Experimental Study of Stress Intensity Factors Km of a Three-Dimensional Crack by the Method of Caustics

In fracture problems, stress intensity factors obtained theoretically or by experimental analyses have been effectively utilized in analytical evaluation of the effect of cracks. The optical method of caustics is a very effective technique for evaluating this factor. Use of the caustics method to determine the stress intensity factors of two-dimensional cracks has been established theoretically and experimentally. However, relatively few experimental analyses of three-dimensional cracks using this method have so far been conducted. In this paper, an experimental technique for determining the stress intensity factor Km for a three-dimensional elastic solid has been presented by using the method of reflected caustics in combination with the photoelastic stress freezing method. The experimental model is a cylindrical bar with a radial crack along the direction of the axis under the load of pure torsion. In order to obtain a caustic pattern, a slice cut from the frozen model is annealed. The results of this experimental method coincided favorably with the theoretical analysis results of Tweed and Rooke.