Foreword to the special section on electromagnetic and ultrasonic nondestructive evaluation

A central aim of nondestructive evaluation (NDE) is to acquire knowledge of material properties and flaws in manufactured components from measurements using sensors. New developments in NDE are motivated by the desire for quality control of manufactured parts and by the need to maintain the safe operation of industrial plant and machinery. Aircraft, nuclear power plants, gas pipelines and many other complex systems supporting the needs of advanced societies rely on inspection for reliable and safe operation. Much of the research effort in NDE is aligned with the interests of the broader community of scientists and engineers who study inverse problems and their applications in areas such as geophysics, medical imaging, remote sensing or underwater acoustics, to mention but a few. Indeed, many of the basic methods adopted for NDE including tomography, synthetic aperture techniques and iterative inversions, under many guises, are widely used in these areas. In view of this common interest, and the benefits of bringing the distinct problems that arise in a particular technical area to the attention of a wider audience, it would seem appropriate and timely to publish this special section on electromagnetic and ultrasonic nondestructive evaluation. Although the mere detection of an anomaly in a newly manufactured component might be sufficient cause to condemn it to the scrapheap, it is often expedient, following in-service inspections, to maintain a defective structure in operation. In the field, the cost of replacement or repair is weighed up against the probability and price of failure, which may, in severe cases, include damage to the environment or loss of life. A full and valid assessment depends on the knowledge of a number of factors such as the risks involved and the current state of the damage. Inversion techniques can play a significant role in damage assessment because the raw inspection data provide only indirect knowledge of defects. The challenge for the researcher is to provide the tools for their improved characterization through the use of imaging and inversion techniques. Typical (yet far from unique) defects in aging components are fatigue cracks and regions of material loss due to corrosion. Ultrasonic and electromagnetic testing (mostly, but not only, at eddy current frequencies) and their combination with other investigation modalities, provide, as a minimum, an indication from sparse data that a defect is present and, at best, a limited data set that is adequate to reconstruct the flaw geometry or to generate a reasonable visual representation of the flaw in the form of an image. Improvements in inspection methods and the ability to acquire large quantities of data automatically will undoubtedly provide the inspiration in the future for refinements in the inversion methods currently used or undergoing development in this field as well as for completely new tools that should be put forth in, probably, unexpected ways. This special section contains twelve articles involving almost forty authors and more than fifteen research groups worldwide. These articles combine theoretical, computational and/or experimental developments, and may be either a review of the most recent advances in a particular area of research or an original and specialized contribution. In short, and all taken together, they attempt to summarize the progress made and the current status of developments after a period of twenty or so years as well as opening up some new paths of investigation at the modeling and/or application level.