| The dissertation is focused on the problem of thin (stress corrosion) crack modeling in the numerical electromagnetic NDT simulations for gas transmission pipelines. Axially oriented cracks are known as the most difficult for electromagnetic detection and a number of methods have been designed to achieve the necessary sensitivity of the output signal. Two recently developed methods, the velocity induced current perturbation (VICP) method and the rotating magnetic field (RMF) method, are considered as possible ways to improve the quality of inspection. In the VICP method the material in the inspected region is magnetized to saturation, so that the changes in output signal are mostly caused by perturbations due to the velocity induced currents. Due to the axial symmetry in geometry, the velocity induced currents are circumferentially oriented, and hence, are normal to the plane of axially oriented cracks. In the RMF method, saturation magnetization is not used, and the main contribution in the changes of output signal is from the magnetic flux perturbations in the pipe, caused by cracks. In this case the magnetic flux lines are normal to the plane of axially oriented cracks.; Conventional modeling requires dense mesh discretization around the crack. In the proposed approach a crack is modeled in such a way, that the material properties are allowed to vary arbitrarily within a single element, which significantly simplifies the mesh. A number of tight crack models have been developed and integrated in the numerical codes, simulating the electromagnetic field redistribution in a pipe with VICP and RMF test bed vehicles. It has been found that better accuracy is achieved if, for each model, a specific shape function is used. 1-D, 2-D (axisymmetric and polar) and 3-D codes were implemented and studied. Initial experimental studies have been made to calibrate the codes. |
