Research on Nondestructive Evaluation of a Crack Existing at Weld by Means of D-C Potential Drop Technique

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The research has been performed to develop a method based on d-c potential drop technique for evaluating nondestructively a crack at weld. The main results are summarized as follows:1. A method was presented for the nondestructive evaluation of a 2-D crack on the back wall of an infinite strip having weldment. The crack geometry in the strip having the weldment was determined by combining simple problems of a cracked strip without the weldment and a welded strip without the crack.2. A new procedure was proposed to evaluate a 3-D crack shape nondestructively with high accuracy by utilizing the d-c potential method. An elliptical embedded crack and a semielliptical surface crack on the back wall in a plate were evaluated especially with respect to location , length and inclination of the crack. Furthermore a nondestructive procedure was developed to evaluate a 3-D shape of a crack in a welded plate, where the electrical and geometrical inhomogeneity of the weldment was considered, and the effect was calibrated. The availability of the presented procedures was verified by experiments stainless steels.3. Effects of closure on an evaluated value of crack size were examined experimentally, where the evaluation was carried out by using d-c electrical potential method. It was found that the potential method is able to give valid evaluation for close cracks, provided that the applied load is free or tensile.4. A simplified method was developed for applying potential drop technique to nondestructieve evaluation of a 3-D crack on the inner surface of a welded pipe. The method was based on 2-D theoretical analysis with modification for considering the actual 3-D nature of current flow.