Electromagnetic NDT has a long history in England. RADAR, an early application,
played an important role during the Second World War. This application involved
detection, with some discrimination. Although this technique was in its infancy,
it had won over major support from scientific and political arenas. In a meeting
at the Air Defence Research committee in July, 1935, Robert Watson-Watt, inventor
of radio direction finding (RDF - The American term for RDF, Radio Direction
and Ranging, was later adopted in an effort to standardize on nomenclature,
hence RADAR), presented his paper 'Detection and Location of Aircraft by Radio
Methods' for the first time to Sir Winston Churchill, who was to become a strong
advocate. At the meeting he passed a four line pencilled note across the table
saying
Seeking, Finding, Following, Keeping,
Is he sure to bless?
Angels, Martyrs, Prophets, Virgins,
Answer: "M, Yes"
The detection of enemy aircraft by radar was one of the major success stories of the war, firmly establishing the utility of this electromagnetic technique.
In the following decade, industry began using electromagnetic techniques for inspection. Most of these techniques were empirical. In an early effort, General Motors was using a quasi-theoretical approach to measure overlay thicknesses. However; the equipment used was analog and based on calibrated standards, advances in computers still restricted quantitative NDT to theoretical work. The theoretical basis upon which quantitative NDT stands, was being laid down at this time. Work by Tai and Wait, to mention a few.
By the mid-60's, advances in computer science had made computational electromagnetics possible and affordable for large corporations or government bodies. Therefore, theoretical results could now be approximated numerically as well as verified by experiment. The theoretical work continued with important contributions on the analytical and computational complexities of the free-space Green's function by Fikioris and Van Bladel. In their important paper Dodd and Deeds provided a theoretical basis for understanding the interaction between an eddy-current probe and planar and cylindrical conductors with up to two layers. This work also showed that numerical results were in good agreement with experiment for a large range of normalized probes geometries. Contributions were also being made from geophysics, where geophysicists were faced with similar problems. This can be seen in the work of Weaver.
The theoretical efforts continued into the next decade. The seventies saw the important work of Dodd and Deeds and Wait on cylindrical conductors. The geophysicists were now looking at volume integral techniques to solve their problems, with important contributions from Raiche and Coggon. During this period quantitative eddy-current NDT became well established as an accepted inspection technique in the field; being used especially in safety related areas in aerospace, oil and power generation industries. In the field calibration standards and manual inspection still were the norm, with very little numerical analysis of data.
There has been an explosion of effort on the computational side of eddy-current NDT since 1980 as computation speed has increased and the price of hardware decreased. Early in this decade there were several results attempting to come to grips with the nature of the singularity in the heart of the Green's function uses for electromagnetic analysis when using integral techniques by Lee, Yaghjian, Wang and Beissner. Finite element techniques were also applied to NDT by Lord and Véritè and others. Beissner and others developed boundary element techniques.
Burke has carried out experimental work for the validation of theory and computer codes. In an effort to establish experimental and theoretical benchmarks for these complicated codes two international organizations were established: Testing Electromagnetic Analysis Methods (TEAM) and Applied Computational Electromagnets (ACES). The NDT validation exercises by these two groups have been used to validate the EddyCentre basic models and the EddyCentre advanced models. These models were acknowledged by winning The Technical Co-operation Program Technical Achievement Award for Verification of Field-Flaw Models in 1992 by the British Ministry of Defense.
The application of the volume integral technique to eddy-current testing has
allowed the implementation of the volume integral technique to run on personal
computers on desk tops and is one of the most exciting aspects of our \ec products.
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