Evaluation of structural integrity of steel components by non-destructive magnetic methods
Magnetic non-destructive methods utilising the Magnetic Flux Leakage (MFL) and Magnetic Barkhausen Noise (MBN) phenomena are widely used in the evaluation of the structural integrity of steel components. The MFL method is effectively applied for in-service flaw monitoring of oil and gas pipelines, fuel storage tank floors and rails; whereas the MBN method, due to high sensitivity of Barkhausen emission to residual and applied stress, has become one of the most popular NDE tools for investigating this condition of steels. Despite the affirming research and successful applications, which helped these methods to gain acceptance as a viable non-destructive tools, there is still a requirement for establishing a quantitative links between magnetic and mechanical properties of steel which would enable their further understanding and optimisation.\ud In this thesis the applications of MFL and MBN methods for flaw and stress detection are analysed via analytical and numerical modelling.\ud A new model relating the MBN amplitude and stress for materials having different magnetostrictive behaviour under load is proposed and validated in the quantitative stress evaluation of different grades of steel. Moreover, a new method for determining depth dependence of stress from measured magnetic Barkhausen signals is presented. A complete set of newly derived equations describing the detected Barkhausen signals in terms of the actual emissions that are generated inside the material and how these appear when they propagate to the surface is given.\ud The results from finite element modelling of magnetic flux leakage signals above unflawed and flawed rails energised in various directions are presented. These results enabled to identify the most effective current injection procedure and optimise the probability of transverse flaw detection in the rail inspection. The agreement between modelled and measured electromagnetic signals indicating presence of transverse rail defects has been justified.