Local Yielding Around a Crack Tip in Plexiglas

An optical method was developed to study the highly strained region surrounding small edge cracks in brittle transparent materials loaded in tension. Interference of the partially reflected coherent monochromatic light beam from the front and back surfaces of the specimen prepared from an optically isotropic and inert material, gave a fringe pattern which depicted the thickness variation of the plate in the neighborhood of the crack. The fringe pattern consisted of a dense pattern surrounding the crack tip which was separated from the rest by a distinct bright envelope. It was shown for the material studied that the limiting envelope was a circle which, when projected to a screen, became an epicycloid. The evolution of the geometric characteristics of the highly strained zone with increasing load was studied, up to the critical load for a rapid crack propagation. The data obtained yielded an insight on the mechanism of fracture in brittle materials.