Electric and magnetic contributions and defect interactions in remote field eddy current techniques

This thesis investigates the remote field eddy current (RFEC) inspection of stress corrosion cracking (SCC) in steel pipes. The results suggest a conquerable gap between the present experimental level and RFEC detection of SCC on a laboratory scale. The investigation also covers defect source modelling and 3-dimensional finite element analysis (FEA) for detailed studies of defect electric and magnetic interactions, and electric and magnetic individual contributions from defects in steel. It was found that there is a considerable difference between far and nearside SCC signals. The dipole radiation model has been developed to interpret and reconstruct the defect responses from conducting magnetic and steel samples.; Naturally formed SCC fine shallow cracks (15-30% full wall) in a 510 mm steel pipe were measured using an inside-excitor and outside-scan electromagnetic field through-wall transmission setup. Clear crack signals were obtained. The data were compared with inside-scan measurements, combined with finite element results on near and farside crack responses, to find a link to RFEC testing. The crack signal improvement using a pipe wall magnetic saturation technique was also investigated. The results presented in this thesis are part of the work under a contract for investigation of the possibility of detecting fine SCC using RFEC techniques.; The dipole radiation model was proposed and tested. This defect source model describes computable electric and magnetic defect sources, and provides a clear frame to simplify the analysis of complicated defect responses. A classification leading to four basic types of electric and magnetic interactions is proposed and a rule for the interactions is discussed. A general approach using FEA to evaluate electric and magnetic individual contributions from a defect in steel is introduced. The evaluation may provide a quantitative guidance to estimate the gain or loss in a signal when the sample material properties or experimental conditions change. Defect source concepts are discussed and applied throughout the thesis. The thesis also investigates the crack signal dependence of crack volume filling rate, and infinitely-thin-slit behaviour. It revealed the existence of significant residual signal responses, using finite element analysis. These residual signals may have a significant influence on SCC detection. The rich and carefully prepared graphs may prove to be useful for reference in further studies.