Method for Measurement of Near Tip Field of Fast Propagating Cracks by Means of Interferometry

An interference method is applied to the measurement of stress field in the vicinity of a fast propagating crack tip. A parallel light beam impinges upon a transparent plate specimen perpendicularly to the surface. A crack is being propagated through the specimen at a speed of five hundred m/s or more. The incident light is partly reflected either by the front surface of the specimen or by the back surface of it. The two reflected light waves interfere each other and interference fringes appear on the specimen. The interference fringes indicate the equi-thickness lines along which the sum of the principal stresses is kept constant. In the directions of 0, ±36 and ±72 degrees from the propagation direction of the crack, the orders of the interference fringes are measured as a function of distance r from the crack tip. And the dynamic stress intensity factor is obtained in each direction, respectively.