2004 Volume 47 Issue 3 Pages 264-273
High-speed holographic microscopy is applied to take three successive photographs of fast propagating cracks at the moment of bifurcation. The cracks are propagated in PMMA plate specimens at a speed about 660m/s. From the photographs, crack opening displacement (COD) is measured along the cracks as a function of distance r from the crack tips. The measurement results show that the CODs are proportional to √r before bifurcation. After bifurcation, the CODs of mother cracks are proportional to √r, however, the CODs of branch cracks are not always proportional to √r. Crack speed is also measured from the photographs. As a result, discontinuous change of crack speed is not observed at the moment of bifurcation. The energy release rate and energy flux toward crack tips are obtained from the COD data, and are found to be continuous across the bifurcation point. The energy release rate and energy flux increase gradually across the bifurcation point.