crack bifurcates is controlled by varying the tensile stress applied to the specimen. When it bifurcates, the fast propagating crack is recorded as three holograms. The holograms reconstruct the real images of the crack, which are photographed through a conventional microscope. From the photographs, crack speed before and after bifurcation is obtained. Crack speed after bifurcation is slightly lower than that before bifurcation. Crack opening displacement (COD) is also measured along a crack both before and after bifurcation. The COD before bifurcation is proportional to square root of the distance from the crack tip. After bifurcation, the COD of a mother crack is proportional to square root of the distance from the nominal tip of the mother crack. However, the CODs of two branch cracks are not always proportional to square root of the distance from each of the crack tips.