Overview of Hawkeye Technique for Non Destructive Testing
Hawkeye utilises BFET (Balanced Field Electromagnetic Technique), and is a surface and slightly subsurface technique used chiefly for the detection of cracking in welds and metal plates.
BFET uses a bridge probe with a differential wire arrangement. The twin coils sit perpendicular to each other and are balanced to the material being tested. Any changes in permeability will cause different responses in each wire as the probe passes over the defects, this causes the field to become unbalanced, and changes are reflected in the graph on the characterize.
Hawkeye is very versatile in terms of what it can inspect, and this is perhaps its greatest strength. Due to its speed it also makes a great screening technique. Ferrous in addition as non-ferrous metals can be inspected, and inspection can also be performed by coatings and linings. Surface preparation required is also minimal, with a high pressure water blast generally being sufficient. As long as there is a fairly clean smooth surface inspection can be carried out.
Hawkeye being an electromagnetic technique is only effective on metals unfortunately, due to the magnetic field needing to be induced into the material. Due to the natural permeability of the metal, inspection thoroughness can vary. 10mm is generally considered the maximum penetration though under ideal conditions in carbon steel.
Usual applications of Hawkeye are weld inspection, HAZ area testing and also for metal plates, such as in storage tanks or columns. Weld/HAZ area inspections in lined tanks are chief candidates for Hawkeye due to ability to inspect by coatings. Stress corrosion cracking in pressure vessels and columns are also good candidates for Hawkeye due to the penetration thoroughness obtainable, and the rapidness with which it can examine. Generally targeting a percentage of the most at risk areas is a good first step.
Results of Hawkeye can be backed up with technologies such as dye penetrant testing, and shearwave ultrasonics. Dye penetrant testing is a viable different in certain situations, however takes much longer, and requires a much cleaner surface, in addition as an inability to inspect by coatings. Hawkeye will also detect subsurface defects that dye penetrant testing would miss.
shared flaws detected include porosity, slag, without of fusion (without of penetration), and crack like flaws.
Overall Hawkeye is a very versatile technique with a lot in shared with other electromagnetic techniques, and is a valuable addition to existing NDT techniques. As with all NDT the more data obtainable the more certain we can be of true results.