Determination of Stress Intensity Factor K_III for Three-Dimensional Crack by Using Caustic Method in Combination with Stress-Freezing and Stress-Releasing Technique

In this paper, an experimental technique to determine the stress intensity factor K_III for three-dimensional cracks using the method of reflected caustics in combination with the photoelastic stress-freezing and stress-releasing techniques is presented. The experimental model is a cylindrical bar with a three-dimensional surface crack under a load of pure torsion. To obtain the caustic pattern, a slice cut from the frozen model is annealed to release the frozen-stress. The results of this experimental method coincide favorably with the theoretical analysis results of Tweed and Rooke. Moreover, the mixed-mode stress intensity factors K_II and K_III of the three-dimensional crack are separately determined using the photoelastic and caustic methods. By using the present experimental technique, the mixed-mode stress intensity factors K_II and K_III for a three-dimensional crack are found to be easily separated.