Keywords: non-destructive testing, rotating magnetic field, finite element analysis, detection of cracks, metal welded joints, quality control.

Non-destructive tests (NDT) play a significant role with special regard where the integrity of the material is strictly required. Within the framework of aging materials inspection, one of the most important aspects is concerned with the detection of defects in metal welded joints. The quality of a welded joint depends on the product allocation. The feature is devised according to the intended use of the jointed material, but it also considers all factors that may affect the welding. Scientists measure the normal component of the magnetic field to the inspected surface. In this paper we analyse an innovative solution, using a rotating magnetic field, for recognizing the presence of cracks in welded joints. This approach can detect cracks even if their orientations are perpendicular to the longitudinal direction of the sensor. For our purpose, a finite element analysis (FEA) code has been used. State-of-the-art NDT systems allow the detection of a defect but without being capable of determining its shape. In particular, porosity bubble cracks are among the typical discontinuities affecting a welded joint. The causes provoking the formation of these defects are, for instance, metal and/or technological, depending on faults during the welding process. Typical defects occur at a temperature of 1000° (C) during the cooling process. They are the result of two different factors: presence of impurities in the metal base and internal tensions. The generation of a rotating magnetic field exploits the so called Ferraris effect [1]. Our FEA code is able to simulate the time dependent electromagnetic system; it utilizes a two-dimensional A-psi formulation [2]. For simulating the rotation effect, Euler rotation formulation has been used. The performance shown by the proposed solution is very encouraging: the use of a rotating magnetic field allows not only the detection of defects irrespective of their orientations, but also an increased ability of detecting very small cracks in welded joints. Subsequently, Shewhart control charts have been drawn. By varying the limits, it has been possible to assess the relevance of defects and the seriousness of the crack presence in welded joints besides the possibility of establishing when a joint is conform to specifications.

1

G. Ferraris, "Rotazioni elettrodinamiche prodotte per mezzo di correnti alternate", (in Italian), Atti dell'Accademia delle Scienze di Torino, vol. 23, pp. 360, 1888. doi:10.1007/BF02730244

2

D. Rodger, P.J. Leonard , H.C. Lai, "Interfacing the general 3D A-psi method with a thin sheet conductor model", IEEE Transactions on Magnetics, vol. 28, no. 2, pp. 1115-1117, 1992. doi:10.1109/20.123879

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