Effect of Higher Order Terms on Near-Tip Field of Cracks Immediately after Beginning of Rapid Propagation. Measurement by Interferometry.

Abstract

A method of interferometry is applied to study the near-tip field of cracks which are propagating in PMMA specimens at a speed of about 550 m/s. The interference fringes around the cracks are photographed with a holographic optical system at 18 μs or at 73 μs after the crack initiation. The interference fringes are measured and analyzed in the directions of ±72 degrees from the propagation direction of the crack in order to compare the measurement results obtained through interferometry with those through the caustic method. It is found that the effect of higher order terms appears in the near-tip field at 18 μs after the crack initiation, but disappears at 73 μs, and the singular field is developed as far as 9 mm from the tip. Consequently, it can be concluded that, in the directions of ±72 degrees from the propagation direction, the effect of higher order terms appears within several tens of microseconds after the beginning of rapid crack propagation where crack speed is more than it few hundred m/s. These results are in good agreement with those obtained through the caustic method.