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Title: Advances in Potential Drop Techniques for Non-Destructive Testing
Author: Sposito, Giuseppe
ISNI:       0000 0004 2681 4539
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2009
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In the field of Non-Destructive Testing, Potential Drop (PD) techniques have beenused for decades, especially in the petrochemical and power generation industries,for monitoring crack growth and wall thickness variations due to corrosion and/orerosion in pipes, pressure vessels and other structures. Inspection is carried out by injecting currents in the specimen to be tested andmeasuring the arising electrical potential di erence between two or more electrodesplaced on its surface. The presence of a defect generally increases the resistance andhence the measured voltage drop; inversion of these data can give information onthe size and shape of the defect. However, while the principle underlying these techniques is relatively simple, somedi culties have been encountered in their practical applications. Many commercialsystems based on PD methods, for instance, require the injection of very largecurrents in order to obtain su ciently large signals; doubts have been raised onthe stability of these methods to variations in the contact resistance between theelectrodes and the inspected material. The present work aims to show that someof these problems can be easily overcome, and to evaluate the capabilities of PDtechniques for crack sizing and corrosion mapping. After a brief review of the advantages, disadvantages and applications of the mainelectromagnetic methods for Non-Destructive Testing, an experimental setup forPotential Drop measurements which was developed for this work and which usessmall alternating currents (AC) is described. The setup is benchmarked against existingPD systems and then used to validate a model that allows AC PD simulationsto be run with a commercial Finite Element code. The results of both numericalsimulations and experimental measurements are used to investigate the possibilityof sizing defects of complex geometry by repeating the analysis at several di erentfrequencies over a broad range, and of reconstructing the depth pro le of surfacebreakingdefects without the need for assumptions on their shape. Subsequently, the accuracy to which it is possible to obtain maps of corrosion/erosion on the far surfaceof an inspected structure is discussed, and results obtained with an array probethat employs a novel arrangement of electrodes are presented. Finally, conclusionsare drawn and suggestions for further research are made.
Supervisor: Cawley, Peter ; Lowe, Michael Sponsor: Research Centre of Non-Destructive Evaluation (RCNDE)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral