Ultrasonic imaging of corrosion and fatigue cracks in multilayered airplane structures

Application of ultrasonic imaging to the inspection of multilayered airplane structures can provide comprehensive information on possible defects such as corrosion and cracks. A quantitative assessment of defects in internal layers of airplane structures using this technique can be accomplished form an external surface of the airplane skin. No disassembly is required for component evaluation. Therefore this ultrasonic technique can be extremely useful for cost-effective maintenance of airplanes. Three different ultrasonic techniques developed for inspection of the DC-9 and DC-10 airplanes are discussed in this paper. One of the techniques was used for evaluation of corrosion and stress corrosion cracks in a DC-9 tee cap, other techniques have been developed for fatigue crack characterization in a DC-9 rear spar and a DC-10 spar-cap/strap connection, respectively. All three techniques were developed to be applied with the same type of data acquisition and imaging system. However for each kind of flaw and structural configuration substantial modifications of the scanning and the data acquisition procedures have been made. Peculiarities of each ultrasonic technique will be discussed. The ultrasonic technique for the inspection of the DC-9 tee cap has been approved by the FAA as an alternate means of compliance to meet the requirements of the DC-9 Service Bulletin.