Static experiment on rapid crack bifurcation with Y-shaped and parallel notches

Static experiments related to rapid crack bifurcation are conducted with PMMA plate specimens with Y-shaped notches. The bifurcation angle of the Y-shaped notch is changed from zero to 45 degrees. Tensile force is applied to the specimen, and a crack arises at one of the two branch notch tips. The crack propagation angle is measured, which is the angle between the crack and the branch notch. The measurement results say that the crack propagates in the same direction as the branch notch when the bifurcation angle is about 14 degrees. This angle is approximately the same as the bifurcation angle of fast propagating cracks. The crack propagation angle is also measured when the bifurcation angle approaches to zero. The crack propagation angle increases with decreasing the bifurcation angle and approaches 14 degrees as the bifurcation angle tends to zero. The experiments with two parallel notches are also carried out, and the crack propagation angle is measured. The crack propagation angle increases with decreasing the distance between the two parallel notches, and has the maximum value of about 22 degrees. When the distance between the two parallel notches decreases further, the crack propagation angle decreases. But the crack propagation angle does not approach zero but remain finite at 14 degrees as the distance of the two parallel notches tends to zero. These results explain why a crack bifurcates with the bifurcation angle of 14 degrees