Evaluation of positions of thinning spots in a dual structure using ECT with DC magnetization

Abstract

Ultrasonic thickness meters are generally used in corrosion evaluations of pipes. On the other hand, there are pipes with gusset plates. Then the meters using a simple reflection cannot be used, because the ultrasonic is attenuated by an air gap between pipes and gusset plates. For the pipes with the dual structure, new method with multiple reflections is being studied. When the method is used, it is important to know number and positions of thinning spots. This work is motivated by the desire to evaluate positions of thinning spots in a Dual Structure of steel nondestructively. Our approach is based on the Eddy Current Testing with DC magnetization. The ECT with DC magnetization has been studied. Recent studies have revealed that the Hall coefficient is sensitive to the presence of elastic strain. However, there are not still studies which applied the ECT to the structures. In this paper, the ECT is applied to an evaluation of thinning spots in the dual structure. This paper attempts a first step toward visualizing magnetized states of specimens with an electromagnet using the 2D nonlinear electromagnetic field analysis. Here, needed magnetic force is obtained to detect the thinning spots on the back surface. Then, the measurement principle is explained. In the next section, the ECT with DC magnetization is applied to a specimen with a thinning spot. From the results, the usability of this method is evaluated.