A BOUNDARY ELEMENT METHOD FOR INVERSE PROBLEMS IN NON-DESTRUCTIVE EVALUATION USING ACFM

Abstract

The alternating current field measurement (ACFM) technique was developed as a non-destructive evaluation tool for the detection and measurement of surface cracks in the 1960s. However, the success of the ACFM relies greatly on the interpretation of the measurement data, and existing analytical methods are not applicable to general shapes of crack profiles. Numerical methods, such as the boundary element method (BEM), are generally more suitable for the prediction of general crack profiles but are normally limited in stability and accuracy to cracks with large aspect ratios (small depths). The present work focuses on the development of BEM techniques which are applicable to a wider range of cracks in the ACFM non-destructive evaluation method. Quadratic boundary elements are introduced to improve the stability and accuracy of the predictions in this class of inverse problems. The standard iterative technique is shown to be efficient in the prediction but not stable and accurate enough for cracks with profiles deeper than a semi-circle. A new isopotential method is proposed, and is found to be a great improvement in the prediction of deep cracks. When the alternating current field is asymmetrical with respect to the crack, a multi-domain BEM based on the standard iterative technique is employed. The problems under consideration cover asymmetrical surface cracks on the plane and around a comer. A uniform domain measurement scheme is also developed for both symmetrical and asymmetrical alternating current fields, and experiments were conducted in order to verify the method. The numerical examples, which are used to assess the standard iterative technique, the isopotential method, the multi-domain BEM and the uniform domain measurement scheme, show that significant improvements have been made possible in the present work. Many different types of crack profiles are predicted successfully by applying the technique proposed in this thesis.